FOOD TOXINS 1Lecture No. 7

Copyright © Mgr. Zuzana Široká, PhD.

Toxic agents in food of plant origin

Glucosides

• This is the largest group of secondary metabolites.• Substances consisting of sugar part and non-sugar part,

so called aglycone. These two parts are connected by a bond of ether character – by glucosidic bond.

• Generally speaking, we can say that these substances usually affect blood and circulatory system.

• These agents are not toxic themselves, but are easily hydrolysed in acid environment and release cyanic acid

Cyanogenic glucosides:

• Very common in plants, found in more than one thousand species in more than one hundred families.

• They are bitter and primarily act as repellents.

• We know more than fifty cyanogenic glucosides.

• In table there are some most known ones:

Glucoside Plants HCN (mg/kg) content in fresh flesh

Linamarin Manioc, white clover, cultivated flax Manioc-bitter type-peel of bulbs 840

-pulp of bulbs 330

Faseolunatin Sugar bean 100-4000

Lotaustralin Bird´s-food trefoil, white clover, manioc

Linustatin Neolinustatin

Cultivated flax -seeds 200-380

Prunasin Plum tree

Sambunigrin Elder – Sambucus nigra

Amygdalin Plum tree, kernels of stone-fruits: bitter almonds, apricot, peach

Kernels of these plants approximately 3 %

Vicianin Bob, vetch

Taxifyllin Common yew, bamboo

Dhurrin Sorghum

• Aglycones (non-sugar parts) are formed by α-hydroxynitriles, which are stabilised by glucosyl bound to hydroxyl group of a sugar.

Aglycone:

R1 OH R1 O- β-glycosyl

C C

R2 C N R2 C N

α-hydroxynitrile cyanogenic glucoside

Manioc (kasawa, cassava):

• Originally south-american species from Euphorbiaceae family

• It is widespread in all tropic areas• Root bulbs contain starch and

hydrocyanic acid, which can be removed by cooking or baking

• Bulbs are used as potatoes, they are dried and flour is made of them

• We distinguish bitter and sweet types of manioc, bitter can contain up to 500 mg/kg of HCN

Sorghum:

• From Poaceae family

• Has been known since ancient Egypt times

• It is an annual plant, about 3 m high with minute grains containing a lot of starch

• Dhurrin is mainly in germs, seeds usually don’t contain cyanic acid

Almond, apricot, peach:

• Contain glucoside called amygdalin• HCN is very poisonous, but in plants occurs in

quite low concentrations• Additionally, releasing process is slow so there is

not sufficient amount cumulated in one time• Lower concentration of amygdalin is found in

plums, blackthorns, cherries, sour cherries and apples.

Mechanism of action:• The carrier of toxicity is HCN, released from glucoside• In blood it binds to haemoglobin and methaemoglobin

and forms cyanhaemoglobin and cyanmethaemoglobin respectively

• Its affinity to methaemoglobin (which is higher than to haemoglobin) is used for treatment

• Cyanides and cyanic acid act as neurotoxins and they also bind to several proteins damaging enzyme systems.

• They decrease tissue breathing, block cytochromoxidase (HCN binds to Fe and Cu), cause tissue asphyxia (suffocation), because oxygen is not passed on to tissues, blood is oversaturated by oxygen and is of bright brick red colour

• In acute poisoning we can see increased respiration due to chemoreceptors´ and respiration centre damage

• Then the decrease of blood pressure, seizures, coma and exitus due to respiratory centre collapse follows

• In chronic intoxications is typical tremor, conjunctivitis, inappetence, neuropathies, headaches

• In manioc it is called tropic ataxic neuropathy

Biotransformation:• Dissociation of cyanogenic glucosides goes on in two

steps• First, β-D-glucopyranose is dissociated due to action of

enzyme called β-D-glucosidase and α-hydroxynitrile is formed

• This substance is dissociated to HCN and aldehyde or ketone due to enzyme hydroxynitrillyase

Dissociation of cyanogenic glucosides: R1 O- β-glucopyranose R1 OH

C C + glucose R2 C= N + H2O R2 C = N cyanogenic glucoside α-hydroxynitrile

β –glucosidase

R1 OH R1

C C = O + HCN R2 C= N + H2O R2

Aldehyde and ketone

α-hydroxynitrile α-hydroxynitrillyase

• In normal plant tissues free HCN doesn’t occur• Described dissociation takes place in mechanically

damaged tissues, where glucosides and enzyme, normally stored separately, come into touch and start to react together

• It can be during cutting of plants, crushing of seeds, if the plant is frozen, in extreme drought

• Efficient β –glucosidase is also a part of bacterial microflora in GIT of mammals

• Released HCN must be detoxified in plant or in animal somehow - product of detoxification is thiocyanate SCN-

• This change is possible thanks to one of two enzymes - sulphurtransferases. Cyanide-sulphurtransferase, also called rhodanase, which is a part of mitochondria and 3 -merkaptopyruvatesulphurtransferase (also in mitochondria)

S2O3 + CN- SO32- + SCN-

thiosulphate nitrile rhodanase sulphite thiocyanate

CH2 - C - COOH + CN- CH3- C -COOH + SCN-

3-merkaptopyruvatesulphur transferase SH O O Mercaptopyruvate Pyruvate thiocyanate

Detoxification of cyanides:

• Similar mechanism is found in animals• It takes place in liver and thiocyanates are excreted by urine or

faeces• Detoxification is dependent on sufficient amount of sulphur

substances in an organism

Treatment:• Administration of nitrites – formation of methaemoglobin, which

binds cyanides – formation of cyanmethaemoglobin – administration of thiosulphates – cyanides released from methaemoglobin react with them and form thiocyanates

• Now a new antidote is tested – hydroxocobalamin – with cyanides in blood forms cyanocobalamin (vitamin B12)

• Lethal doses:- In fruit containing amygdalin they are 5-25 pieces of seed for

children. - Lethal dose for humans is 0,5 – 3,5 mg/kg HCN

Glycoalkaloids of potatoes (Solanum tuberosum, Solanaceae)

• Potatoes are fourth most important crop-plant in the world• Toxic glycoalkaloids are found in the whole plant of potato

Solanum tuberosum, and also in majority of other species from family Solanaceae, e.g. tomato or aubergine - Solanum melongena

• A)These alkaloids are so called steroid glycoalkaloids (SGA), or pseudoalkaloids, R)because their precursors are not aminoacids like in genuine alkaloids

• They can be also classified as glucosides – they contain glucosidic bond in their molecule

• Glycoalkaloids are formed of:- polar, in water soluble saccharide part – mono-tetrasaccharide- non-polar steroid (aglycone)

Potatoe:• The whole overground part of the plant is poisonous• In green parts, germs and berries there is solanin

and less potent solanidin, in tubers, there are solanin and chaconin

• In general we call all these alkaloids Solanin• The concentration of alkaloids varies according to

many conditions (warm, dryness, humidity). Also quality of soil, fertilizers added, stress factors (e.g. insect attacks, mechanical damage or light) influence the concentration

• These stress factors and increased amount of magnesium in soil increase the concentration of alkaloids, on the other hand, molybdenum in soil decreases their concentration

• In starch parenchyma of tubers, there is only about 1 % of solanin

• Glycoalkaloids are termostable, so you cannot destroy them by cooking, frying or drying. They bear temperatures higher than 300°C. But during cooking potatoes in water majority of alkaloids pass to water

• Solanin – in fresh unpeeled tubers there can be concentration of solanin more than 100 mg/kg. The limit for potatoes determined to eat is 200 mg/kg. But at about 150 mg/kg, the taste starts to be bitter and in higher concentration this taste disallows to eat them.

• In tubers for consumption , the vast majority of glycoalkaloids is in 1.5 mm thick part of cortex, of peel

• By peeling we remove 60-95 % of solanin• It is recommended not to expose them to light. Tuber

reacts with rapid synthesis of alkaloids as a defence against stress

• Once formed, these alkaloids cannot be removed during storage in dark

• Another danger is mechanical damage which also leads to alkaloid synthesis

• Data on temperature and humidity differs among sources, low temperatures lengthen high quality of tubers, but cold can cause increased synthesis of alkaloids

• A)For decreasing of alkaloid formation we use chemical retardants or vacuum package, R)because lack of oxygen and decreased respiration block metabolism in tubers

• They posses two mechanisms of action: - First: they disturb cell membranes containing steroid substances (an effect

similar to those in saponines) - Second: they inhibit serum cholinesterase too (similar to organophosphates)• Glycoalkaloids cause gut mucosa and liver damage• Early signs of intoxication are colic, diarrhoea with blood in excrements,

apathy, halucinations. Then follow seizures, coma and respiration paralysis• We suppose, that many of slight intoxications are considered as common gut

problems and we dont attribute them to potatoes• For humans, toxic dose is 2-5 mg/kg of body weight and lethal dose between

3-6 mg/kg body weight • Other group of glycoalkaloids in potatoes are leptins. They are natural

pesticides and genetics try to put the gene for leptins to other plants to protect them

• Tubers also contain so called protease inhibitors, which dissociate trypsine, chymotrypsine, kathepsine D and karboxypeptidases. These inhibitors can be removed and destroyed by exposure to heat

• BEWARE - Inhibitors are also in: - peanuts, wheat, red beet, onion, soya beans

Toxic effects of glycoalkaloids:

Organ, tissue Content (mg/kg)

Germs 2000-4000

Roots 180-400

Stalk 20-30

Leaves 400-1000

Flowers 3000-5000

Berry 4200

Whole tuber (bulb) 75

Peel 300-500