TOXINS AND TOXICS

TOXINS AND TOXICS

DEFINITIONS

TOXIN:

Technically, a toxin is a poisonous substance produced by living organisms.
By this definition, man-made substances produced artificially are not considered to be toxins.
Toxins are different from biological agents such as bacteria, viruses etc. According to the International Committee of the Red Cross definition of toxins, toxins are chemical agents and not biological agents: "Toxins are poisonous products of organisms; unlike biological agents, they are inanimate and not capable of reproducing themselves."
Toxins are usually small molecules, peptides, or proteins that are poisonous to organisms when they interact with cellular components such as macromolecules, enzymes or receptors. Toxins differ in their severity, ranging from minor (eg ant sting) to acute (e.g bee sting) to deadly (e.g botulinum toxin).

EXOTOXINS:

These are toxins excreted by an organism.

ENDOTOXINS:

These are toxins released only when an organism such as bacteria is lysed.

TOXOID:

This is a weakened or suppressed toxin.

VENOMS:

These are toxins in the sense of use by some types of animals.

BIOTOXINS:

This word is used to emphasize the biological origin of a toxin. Biotoxins are produced spiders, snakes, scorpions, jellyfish and wasps for the purpose of predation. Biotoxins are also produced ants, bees, termites, honeybees and some frogs for the purpose of defense.

TOXICANT:

This is a toxic substance that is not produced by living organisms. They are not biologically produced from living organisms, but are artificially produced. They are synthetic products, and can also be called xenobiotics.
Plural for toxicant is toxics.

CATEGORIES OF TOXINS.

There are three broad categories of toxins. The term “toxin” is however used in this context in the non-technical sense to include any chemical substance that is injurious or detrimental to a living organism. Here it includes both toxins and toxics.

A) ENVIRONMENTAL TOXINS:

These are toxins that are produced artificially by human beings. They are environmental pollutants of land, water and air. They are also called xenobiotics because they are foreign and synthetic. Examples include:
1. Benzene
2. Benzoic acid
3. Phenobarbitones
4. Chlorinated hydrocarbons such as the chlorobenzoates
5. Xylenes
6. Nicotine
7. Pesticides etc

BENZENE

BENZOIC ACID

DIQUAT (organo-nitrogen pesticide) (NOTE:  insert N+ at the base of the Benzene structures)

PARAQUAT (NOTE:  insert N+ at the ends of the Benzene structures)

B) FOOD TOXINS: 

These are toxins found in food stuff. They are:
1. Chemical food toxins.
2. Biological food toxins.

CHEMICAL FOOD TOXINS.

These include the following:

1. COAL TAR: All artificial color and flavor additives in soft drinks and cosmetics are produced from coal tar. Coal tar dyes give foods bright colour and strong flavour. They are very carcinogenic.

2. ARTIFICIAL SWEETENERS: Cyclamates and saccharin, artificial sweeteners, can cause cancer of the gastrointestinal tract.

3. NITROSAMINES: Nitrosamines are produced in the body from three possible sources:
a) From the ingestion of chemical preservatives and colour enhancers.
b) From the nitrites and nitrates that are added to meat during processing.
c) From the nitrites and nitrates from chemical fertilizers which get into water bodies through runoff water and from thence into living organisms.
Nitrosamines are highly carcinogenic, causing cancer of the liver, stomach, brain, bladder and kidneys.

4. ADDITIVES LIKE BUTYLATED HYDROXYLTOLUENE (BHT), BUTYLATED HYDROXYLANISOLE (BHA), MONOSODIUM GLUTAMATE (MSG) AND DIETHYLSTILBESTROL (DES): BHT and BHA are added to foods as antioxidants in order to preserve the fat in them. MSG is added to food as a taste enhancer. DES is added especially to meat in the USA as artificial sex hormone.These are all carcinogenic poisons.

5. ALUMINUM: Aluminium used as a cookware (i.e., as a cooking utensil) is a kind of food additive, which is poisonous to the body. Small particles of aluminium enter the food as it is being cooked. It has been banned in Sweden. It is safer to use stainless steel for cooking.

6. CHEMICALS SUCH AS HEXACHLOROPHENE, BENZIDINES, ANILINE DYES, NAPHTHALENE, ASBESTOS AND PESTICIDES: These may produce free radicals in the body and thus become strongly carcinogenic to bladder, kidney and lungs.

7. HEAVY METALS SUCH AS NICKEL, LEAD, MERCURY, AND CADMIUM: These could accumulate over a long period of time in the body and become very poisonous.

BIOLOGICAL FOOD TOXINS:

This includes:
1. Food poisoning by toxic plants and animals.
2. Food poisoning by bacteria.
3. Food poisoning by moulds.

FOOD POISONING BY TOXIC PLANTS AND ANIMALS.

PLANTS:

1. Certain mushrooms are very toxic, especially when a poisonous mushroom is mistaken for a non-poisonous one. Some are deadly.

2. Red kidney beans is toxic when eaten raw or when it is not properly cooked. The substance haemagglutinin must be boiled for about 10 minutes to inactivate it. It causes nausea, vomiting and diarrhea.

3. Cyanic acid in improperly processed cassava is poisonous. It is only careful fermentation and processing of cassava that can eliminate its cyanic acid load. Cyanic acid blocks enzymes of the respiratory chain.

4. Ricin is a toxin found in the castor bean plant.

ANIMALS:

1. Puffer fish poisoning.
The intestines of puffer fish (sphaeroides rubripes) contain the toxin tetrodotoxin.

The poisonous part has to be carefully removed before the fish can be eaten. The fish is an ingredient of a Japanese delicacy. This toxin acts by binding to Na+ channels (protein channels) of neurons, and thus preventing normal action potential. This blocks signals from nerves to muscles. In this way it causes paralysis and death.

2. Paralytic shellfish poisoning.

Shellfish such as oysters, mussels or clams that have fed on the phytoplankton, the marine dinoflagellate Gonyaulax become toxic because they accumulate the toxin saxitoxin produced by the plankton in their muscles. The shellfish itself is not sensitive to the toxin, but organisms higher up the food chain that consume it are harmed by a mode of action similar to that of tetrodotoxin.

FOOD POISONING BY BACTERIA.

This may be:
1. Food borne infection: The organism itself is the causative agent of the infection e.g Listeria monocytogenes and salmonella sp. They may cause gastrointestinal tract (GIT) disorders.

2. Food borne intoxication: This is caused by ingestion of toxins of microorganisms. The toxins of staphylococcus aureus and clostridium botulinum are heat stable even though the microorgainsm itself is already killed by heat. The toxin of clostridium botulinum is a protease that prevents the release of neurotransmitter at the synapse of two neurons. It leads ultimately to death.

3. Food borne disease: The organism does not grow in the food, but simply use it as a means of transfer from host to host, e.g faecal-oral route such as salmonella typhi.

FOOD POISONING BY MOULDS.

Toxins of moulds are called mycotoxins. Some mycotoxins are carcinogenic while some others attack specific organs and tissues. An example is aflatoxin found in groundnut infected by Aspergillus flavus. Aflatoxin can also be found in other foods like fresh beef, milk, beer or cocoa.

OTHER NON-FOOD SOURCES OF TOXINS.

1. The venom of the black mamba snake contains the toxin dendrotoxin which hinders K+ channels.

2. The venom of the rattlesnake contains the toxin hemotoxin, which targets and destroys red blood cells.

3. Necrotoxins, which cause necrosis (i.e. death) in the cells they come in contact with are produced in the venom of the puff adder (Bitis arietans).

4. Neurotoxins, which attack the nervous systems of animals, are produced in the venom of Elapid snakes, most scorpions, widow spiders and the cone snail.

5. Apitoxins are produced by the honey bee in its venom.

6. Other reptile/snake venom toxins are bungarotoxin, cobrotoxin, calciseptine, taicatoxin, calcicludine, cardiotoxin III.

7. Amphibian toxins include Allopumiliotoxin 267A, Batrachotoxin, Bufotoxins (Arenobufagin, Bufotalin, Bufotenin), Cinobufagin, Epibatidine, Histrionicotoxin Tarichatoxin.

8. Curare is a substance used as an arrow head poison. The active component is the toxin tubocurarine.

DETOXIFICATION OF ENVIRONMENTAL TOXICS.

Toxics are not of use to the organism, (infact they are harmful) and their absorption is not always prevented; hence it is necessary to neutralize reactive groups in these compounds and to expedite their excretion by active transport.
The metabolism of toxins is biphasic:
1. This involves the introduction of functional groups to the toxin, such as epoxides, sulphydryl (-SH), hydroxylamine (-NHOH), hydroxyl (-OH), carboxyl (-COOH) groups etc. The reactions are epoxidation, reduction, oxidation, hydrolysis etc. Sometimes a reaction may occur to expose reactive functional groups in the toxic. For example, in the detoxification of benzene, a hydroxyl group may be introduced in an oxidation reaction to form phenol. The purpose of this reaction step generally is to make the xenobiotic, especially the lipophilic (i.e., water insoluble) substances, water soluble in order to facilitate their excretion through the urine

2. The second step involves the conjugation of the phase I product with an endogenous substance in order to further enhance its water solubility and excretion via the urine. For example, phenol may be conjugated with glucouronic acid to form the easily excretable phenyl β-D-glucouronide.
Sometimes, however, the metabolism of a xenobiotic need not be biphasic. It may be monophasic, involving only conjugation. For example, the metabolism of benzoic acid involves direct conjugation with glycine or glucouronic acid to form hippuric acid or benzoyl glucouronide respectively.

ASSIGNMENT.

1. Differentiate between toxins and toxics.

2. Differentiate between technical usage and non-technical usage of the term toxin.

3. Discuss toxins of plant origin.

4. Discuss toxins of animal origin.

5. Discuss toxins of bacterial origin.

6. Discuss a known toxin of mould origin.

7. Explain the various ways snake venoms act.

8. What is biphasic metabolism of environmental toxins.

9. What is monophasic metabolism of environmental toxins.

10. Explain the following terms: toxin, toxicant, toxoid and biotoxin.

11. What are chemical food toxins?

12. Draw the structures of diquat, benzoic acid and tetrodotoxin.

13. List examples of environmental toxins.