First draft prepared by
Dr G. Speijers,
National Institute of Public Health and Environmental Protection
Laboratory for Toxicology
Bilthoven, The Netherlands
Cyanogenic glycosides are phytotoxins which occur in at least
2000 plant species, of which a number of species are used as food in
some areas of the world. Cassava and sorghum are especially
important staple foods containing cyanogenic glycosides (Conn,
1979a,b; Nartey, 1980; Oke,1979, 1980; Vennesland et al., 1982;
Rosling, 1987). There are approximately 25 cyanogenic glycosides
known. The major cyanogenic glycosides found in the edible parts of
plants used for human or animal consumption are summarized in Table
1. The potential toxicity of a cyanogenic plant depends primarily on
the potential that its consumption will produce a concentration of
HCN that is toxic to exposed animals or humans (see Table 2).
Several factors are important in this toxicity:
The first aspect is the processing of plant products containing
cyanogenic glycosides. When the edible parts of the plants are
macerated, the catabolic intracellular enzyme ▀-glucosidase can be
released, coming into contact with the glycosides. This enzyme
hydrolyzes the cyanogenic glycosides to produce hydrogen cyanide and
glucose and ketones or benzaldehyde. The hydrogen cyanide is the
major toxic compound causing the toxic effects. Plant products
(notably cassava), if not adequately detoxified during the
processing or preparation of the food, are toxic because of the
release of this preformed hydrogen cyanide.
The second aspect is the direct consumption of the cyanogenic
plant. Maceration of edible parts of the plants as they are eaten
can release ▀-glucosidase. The ▀-glucosidase is then active until
the low pH in the stomach deactivates the enzyme. Additionally, it
is possible that part of the enzyme fraction can become reactivated
in the alkaline environment of the gut. At least part of the
potential hydrogen cyanide is released, and may be responsible for
all or part of the toxic effect of cyanogenic glycosides in the
cases of some foods.
Table 1: The occurrence of cyanogenic glycosides in major edible plants
Cyanogenic Plant species
glycosides Common name Latin name
Amygdalin almonds Prunus amygdalus.
Dhurrin sorghum Sorghum album,
Linamarin cassava Manihot esculenta,
lima beans Phaseolus lunatus.
Lotaustralin cassava Manihot carthaginensis
lima beans Phaseolus lunatus.
Prunasin stone fruits Prunus species e.g.,
P. avium, P. padus,
P. persica, P. macrophylla.
Taxiphyllin bamboo shoots Bambusa vulgaris.
The third aspect is that the cyanogenic glycosides taken up
intact with the food are (partly) hydrolyzed by the ▀-glucosidase
activity of the bacteria of the gut flora of animals or humans
(Conn, 1979a,b; Oke, 1979, 1980; Nartey, 1980; Rosling, 1987;
Gonzales & Sabatini, 1989).
Cyanide, released from a cyanogenic glycoside in food by
▀-glucosidase either of plant or from gut microflora origin and
taken up, follows the known cyanide metabolic pathway and
toxicokinetics both for animals and man. Cyanide is detoxified by
the enzyme rhodanese, forming thiocyanate, which is excreted by
urine (Conn, 1979a,b; Oke, 1979, 1980). Due to several factors
influencing hydrolysis of cyanogenic glycosides and the confounding
influence of nutritional status (such as riboflavin, vit. B12,
sodium, methionine intake) human case studies and epidemiological
studies of the chronic toxicological effects have shown very
variable results and were not conclusive. In addition, the data in
these studies are rarely of a quantitative character (Conn, 1979a,b;
Oke, 1979, 1980; Nartey, 1980; Rosling, 1987). In several studies
both in animals and man the toxicity of cyanogenic glycosides is
often expressed as mg releasable cyanide.
Table 2: Concentration of cyanide in some tropical foodstuffs
(Summarized in Nartey, 1980).
Plant/tissue mg HCN/kg
Cassava(bitter)/dried root cortex 2450
Cassava(bitter)/whole tubers 395
Cassava(sweet)/whole tubers 462
Sorghum/whole immature plant 2500
Bamboo/immature shoot tip 8000
Lima beans from Java (coloured) 3120
Lima beans from Puerta Rico (black) 3000
Lima beans from Burma (white) 2100
Because this monograph first discusses the toxicity of cyanide
as a basis for understanding that of cyanogenic glycosides, a
modified form of the general monograph format has been used,
presenting first biological data for cyanide, then that for
2. BIOLOGICAL DATA
2.1 Biochemical aspects
22.214.171.124 Absorption, distribution, and excretion
Hydrogen cyanide after oral administration is readily absorbed
(it is also readily absorbed after inhalation exposure and through
skin and eyes). After absorption, cyanide is rapidly distributed in
the body through the blood.
The concentration of cyanide is higher in erythrocytes than in
plasma. It is known to combine with iron in both methaemoglobin and
haemoglobin present in erythrocytes. The cyanide level in different
human tissues in a fatal case of HCN poisoning has been reported:
gastric content; 0.03, blood; 0.50, liver; 0.03, kidney; 0.11,
brain; 0.07, and urine; 0.20 (mg/100 g) (EPA, 1990).
The pharmacokinetics of 14CN- and S14CN- in rats
exposed to these agents in diet for 3 weeks was investigated. All
tissues contained radioactivity 9 h after intraperitoneal injection
of 14CN-; highest radioactivity was found in the stomach (18%).
Eighty per cent of this activity was in the form of thiocyanate. At
this point 25% of the dose had already been eliminated in the urine
and 4% in the expired air. When S14CN- was given per os to
rats with elevated plasma thiocyanate levels due to chronic oral
exposure to cyanide, most of the activity was eliminated in the
urine and only small amounts were found in the faeces. This
indicated the existence of a gastrointestinal circulation of
thiocyanate (Okoh & Pitt, 1981).
The excretion of an acute oral dose of 14C-labelled cyanide
in urine, faeces and expired air was studied in rats (12
animals/group) pretreated orally for 6 weeks with unlabelled KCN or
a control diet. Urinary excretion was the main route of elimination
of 14C-labelled cyanide in these rats, accounting for 83% of the
total excreted radioactivity at 12 h and 89% of the total excreted
radioactivity at 24 h. The major metabolite of cyanide excreted in
urine was thiocyanate, and this metabolite accounted for 71% and 79%
of the total urinary activity at 12 h and 24 h, respectively. Only
4% of the mean total activity excreted was found in expired air
after 12 h, and this value did not change after 24 h. Of the total
activity in expired air in 24 h, 90% was present as carbon dioxide
and 9% as cyanide. When these results were compared with those
observed for control rats, it was clear that the mode of elimination
of cyanide carbon was altered in neither urine nor breath by the
chronic intake of cyanide (Okoh, 1983).
Golden hamsters exposed to cyanide by subcutaneous infusion
appeared to excrete only a relatively low percentage (10-15%) of the
dose as thiocyanate in the urine (Doherty et al., 1982).
The major defence of the body to counter the toxic effects of
cyanide is its conversion to thiocyanate mediated by the enzyme
rhodanese. The conversion of cyanide to the less toxic thiocyanate
by rhodanese was discovered by Lang (1933).
Thiosulfate and 3-mercapto-pyruvate can act as sulfur donors,
but neither free cystine nor cysteine can. The enzyme contains an
active disulfide group which reacts with the thiosulfate and
cyanide. The trivial name rhodanese is more widely used than that
assigned by the Enzyme Commission [thiosulfate-cyanide sulfur
transferase, EC. 126.96.36.199]; it has been inappropriately called
rhodanase in several reports.
The rhodanese-catalyzed irreversible conversion of cyanide to
thiocyanate, in the presence of thiosulfate, provides a means for
the treatment of cyanide poisoning. Since the enzyme, which is
usually localized in the mitochondria in different tissues, is
relatively abundant, but in sites which are not readily accessible
to thiosulfate, the limiting factor for the conversion of cyanide is
thus thiosulfate (EPA, 1990).
The overall rate of in vivo detoxification of cyanide may be
influenced by several minor reactions. Cystine may directly react
with cyanide to form 2-imino-thiazolidine-4-carboxylic acid which is
excreted in saliva and urine. Traces of hydrogen cyanide may be
found in expired air, saliva, sweat and urine. A minor amount may be
converted into formic acid which may be excreted in urine or
participate in the metabolism of one carbon compound. One minor
detoxification route is the combination of cyanide with
hydroxycobalamine (vitamin B12) to form cyanocobalamine which is
excreted in urine and bile. It may be reabsorbed by the intrinsic
factor mechanism at the level of the ileum allowing effective
recirculation of vitamin B12. Methaemoglobin effectively competes
with cytochrome oxidase for cyanide and its formation from
haemoglobin, effected by sodium nitrite or amylnitrite, is exploited
in the treatment of cyanide (EPA, 1990). It has been reported that
other species have lower rhodanese activity than the rat and hence
the rat may be able to convert cyanide to thiocyanate more easily
than other species (Himwich & Saunders, 1948).
No information available.
2.1.3 Effects on enzymes and other biochemical parameters
Cyanide causes a decrease in the utilization of oxygen in the
tissues, producing a state of histotoxic anoxia. This occurs through
inactivation of tissue cytochrome oxidase by cyanide, which combines
with Fe3+/Fe2+ contained in the enzyme. The enzyme-cyanide
complex dissociation constant has been found to be 1 * 10-6 and 1
* 10-4 (moles/l) for the oxidized and reduced form of the enzyme,
respectively. Thus, the affinity of cyanide for the oxidized form of
the enzyme is two orders of magnitude higher than for the reduced
form. However, the rate of reaction of cyanide with the reduced
enzyme is twice the rate of reaction with the oxidized form.
Cyanide can inhibit several other metalloenzymes most of which
contain iron, copper or molybdenum (e.g., alkaline phosphatase,
carbonic anhydrase), as well as enzymes containing Schiff base
inter-mediates (e.g., 2-keto-4-hydroxyglutarate aldolase). The
effect of sublethal doses of cyanide on the metabolism of glucose in
mice has been studied using radiorespirometric techniques
Cyanide causes an increase in blood glucose and lactic acid
levels and a decrease in the ATP/ADP ratio indicating a shift from
aerobic to anaerobic metabolism. Cyanide apparently activates
glycogenolysis and shunts glucose to the pentose phosphate pathway
decreasing the rate of glycolysis and inhibiting the tricarboxylic
acid cycle (EPA, 1990).
2.2 Toxicological studies
2.2.1 Acute toxicity studies
Lethal doses of HCN in mg/kg bw were reported for mouse, 3.7;
dog, 4.0; cat, 2.0 and for cattle and sheep 2.0 (Summarized by Conn,
Table 3. Acute toxicity of cyanide
Species Route LD50 References
Mouse i.v. 0.99 (HCN) EPA 1990*
Rat i.v. 0.81 (HCN) EPA 1990*
Guinea-pig i.v. 1.43 (HCN) EPA 1990*
Rabbit i.v. 0.66 (HCN) EPA 1990*
Cat i.v. 0.81 (HCN) EPA 1990*
Dog i.v. 1.34 (HCN) EPA 1990*
Monkey i.v. 1.30 (HCN) EPA 1990*
Mouse s.c. 6.0 (KCN) WHO, 1965*
i.v. 2.5 (KCN) WHO, 1965*
Rat oral 10-15 (KCN) WHO, 1965*
i.v. 2.5 (KCN) WHO, 1965*
Dog oral 5.3 (KCN) WHO, 1965*
Rabbit s.c. 2.2 (NaCN) WHO, 1965*
Guinea-pig s.c. 5.8 (NaCN) WHO, 1965*
Dog i.v. 2.8 (NaCN) WHO, 1965*
Mouse oral 598 (NaSCN) WHO, 1965*
i.v. 484 (NaSCN) WHO, 1965*
Rat oral 765 (NaSCN) WHO, 1965*
i.p. 540 (NaSCN) WHO, 1965*
* as summarized in
2.2.2 Short-term toxicity studies
In a 13-week toxicity study male Sprague-Dawley rats
(approximately 30 rats/group) were administered KCN in the drinking
water. The dose levels were 40, 80 and 160/140 mg KCN/kg bw/24 h.
Three control groups were used, respectively a normal drinking water
ad libitum, a "paired drinking" group (parallel to the high dose
level KCN) and a group receiving drinking water with 10% ethyl
alcohol. In addition, one group received drinking water with KCN
(80 mg/kg bw) and 10% alcohol.
Behaviour, external appearance, body weight, food consumption
(daily) and drinking water consumption (twice weekly) were recorded
frequently. Extensive haematological, clinical chemical (in serum)
and urine analyses were carried out in 5 animals per group in week 6
and week 13. Autopsy and macroscopy were performed after 13 weeks
(approximately 20 animals/group) and 11 organs were weighed.
Histopathological examination was performed in brain, kidneys,
heart, liver and testes of these animals. In addition thyroids of
the control, the "pair drinking" control and the high-dose group
(160/140 mg/kg bw/day) were examined. There was a clear indication
that reduced food consumption and body weight in the KCN groups were
caused by a decrease in water consumption due to decreased
palatibility. Urinalyses revealed a higher level of protein in the
animals receiving KCN. The amounts of protein determined showed a
clear correlation to the increasing doses of KCN, as did the
drinking water. Several changes in absolute organ weights were seen
in the 160/140 mg KCN/kg bw/day group. Relative weights of organs
were very slightly increased in the 40, slightly increased in the
80 mg and clearly increased in the 160/140 mg KCN/kg bw groups. The
thymus weight was, however, reduced in the high-dose group.
Histopathological examination revealed no indication of damage to
the brain, heart, liver, testes, thyroids nor kidneys due to
treatment with KCN (Leuschner et al., 1989b).
2.2.3 Long-term carcinogenicity studies
No data on the carcinogenicity of hydrogen cyanide have been
published. However, anticarcinogenic effects of cyanide have been
reported. Longevity of mice with transplanted Ehrlich ascites
tumours and Sarcoma 180 was increased 20 to 70% on i.p. injection of
sodium cyanide in the dose range 0.75 to 2.0 mg/kg bw (EPA, 1990).
2.2.4 Reproduction studies
A short-term reproductive study (49 day study in adults and 28
day study in pups) was performed to evaluate the cumulative effects
of adding 500 mg KCN/kg to cassava root flour-based diet in pregnant
rats. This meal was prepared from a low-HCN cassava variety (21 mg
High dietary level of KCN did not have any marked effect in
gestation and lactation performance of female rats. No carry-over
effect of high cyanide-containing diet fed during gestation was
observed on lactation performance. The high cyanide-containing diet,
however, significantly reduced feed consumption and daily growth
rate of the offspring when fed during post-weaning period. Protein
efficiency ratio was not only reduced by the cyanide diet during
post-weaning growth phase but there was an additional carry-over
effect from gestation. Serum thiocyanate was significantly increased
in lactating rats and their offspring during lactation and in the
postweaning growth phase of the pups. No apparent carry-over effect
was noticed on this parameter. Rhodanese activity in liver and
kidneys was unaffected by feeding the high cyanide diet during
gestation, lactation, nor during postweaning growth (Tewe & Maner,
2.2.5 Special studies on embryotoxicity and teratogenicity
Pregnant golden hamsters were exposed to sodium cyanide on days
6-9 of gestation by infusion via subcutaneously implanted osmotic
minipumps. Cyanide (0.126-0.1295 mmol/kg/h) induced high incidences
of resorptions and malformations in the offspring. The most common
abnormalities observed were neural tube defects (Doherty et al.,
2.2.6 Special studies on the thyroid gland
A group of 10 male rats was fed a 10% casein diet containing
added methionine, vitamin B12, iodine and potassium cyanide
(1500 mg/kg feed) for nearly one year. Compared to a control group
not receiving cyanide, depression of body-weight was observed
throughout the study period, but there were no deaths nor clinical
signs of toxicity. Depression of both plasma thyroxine and thyroxine
secretion rate suggestive of depressed thyroid function were evident
at 4 months but less so after 1 year. At autopsy the animals were
found to have enlarged thyroids and this may have been the mechanism
of adaptation. Some differences in the histopathology of the spinal
cord, notably the white matter, were also found between controls and
cyanide-treated animals (Philbrick et al., 1979).
Performance and metabolic and pathological changes were
evaluated in 48 growing pigs fed different levels of dietary protein
(9 and 16%), cyanide, and iodine (0 and 0.36 mg iodine/kg feed)
during 56 days. Protein deficiency reduced urinary iodine excretion
and the concentrations of protein, protein-bound iodine (PBI) and
thiocyanate in serum. It also reduced liver rhodanese activity and
caused a decrease in urinary thiocyanate excretion which was not
significant. Dietary cyanide increased urinary thiocyanate and
iodine excretion and serum PBI. Pathological studies showed that
cyanide treatment had no marked effect on the microanatomy of the
tissues examined. Dietary iodine deficiency caused histological
changes in the thyroid gland and bone which suggested a decline in
metabolic activity. Iodine deficiency caused hyperplastic goitre in
the experimental animals (Tewe & Maner, 1980).
Two negative and one marginally positive genotoxicity studies
for cyanide have been reported. Potassium cyanide was not mutagenic
in Salmonella typhimurium strains TA1535, TA1537, TA1538, TA98 nor
TA100 with or without S-9 liver microsomes. Cyanide was also
negative in recombinant-assay in Bacillus subtilis. One study
reported marginally mutagenic activity of HCN to Salmonella
typhimurium strain TA100 in the absence of S-9 mix. In the same
study no mutagenic activity in strain TA98 with or without S-9 mix
was observed (EPA, 1990).
An in vitro Ames test with HCN in Salmonella strains
TA1537, TA1538 and TA98 for detection of frame shift mutation and
TA1535 and TA100 for base-pair substitutions was performed with and
without metabolic activation of a S-9 microsome mixture. There was
no indication of mutagenic properties under these conditions
(Leuschner et al., 1983a).
An in vivo mutagenicity study in Chinese hamsters detecting
chromosomal aberrations with HCN orally administered to Chinese
hamsters was carried out. Preparations of metaphase cells were
studied for structural chromosome aberrations after 6, 24 and 48 h
after oral administration of 0.4 mg HCN/kg bw. The incidence of
aberrations or gaps was within the spontaneous range. Neither
multiple aberrations nor pulverised metaphases were found. There was
no indication of mutagenic properties relative to structural
chromatid or chromosome damage (Leuschner et al., 1983b).
A gene mutation assay in cultured Chinese hamster cells, V79
(genetic marker HGPRT) both in the presence and absence of metabolic
activation system was carried out with KCN. The duration of the
exposure with the test substance was 24 h in the experiments without
S-9 mix and 2 h in the experiments with S-9 mix. The test compound
dose levels employed were chosen following a preliminary toxicity
experiment. The dose-levels for the main study were 400, 800, 1000,
2000 and 3000 Ág/ml without S9 mix and 1000, 2000, 3000, 4000, 6000,
8000 and 10 000 Ág/ml with S9 mix. KCN was tested up to a high
cytotoxicity in the absence and presence of metabolic activation.
Under the present test conditions KCN was negative in the V79
mammalian cell mutagenicity test (Leuschner et al., 1989a).
2.2.8 Special studies on nervous system
A special study on the behavioural effects of chronic sublethal
dietary cyanide (KCN; 0.4, 0.7 and 1.2 CN-/kg bw) in juvenile
swine, mimicked the situation of free CN- intake in Liberia due to
eating cassava-based foods. There were two clear behavioural trends:
1) increasing ambivalence and slower response time in reacting to
various stimuli and 2) an energy conservation gradient influencing
which specific behaviours would be modified in treated animals.
Serum SCN- was positively correlated with daily CN- intakes.
CN- treatment diminished T3 and T4 levels but elevated fasting
blood glucose values (Collier-Jackson, 1988).
Neuronal lesions in several animal species have been produced
by chronic cyanide intoxication either by injection of unbuffered
alkaline cyanide salts or by inhalation of hydrogen cyanide. The
neuropathological changes include areas of focal necrosis especially
around the centrum ovale, corpus striatum, corpus callosum,
substantia nigra, anterior horn cells, and patchy demyelination in
the periven-tricular region. In some species, the earliest effects
may be on the oligodendroglia and hence myelin lesions may precede
neuronal damage. Bass (1968) showed that in rats chronic cyanide
intoxication produces myelin loss by its primary effect on glial
cells followed by breakdown of myelin. Brierly et al. (1977)
reported myelin damage and changes in the oligodendroglia in cyanide
poisoning in rats. Clark (1936) described fatty degeneration of the
liver and secretory tubules of the kidney in rats subjected to
chronic cyanide intoxication. In these animal experiments relatively
large doses of cyanide were given, often sufficient to cause partial
asphyxia. Therefore it was doubtful whether the neuropathological
effects demonstrated were due to asphyxia or chronic cyanide
intoxication so they did not appear to have an obvious parallel to
human exposure at lower levels. It is noteworthy, however, that
changes in optic nerves and tracts occur consistently only in
primates (Ferraro, 1933; Hurst, 1940; Lessell, 1971). In a carefully
controlled experiment in which small weekly doses of cyanide were
administered over several months to rats, neuronal degeneration and
demyelination were reported (Smith et al., 1963). Williams and
Osuntokun (1969) found that the demyelination of peripheral nerves
induced in rodents by cyanide injection bore a striking resemblance
to the lesions found in biopsy specimen of peripheral nerves of
Nigerian patients who suffer from tropical neuropathy (reviewed by
2.3 Observations in humans
2.3.1 Acute toxicity studies in humans
The acute oral lethal dose of HCN for human beings is reported
to be 0.5-3.5 mg/kg bw corresponding to 1.0-7.0 mg/kg bw of KCN. The
clinical signs are well described (Montgomery, 1969; Gosselin
et al., 1976) and include headache, dizziness, mental confusion,
stupor, cyanosis with twitching and convulsions, followed by
terminal coma (Conn, 1979a).
The acute oral lethal dose of HCN for man was reported to be
60 mg (Sinclair & Jeliffe, 1961). For man the acute oral dose of HCN
is usually given as 50-90 mg and for potassium cyanide as 200 mg,
corresponding to 81 and 110 mg HCN respectively (Lehman, 1959). Data
on the oral lethal dose of cyanide for man in four cases of suicide,
calculated from the amount of HCN absorbed in the body at the time
of death, and from the amount of HCN found in the digestive tract,
differed considerably ((calculated as mg HCN: 1450 (62.5 kg bw),
556.5 (74.5 kg), 296.7 (50.7 kg) and 29.8 (51 kg)) (Geitler & Baine,
1938). This corresponds to doses varying from 0.58-22 mg/kg bw (in
2. Biological data
2.1 Biochemical aspects
2.1.1 Absorption, distribution, and excretion
Wistar rats (4 animals/sex/group) were given 50 mg
amygdalin/rat respectively, intravenously and orally after an
overnight fast. During the experiment and the night before they were
kept in metabolic cages which allowed collection of urine without
faeces and minimized coprophagy.
After intravenous and oral administration fractions of the dose
excreted by the rat as unchanged amygdalin were 70% and 0.8%,
respectively. The fraction excreted as prunasin after intravenous
administration was 6.6%, whereas it was 39% of the total dose after
oral administration (Rauws et al., 1982).
In a toxicokinetic study 10 male rats (100-120 g) were given
50 mg pure linamarin (dissolved in water, volume 0.5 ml) and 6 male
rats were given water alone by stomach tube. Seven rats dosed with
50 mg linamarin died within 4 h. In a second trial 6 male rats were
given 30 mg linamarin. Following dosing, urine and faeces were
collected after 24, 48 and 72 h, and heparinized blood samples were
taken from the optic vein or lateral tail vein. Blood was taken
after 30, 40, 60, 80 and 100 min and at 2, 4, 8, 24 and 48 h.
No intact linamarin was detected in faeces nor blood of the
rats dosed with 30 mg (300 mg/kg bw). No linamarin in the faeces of
the three surviving rats dosed with 50 mg (500 mg/kg bw) was
detected, however blood and urine were not examined in these
animals. Linamarin was excreted in the urine at a level of 5.65 mg
(cumulated after 72 h) along with 0.823 mg of thiocyanate. These
findings indicate that linamarin was absorbed intact in considerable
proportion and was partially hydrolyzed (Barrett et al., 1977).
In a toxicokinetic study beagle dogs (4 animals/sex/group) were
administered 500 mg amygdalin in 10 ml solution respectively,
intravenously and orally after overnight fast. Blood was sampled
from jugular vein and urine was collected by a funnel. Faeces were
removed from the funnel.
The major part of the dose (71%) was recovered in the urines
collected during 6 h following intravenous amygdalin administration.
The fraction of the dose excreted by glomerular filtration was
calculated using the ratio of diatrizoate (which was administered
simultaneously) clearance to amygdalin clearance, showing that 97%
of the amount of amygdalin to be expected was recovered from the
urine. The result of the experiments after intravenous
administration were analyzed assuming a two-compartment model. The
distribution T┤ alpha was 0.10 and the elimination T┤ ▀ 0.57. No
prunasin was detected in urine (detection limit = 0.2% of dose).
After oral administration of amygdalin a very low maximal
plasma level is found after approximately 0.75 h. Only 2.3% of the
amygdalin was systematically available (absolute bioavailablity).
Prunasin was found in plasma and urine of dogs. In the urine
collected during 6 h following amygdalin administration only about
1% of the dose was recovered unchanged and 21% of the dose was
identified as prunasin (Rauws et al., 1982).
In a toxicokinetic study prunasin was administered
intravenously and orally as 100 mg doses to female dogs (2/group;
10 kg) after an overnight fast. Blood was sampled from jugular vein
and urine was collected by a funnel.
The results of the experiment after intravenous administration
were analyzed assuming a two-compartment model. The prunasin results
were compared to those obtained with amygdalin in a earlier
experiment. The distribution T┤ alpha was 0.08 h and elimination
T┤ ▀ was approximately 0.64 h.
Prunasin was absorbed to a large extent after oral
administration. The absolute bioavailability after oral
administration was 50% of the administered dose. The volume of
distribution (0.34 L/kg) and the clearance (0.55 L/kg h) are larger
than those of amygdalin (respectively 0.19 L/kg and 0.39 L/kg h).
The oral bioavailability of prunasin is considerably greater,
whereas amygdalin is only slightly (2.3%) absorbed (Rauws et al.,
In a toxicokinetic study pure linamarin, at a dose level of
300 mg/kg bw, was administered in food to a group of Wistar rats
maintained on vitamin B2-deficient, -sufficient, and -excess diets
for 5 weeks and to another group of kwashiorkor rats. Free and total
cyanide, intact linamarin and thiocyanate levels were estimated in
urine and faeces obtained at 0, 24, 48 and 72 h periods and in blood
samples obtained 72 h after the compound had been administered.
There was no detectable cyanide nor intact linamarin in the faecal
samples. Rats on vitamin B2-sufficient and B2-excess diets excreted
higher total and free cyanide in urine than the respective vitamin
B2-deficient groups. Most of the linamarin was degraded after 24 h.
The rate of breakdown of the glycoside within the first 24 h was
slowest for zero and half normal vitamin B2 status rats as evidenced
by appearance of the glycoside in large quantities in the urine. The
kwashiorkor rats, on the other hand, excreted less thiocyanate than
the controls. In addition, their control group excreted most of the
thiocyanate in the first 24 h, whilst the kwashiorkor rats excreted
most of the thiocyanate in the first 48 h. Dietary protein
deficiency prolongs the time of metabolism and hence increases the
toxicity of cyanogenic glycosides in the body (Umoh et al., 1986).
Strained ruminal fluid was collected from cattle fed five diets
(concentrate diet, freshly harvested alfalfa, cubed alfalfa, alfalfa
hay and orchard grass) to determine in vitro rates of cyanogenesis
from the glycosides amygdalin, prunasin and linamarin. Rates of
dissociation for the corresponding aglycones, benzaldehyde
cyanohydrin and acetone cyanohydrin were also determined. Hydrogen
cyanide (HCN) in ruminal fluid was determined with a modified method
of HCN analysis that independently measured the overall rate of
cyanogenesis and the non-enzymatic dissociation of cyanohydrins, the
intermediate products in the degradation of cyanogenic glycosides to
HCN. Rate of dissociation of cyanohydrins in ruminal fluid was
pH-dependent, with high rates of dissociation (as expressed by the
rate constant or half-life of reaction) occurring at pH >6 and
slower rates at pH 5 to 6. Cyanohydrin dissociation was most rapid
when cattle were fasted for 24 to 48 h and ruminal pH was high; rate
of dissociation was much slower during feeding and digestion. When
the glycosides were examined, highest rates of cyanogenesis
(mg HCN/L/s) were observed after a 24 h postprandial period. The
rates were highest after feeding hay: 0.019 for amygdalin, 0.033 for
linamarin and 0.048 for prunasin. Hence cattle are most susceptible
to poisoning by cyanogenic plants when the pH of ruminal fluid is
elevated, leading to rapid dissociation, and also when the activity
of ▀-glucosidase is adequate for rapid hydrolysis of glycosidic
bonds. Rates of cyanogenesis were higher when ruminal inocula were
from cattle fed fresh alfalfa or cubed alfalfa hay rather than from
those fed grain or long hay. Rates of HCN production were lowest
using inocula from cattle fed grain; rates for the three glycosides
were negligible at the 3 and 6 h postprandial sampling times. In
agreement with previous studies, prunasin was degraded in ruminal
fluid much more rapidly than linamarin or amygdalin (Majak et al.,
In a comparative metabolism study rates of cyanide liberation
resulting from hydrolysis of the cyanogenic glycosides linamarin,
amygdalin and prunasin by a crude ▀-glucosidase prepared from
hamster caecum were studied in vitro. In addition, hamster blood
cyanide and thiocyanate concentrations were determined at 0.5, 1, 2,
3, and 4 h after oral dose of 0.44 mmol linamarin or amydalin/kg bw.
Plots of cyanide liberated versus time for linamarin and
prunasin yielded straight lines, whereas for amygdalin the plot was
curvilinear; the rate of cyanide release increased with time. At
10-3 M substrate concentrations, the averaged rates of hydrolysis
of prunasin, amygdalin and linamarin were 1.39, 0.57 and
0.13 nmol/min/mg protein, respectively. Lineweaver-Burk plots
yielded apparent Km and Vmax values of 3.63 * 10-5 M and
0.13 nmol/min/mg protein, respectively for amygdalin, and 7.33 *
10-3 M and 1.04 nmol/min/mg protein, respectively for linamarin.
Blood cyanide concentrations following amygdalin treatment of
the hamster reached their highest level (130 nmol/ml) 1 h after
dosing and remained elevated until 3 h after treatment. Blood
cyanide concentrations following linamarin treatment reached their
highest level (116 nmol/ml) after 3 h and then declined immediately.
Area under the blood cyanide concentration-time curve was
395 nmol h/ml for amygdalin and 318 nmol h/ml for linamarin. The
results suggest a faster rate of enzymatic hydrolysis and cyanide
absorption for amygdalin than for linamarin (Frakes & Sharma, 1986).
In humans, pharmacological studies have shown that amygdalin is
broken down to HCN, benzaldehyde and glucose by enzymes found in gut
bacteria, but not intracellularly in humans. Animal and human
tissues contain no significant concentrations of ▀-glucosidase, the
only known activating enzyme of hydrolysis of cyanogenic glycosides
in vivo (Dorr & Paxinas, 1978).
Cyanogenic glycosides are hydrolyzed by ▀-glucosidase produced
by intestinal bacteria to glucose, HCN and benzaldehyde or acetone.
Benzaldehyde is oxidized to benzoic acid (and subsequently to
hippuric acid) or salicylic acid isomers. Thiocyanate is present in
body fluids; blood, urine, saliva, sweat and tears (Oke, 1979).
The cyanide-yielding capacity of insufficiently processed
cassava probably occurs as linamarin, or an intermediate break-down
product, from which cyanide may be yielded in the gut by action of
microbial enzymes. Significant amounts of linamarin are observed in
the urine after consumption of insufficiently processed cassava as
well as after consumption of other plants containing linamarin.
These results indicate that linamarin, if not metabolized in the
gut, will be absorbed and excreted in the urine without causing
exposure to HCN. About 80% of ingested cyanide will be turned into
thiocyanate and is excreted in the urine after a short period
2.1.3 Effects on enzymes and other biochemical parameters
No information available.
2.2 Toxicological studies
2.2.1 Acute toxicity studies
Table 4. Acute toxicity studies of cyanogenic glycosides or cyanogenic plant
Species Sex Route LD50 References
Mouse ? i.p. 0.1 mmole Solomonson, 1981
Rat ? i.v. 20 000 Oke, 1979
Rat ? oral 450 linamarin Oke, 1979
A dose of 25 mg linamarin (250 mg/kg bw) fed to rats
(100-120 g bw) caused clinical signs of toxicity, including apnoea,
ataxia and paresis. These symptoms were very marked in the absence
of methionine supplementation, 50% of these rats died within 4 h. In
the presence of adequate methionine supplementation, 10% of rats
died and about 40% showed no signs of toxicity. The activity of
Na+K+-dependent ATPase was reduced in much the same way as it
was by the glycoside, digitalis (reviewed by Oke, 1980).
In a toxicokinetic study 7 out 10 rats (100 g bw) died after
administration of 50 mg linamarin by stomach tube (Barrett et al.,
Oral doses of 100, 120 and 140 mg linamarin/kg bw given by
stomach tube to hamsters (90 g bw) produced signs of cyanide
intoxication in a large percentage. The signs appeared within 1 h
after dosing included dyspnoea, hyperpnoea, ataxia, tremors and
hypothermia. Two animals dosed with 140 mg/kg bw and one animal
dosed with 120 mg/kg bw died within 2 h of dosing. The signs of
poisoning were greatly reduced or gone within 3 h after treatment in
the surviving animals. No relationship between length of
intoxication and dose was observed (Frakes et al., 1985).
Hamsters have been reported to be more susceptible than rats to
the acute toxic effects of orally administered amygdalin and
prunasin (Willhite, 1982).
A species difference in reaction to cyanogenic glycosides is
observed due to a difference of detoxifying ability due to
anatomical structure. Ruminants, e.g., cattle and sheep are supposed
to be more susceptible to the acute toxic effects because of their
larger flora of microorganisms and considerable quantities of the
enzyme emulsine which hydrolyzes the glycoside (Oke, 1979).
2.2.2 Short-term toxicity studies
Albino female rats (10 animals/group) were fed ad libitum one
of the following diets, A; a normal laboratory diet (control), B; a
50% gari diet (Nigerian preparation of cassava), C; a raw cassava
diet, D; a diet containing 5 g KCN/100 g and E; a diet containing
10 g KCN/100 g during a 14-day period. The 50% gari diet caused no
significant biochemical nor haematological changes in the female
rats, whereas for both the raw cassava diet and the KCN diets a
decrease of Hb, PCV, total serum protein concentration and T4
concentration was observed. In the 50% gari diet group, and to a
greater extent, in the other treatment groups, the serum thiocyanate
levels were increased. The body weight gain was not significantly
decreased in the 50% gari group, whereas the other treatment groups
showed instead of gain a loss of body weight (Olusi et al., 1979).
In a 24-day toxicity experiment guinea-pigs (8 animals/group)
were dosed daily with, respectively, laetrile (10 mg amygdalin) and
8 mg KCN/kg bw with and without ascorbic acid (100 mg). No
significant effect on the body weight nor liver weight was observed.
However, treatment with laetrile alone for 4, 16 or 24 days,
respectively, resulted in a significant increase in urinary levels
of thiocyanate. The increase was less in animals treated with
vitamin C. In guinea-pigs treated with 8 mg KCN/kg bw, toxic effects
were seen as evidenced in slight tremors in 3 of the 8 animals,
which recovered within 5 min. All animals in the KCN group which
were supplemented with ascorbate showed severe tremors, motor
ataxia, bizarre neuromuscular manifestations and rhythmic head
movements. The toxicity of KCN increased with elevation of vitamin
C, whereas urinary excretion of thiocyanate decreased (Basu, 1983).
In two feeding experiments (respectively 63 and 56 days) one
day old broiler chickens (male and female) were fed a diet
containing 0, 10, 20 or 30% cassava, respectively. The animals were
studied for haematological and histopathological effects. The
cassava diet studied in the 1st experiment consisted of a
high-cyanide-containing cassava root meal (CRM) supplying 300 mg of
total cyanide/kg, most of it in the form of cyanogenic glycosides.
The cassava diet in the 2nd experiment also contained cassava
foliage meal (CFM) supplying 156 mg total cyanide/kg. In the 1st
experiment 26 chickens per group were used and in the 2nd experiment
160 chickens were used for the cassava groups and 80 for the control
No changes in the haematological parameters due to cassava were
seen. Addition of up to 30% CRM failed to adversely affect broiler
survival, performance nor feed efficiency, but the inclusion of CFM
in the experimental diets increased mortality, decreased weight gain
and decreased feed efficiency. In both experiments, increased
quantities of dietary cassava cyanate were associated with increased
(P < 0.05) blood serum thiocyanate concentrations.
Histopathological examination of thyroid, liver and kidney revealed
no appreciable alterations due to the cassava feeding, however there
was no conclusive evidence of cyanide or thiocyanate effects on
thyroid activity. Aflatoxin contamination appeared to have
contributed to the high mortality rate associated with CFM diets.
The results showed that broilers were tolerant of relatively high
levels of dietary cyanogenic glycosides (Gomez et al., 1988).
2.2.3 Long-term/carcinogenicity studies
No information available.
2.2.4 Reproduction studies
In a one-generation reproduction study albino female rats (10
rats/group) were fed ad libitum one of the following diets: A, a
normal laboratory diet, B, a 50% gari diet (Nigerian preparation of
cassava), C, a raw cassava diet, D, a diet containing 5 g KCN/100 g
and E, a diet containing 10 g KCN/100 g. After 2 weeks rats in each
group were mated with 5 adult males fed normal diet. Pregnant rats
from each group were maintained on their respective diets. After
littering, the newborn rats were studied for postnatal development.
After 21 days F1 rats were put for another 4 weeks on their
The offspring of the rats fed the 50% gari diet had
significantly lower birth weights and brain weights and never
attained the same adult weights as those of the controls. The adult
female rats fed a diet consisting entirely of raw cassava had
significantly reduced haematological and biochemical parameters
(Hb, PCV, serum protein and T4 concentration). This diet also
caused an increased incidence of cannibalism and a significant
reduction in the frequency of pregnancy, the average number of pups
per litter and birth weights among these pups. In addition there was
an increased incidence of neonatal deaths among the offspring which
also had poor development, reduced brain weights and an increased
tendency of aggression towards their litter mates. Adult female rats
fed diets containing 5 and 10 g KCN/100 g laboratory diet survived
for more than three months but never became pregnant. They developed
enlarged thyroid glands and tumours of the large intestine. The
usual content of cyanide in cassava varies from 70 to 500 mg/kg
which is much less than the levels used in these experiments; thus
the rats were able to cope with the 50% gari diet and detoxify the
glycoside present (Olusi et al., 1979).
General toxicity and reproductive effects were studied for
cassava in combination with added cyanide. In a 110-day feeding
experiment 18 pregnant Yorkshire gilts were allocated to three equal
groups and fed fresh cassava (containing 40.2 HCN/kg) supplemented
with 0, 250, and 500 mg cyanide (KCN) per kg of fresh cassava
offered. Serum thiocyanate concentration was slightly but not
significantly increased in the 500 mg KCN/kg group and serum protein
bound iodine decreased during gestation in all groups. Fetal serum
thiocyanate concentration was significantly (p <0.05) higher in the
group fed 500 mg KCN/kg. A small increase in maternal thyroid weight
with increasing levels of cyanide was observed. Pathological studies
showed proliferation of glomerular cells of the kidneys in gilts of
all groups and reduced activity of the thyroid gland in gilts fed
500 mg KCN/kg group. Cyanide fed during gestation did not affect
performance during lactation. Milk thiocyanate and colostrum iodine
concentrations were significantly higher in the group fed 500 mg
KCN/kg feed. No effects of cyanide were reported on indices of
reproduction performance (Tewe & Maner, 1981a).
2.2.5 Special studies on embryotoxicity and/or teratogenicity
In a teratogenicity study pregnant hamsters received oral doses
of 70, 100, 120 or 140 mg linamarin/kg bw or an equivalent volume
(0.5 ml/100 g) of isotonic saline during the early primitive streak
stage of gestation (day 8 of gestation). The hamsters were killed on
the morning of day 15 of pregnancy. Fetuses were removed by
caesarian section and the numbers of resorption sites, dead fetuses,
and living fetuses were recorded. Living fetuses were examined for
gross external malformations and by means of histopathological
methods for internal malformations.
A dose of 120 or 140 mg linamarin/kg bw was associated with an
increased incidence of vertebral and rib anomalies as well as the
production of encephaloceles in the offspring. These larger doses of
linamarin also resulted in obvious maternal toxicity (dyspnoea,
hyperpnoea, ataxia, tremors and hypothermia). Two animals dosed with
140 mg and one animal dosed 120 mg/kg bw died. In surviving animals
the signs of poisoning were greatly reduced or gone within 3 h after
Linamarin treatment had no effect on fetal body weight,
ossification of skeletons, embryonic mortality, nor litter size.
Although ingestion of the cyanogenic glycoside was associated with a
significant teratogenic response, the effects occurred only at doses
that elicited signs of maternal intoxication (Frakes et al.,
In a teratogenic study groups of pregnant hamsters
(8 dams/group) were fed diets consisting of cassava meal:laboratory
chow (80:20) during days 3-14 of gestation. One low cyanide (sweet)
cassava meal and one high cyanide (bitter) cassava meal were
studied. An additional group was fed a diet which resembled cassava
in nutrtional value, but which lacked cyanogenic glycosides.
Thiocyanate concentrations in the urine and blood of dams fed
cassava diets increased significantly. Increased tissue thiocyanate
concentrations were observed in fetuses recovered from cassava-fed
dams. Cassava-fed dams gained significantly less weight than did
control animals and their offspring showed evidence of fetotoxicity.
Reduced fetal body weight and reduced ossification of sacrocaudal
vertebrae, metatarsals and sternebrae were associated with cassava
diets. High cyanide cassava diets were also associated with a
significant increase in the numbers of runts compared to litters
from dams fed either low protein or laboratory stock diets (Frakes
et al., 1986).
2.2.6 Special studies on the thyroid gland
In a study cited by Oke (1980), the influence of a 100% cassava
diet on the thyroid in a 7-day experiment with rats. A significant
decrease in glandular stores of stable iodine, significantly higher
thyroid weight and higher thyroidal 131I uptake were observed.
Each effect is due to a synthetic block in the conversion of
monoiodothyronine to diiodothyronine.
2.3 Observations in humans
2.3.1 Acute toxicity studies in humans
One to 10 g of amygdalin have been given parenterally in
humans, apparently without acute toxicity. This indirectly suggests
that there is no significant metabolism of the intact injected
glycoside. The cyanide-containing breakdown products possess
well-defined toxicities, and 50 mg of hydrogen cyanide can be fatal.
With oral dosing of amygdalin, a toxic potential is manifest.
▀-Glucosidase is present in the gastrointestinal lumen, a
contribution of intestinal microflora. According to Eyerly (1976)
oral laetrile (amygdalin) could be 40 times more toxic than
parenterally administered doses. This is probably due to the free
HCN released by the ▀-glucosidase enzyme present in the gut (Dorr &
The lethal dose of amygdalin for man when ingested is reported
to be in the range of 0.02-0.13 mmol/kg bw (Solomonson, 1981).
If it is assumed that about 100-2000 mg HCN is the lethal dose
for man, as much as 10-20 kg of Lafun cassava (10-20 mg cyanide/kg)
will have to be consumed at a sitting to produce toxicity
Well-nourished individuals have ingested 1000 mg or more of
pure amygdalin every day without any evidence of "side effects"
In a case study an 11-month-old girl was reported accidentally
to have ingested 1-5 amygdalin tablets (500 mg). The patient became
listless within one half hour of ingestion and vomited. Breathing
became irregular and her state of consciousness became altered. An
hour after ingestion she was in shock and died approximately 72 h
following ingestion in spite of hospital treatment (Humbert et al.,
In a case-study a 17-year-old girl suffering from cancer made a
practice of taking, instead of radiotherapy, four ampoules of
laetrile (3 g amygdalin) intravenously. One day she swallowed three
1/2 ampoules of laetrile. Shortly after ingestion, a severe headache
and dizziness developed, and she collapsed. Laboured breathing
developed, her pupils became dilated, and she became comatose. All
symptoms occurred within 8-10 minutes after ingestion. She died 24 h
after ingestion (Sadoff et al., 1978).
In Anatolia (Turkey) 9 cases of cyanide intoxication of
children due to the ingestion of wild apricot seeds (217 mg
HCN/100g) were reported. The victims had probably eaten more than 10
seeds. Also in studies of Jeanin et al. (1961) and Pijoun (1942)
poisoning after consuming a relative large amount of peach seeds or
bitter almonds are reported. Quantitative figures on cyanogenic
glycoside or cyanide intake are not given (Sayre & Kaymakcalavu,
The toxicity of cassava and cassava processing products was
until recently assumed to be associated with free cyanide, this was
50-60 mg which constitutes a lethal dose for an adult man. The
cyanogenic glycosides were at first thought to be of little
consequence to mammals if cassava hydrolytic enzymes have been
inactivated. The possibility of hydrolysis during digestion,
however, is also important (Cooke & Coursey, 1981).
In a case-study a 67-year-old woman collapsed after ingestion
of a slurry of 12 bitter almonds ground up and mixed with water. She
recovered after treatment in the hospital. The average cyanide
content was 6.2 mg HCN/bitter almond (Shragg et al., 1982).
The consumption of 60 bitter almonds is deadly for an adult.
For young children, however 5-10 almonds or 10 droplets of bitter
almond oil are fatal (Askar & Moral, 1983).
2.3.2 Long-term toxicity studies in humans with cyanogenic
glycosides and cyanides
A study to evaluate the possible association of high cyanide
and low sulfur intake in cassava-induced spastic paraparesis was
performed. The north-eastern part of Mozambique suffered a severe
drought in 1981: the only crop available was the most toxic variety
of cassava and, due to lack of food during the harvest period, the
roots were eaten after only a few days of sun drying. A field survey
revealed 1102 cases of spastic paraparesis. In 1982 urine was
collected from 30 apparently healthy children (age 8.1 years). As
reference 17 Swedish children (age 8.6 years) were used. In a second
stage urine was sampled in 1983 (when the nutritional situation was
improved but still unsatisfactory) from 31 children (9.0 years) in
the same village and 30 schoolchildren (8.1 years) in a nearby
district where no cases of paraparesis were seen in 1981 and from 28
children (7.1 years) of the city who ate virtually no cassava. The
children from the village had increased thiocyanate and decreased
inorganic sulfate excretion, indicating high cyanide and low
sulfur-containing amino acid intake. Children from a neighbouring
cassava-eating area, where no cases of spastic paraparesis had
occurred, had lower thiocyanate excretion but higher organic sulfate
excretion. These results support the hypothesis that the epidemic
was due to the combined effects of high dietary cyanide exposure and
sulfur deficiency (Cliff et al., 1985).
Several studies were performed, including epidemiological
studies, on the role of chronic cyanide intoxication caused by the
consumption of cassava diet in the etiology of tropical (ataxia)
neuropathy (TAN) in Nigerian populations. As described in over 400
Nigerian patients the essential neurological components of the
disease are myelopathy, bilateral optic atrophy, bilateral
perceptive deafness and polyneuropathy. The initial and most common
symptoms consist of various forms of paraesthesia and dysaesthesia
usually starting in the distal part of the lower limbs. The next
most common finding is blurring or loss of vision. Other common
symptoms in order of frequency are ataxia, tinnitus, deafness,
weakness and thinning of the legs.
In about a third of the patients stomatoglossitis is present,
additionally motor neurone disease, Parkinson's disease, cerebellar
degeneration, psychosis and dementia have been associated with the
disease. TAN affects males and females in all age groups equally,
but occurs only rarely in children under 10 years. Patients usually
give histories of almost total dependence on a monotonous diet of
cassava derivatives; occasional dietary supplements include yam,
maize, rice, vegetables and animal protein. Analysis of family
relationships among patients showed no evidence of a genetically
determined predisposition. The families were usually poor and
members lived communally.
Clinical evidence of malnutrition was frequently absent. The
occurrence of cyanide intoxication in Nigerian patients was
indicated by the significantly higher cyanide and thiocyanate plasma
levels and higher excretion of thiocyanate than controls. Hepatic
rhodanese activity was not different from that in controls and
histology of liver biopsy specimens showed no abnormality. Total
plasma vitamin B12 levels are normal or high in patients and healthy
Nigerians but plasma concentration of cyanocobalamin was highly
significantly raised in patients. A small proportion of
cyanocobalamin was found in the liver of patients. Methylmalonic
acid excretion was normal in patients, indicating that there was
physiological vitamin B12 adequacy at tissue or cellular level in
In Nigeria, endemic foci of the disease (in epidemiological
studies) recognized since the early 1930's, correspond with the
areas where cassava is intensively cultivated and consumed as the
major or the sole dietary source of carbohydrate. There was evidence
of increased exposure to cyanide in members of families where
multiple cases were found. In biochemical studies no biochemical
evidence of protein-calorie malnutrition was seen and serum
transferrin, said to be sensitive index of protein nutritional
status, was normal. No haematological abnormalities were seen in
patients. Investigation of patient nutritional status with regard to
the water-soluble vitamins showed no abnormality, except in
riboflavin intake. Plasma concentrations of calcium, phosphate,
sodium, potassium, chloride, bicarbonate, and cholesterol, and tests
of thyroid, hepatic, and renal functions were normal. Amino acids
and porphyrobilinogen and other porphyrins were absent in urine.
There was no biochemical nor other evidence of malabsorption.
Glucose tolerance tests were normal. Histamin-fast achlorhydria was
very rare. Serologic tests for syphilis, typhoid, typhus,
brucellosis and screening for prevalent viral infections gave
negative or insignificant results. There was no increased prevalence
of malaria and urinary tract infections were not encountered in
patients. ECG's were normal. Diminished urinary excretion and
riboflavin (vitamin B2) and low serum riboflavin and caeruloplasmin
levels in patients compared with healthy persons were the only
significant abnormalities found. Riboflavin deficiency is, however,
widespread in many parts of Nigeria and especially in areas endemic
for TAN, probably because cassava is a poor source of the vitamin.
The daily intake of HCN from cassava derivatives in areas in Nigeria
endemic for TAN may be as high as 50 mg, which is nearly an acutely
sublethal amount, and can conceivably produce cyanide intoxication.
This is particularly plausible as cassava root is a major item of
food, often the main source of carbohydrate, and is poor in
sulfur-containing amino acids which are essential for detoxification
of cyanide. A high prevalence (2%) of goitre in populations with a
high incidence of TAN is seen; this appears to be related to cassava
diet and high plasma thiocyanate. The effects of riboflavin
insufficiency may combine with those of chronic cyanide intoxication
in the etiology of TAN (Osuntokun, 1981).
Several epidemiological and experimental studies revealed that
TAN resulted from chronic cyanide poisoning due to the release of
HCN from cyanogenic glycosides present in certain cassava food
products. Since TAN invariably occurs among poorly nourished people,
the condition may result from unidentified nutritional deficiencies
or excesses as well as cyanide toxicity. Among patients showing the
TAN syndrome the frequency of goitre was increased (Conn, 1979a,b).
From both experimental and epidemiological studies there is
strong, but not conclusive, evidence that cassava toxicity is a
causative factor in some neurological disorders like TAN and endemic
spastic paraparesis. As well, different deficiencies such as
deficiency of sulfur-containing amino acids may play a role. The
geographical distribution of malnutrition-related diabetes coincides
with that of cassava consumption. Dietary exposure to cyanide has
been proposed as a possible cause, but has not been proven. The poor
nutritional quality of cassava seems a very likely causative factor
Workers in certain occupations are exposed to HCN additional to
sources encountered by the general public. These individuals have
been studied to determine whether chronic cyanide poisoning is a
clinical entity. Symptoms reported were headache, vertigo, tinnitus,
nausea, vomiting, and tremors. Although these symptoms are
sufficiently documented and characteristic, they are transitory in
that exposure to fresh air causes recovery. These do not seem to
produce the outward symptoms of TAN, the pathological condition that
has been attributed in part to exposure to cyanide or cyanogenic
glycosides in certain preparations of cassava (Conn, 1979b).
Neurological disorders such as ataxic neuropathy and cretinism
have been associated directly with the intake of cyanogenic
glycoside-contaminated diets. There are grounds to suspect that
cyanogenic glycoside-contaminated foodstuffs such as cassava and
pulses are directly implicated in acute and chronic cyanide toxicity
in the tropics. Although correlation between dietary
cyanogen-contaminants and disease such as TAN, goitre, cretinism and
mental retardation exists and is based on experimental evidence, the
mechanism of action of cyanide and thiocyanate on the cellular level
is not well understood (Nartey, 1980).
Based on several studies in humans consuming as their main food
cassava or other food products rich in cyanogenic glycosides, it
seems that chronic cyanide intoxication in combination with
deficient intake of riboflavin and/or a poor quality of protein and
hence methionine deficiency is/are responsible, to a large extent,
for the etiology of TAN in cassava-eating areas, whereas chronic
cyanide intoxication in combination with a deficient iodine intake
is responsible for goitre (Oke, 1979, 1980).
Epidemiological and experimental studies show that cyanogenic
glycosides in food products play a important role in the development
of goitre. Thiocyanate, the detoxification product from the HCN
derived from cyanogenic products, is responsible for interference
with thyroid function. Studies on endemic goitre in Africa have
identified iodine deficiency and an antithyroid activity of cassava
diets as major etiological factors of the disease (Conn, 1979a,b).
Extensive studies in Zaire have established that goitre and
cretinism due to iodine deficiency can be considerably aggravated by
a continuous dietary cyanide exposure from insufficiently processed
cassava. This effect is caused by thiocyanate. Thiocyanate has a
similar size to the iodine molecule and interferes with the iodine
uptake in the thyroid gland. Thiocyanate levels which can occur
after exposure to cyanide from cassava can only affect the gland
when the iodine intake is below 100 micrograms/day, which is
regarded minimal for normal function. Several studies have shown
that populations with considerable cyanide exposure from
inadequately processed cassava are free from goitre as long as their
iodine intake is sufficient. Populations in northern Zaire with very
low iodine intake and in addition high thiocynate levels resulting
from the consumption of cassava products, show very severe endemic
goitre. When the population was given iodine supplementation, the
goitre decreased (reviewed by Rosling, 1987).
In persons who ingested cassava a decreased 131I-uptake by
the thyroids was seen, confirming the goitrogenous nature of cassava
(De Lange, 1973 as cited by Oke, 1980).
In one study more than 50% of workers in a processing factory
revealed thyromegaly and an increased 131I-uptake. All workers had
increased blood haemoglobin concentration as well as increased
lymphocyte counts. Two of the employees developed psychosis,
although it is difficult to know whether this was cause or effect.
Abnormal thyroid function has also been found in workers from a
photographic plant in which a cyanide extracting process was used to
recover silver from X-ray films (El Ghawabi et al., 1975).
Alterations in vitamin B12 and folate metabolism have also been
noted to play a role, although the significance of these
observations are not known. Short of having an elevated whole blood
cyanide level, the laboratory findings of cyanide poisoning are
fairly non-specific. Perhaps the most valuable clue is the finding
of a severe metabolic acidosis with a high anion gap. In clinical
medicine there have been reported only 7 causes of anion gap
metabolic acidosis. In most of these disorders lactic acidosis is
present owing to severe tissue underperfusing and hypoxia. In the
patient with cyanide poisoning tissue hypoxia is universally present
(Gonzales & Sabatini, 1989).
3. COMMENTS AND EVALUATION
The potential toxicity of a cyanogenic plant depends primarily
on its capacity to produce a concentration of hydrogen cyanide toxic
to animals and humans. The release of hydrogen cyanide can occur
either following maceration of the plant material - this activates
the intracellular ▀-glucosidase which in turn hydrolyses glycoside -
or by hydrolysis of glycoside by the ▀-glucosidase produced by the
microflora of the gut. The level of ▀-glucosidase activity in the
gut depends on the pH and the bacterial composition. The cyanogenic
glycoside content of a foodstuff, when known, is usually expressed
in terms of the amount of cyanide released by acid hydrolysis; exact
figures for the concentration of the glycosides themselves are very
Hydrogen cyanide absorbed from the gut can be detoxified by
metabolic conversion to thiocyanate; this depends on the presence of
nutritional factors, such as sulfur-containing amino acids and
vitamin B12. Acute toxicity results when the rate of absorption of
hydrogen cyanide is such that the metabolic detoxification capacity
of the body is exceeded.
Available reports of toxicological studies lack information on
the level of intake of cyanogenic glycosides or on the amount of
hydrogen cyanide potentially released. No long-term toxicity or
carcinogenicity studies were available to the Committee. However,
in vitro and in vivo genotoxicity were negative. Teratogenic and
adverse reproductive effects attributable to linamarin (cassava) and
hydrogen cyanide were seen only at doses that also caused maternal
The toxic effects of cyanide on the thyroid (via its metabolite
thiocyanate) depend on the iodine status of the test animals, as
indicated in the twenty-fifth report of the Committee (Annex 1,
On the basis of epidemiological observations, associations have
been made between chronic exposure to cyanogenic glycosides and
diseases such as spastic paraparesis, tropical ataxic neuopathy, and
goitre. However, these observations were confounded by nutritional
deficiencies, and causal relationships have not been definitely
Traditional users of foods containing cyanogenic glycosides
usually have a basic understanding of the treatment required to
render them safe for consumption. However, some products are sold
commercially and are consumed by people who may not be familiar with
such procedures. The Committee therefore recommended that guidelines
be developed to provide reliable and sensitive methods for the
analysis of these foodstuffs for hydrogen cyanide releasable from
cyanogenic glycosides, in order to ensure that amounts in foods as
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