Toxin

A toxin is a chemical substance that is capable of causing injury, illness, or death to an organism (poison) and that is produced by living cells or another organism. The term is sometimes used in a broader sense to refer to any substance that is poisonous to an organism, but generally the usage is limited to poisons produced via some biological function in nature, such as the bacterial proteins that cause tetanus and botulism. While the term is especially applied to substances of bacterial origin, many diverse taxa produce toxins, including dinoflagellates, fungus, plants,a nd animals.

Toxins are nearly always proteins that are capable of causing disease on contact or absorption with body tissues by interacting with biological macromolecules such as enzymes or cellular receptors. Toxins vary greatly in their severity, ranging from usually minor and acute (as in a bee sting) to almost immediately deadly (as in botulinum toxin).

Biotoxins vary greatly in purpose and mechanism, and can be highly complex (the venom of the cone snail contains dozens of small proteins, each targeting a specific nerve channel or receptor), or a single, relatively small protein.

Functions of toxins

Biotoxins in nature have two primary functions:

• Predation or invasion of a host (spider, snake, scorpion, jellyfish, wasp)
• Defense (bee, poison dart frog, deadly nightshade, honeybee, wasp)

For example, a toxin may be used in assisting bacterial invasion of a host's cells or tissues or to combat the defense system of the host. A spider may use toxin to paralyze a larger wasp prey, or snake may use to subdue its prey. On the other hand, a honeybee sting (while of little benefit to the honeybee itself, which usually dies as a result of part of the abdomen ripping lose with the stinger) can help in discouraging predation on the bees or their hive products.

Sometimes, however, action of a toxin on an organism may not correlate to any direct benefit to the organism producing the toxin, but be accidental damage.

Types of organisms producing toxins

Numerous types of organisms produce toxins. Some of the well-known organisms are listed below.

Bacteria

The term toxin especially is used in terms of poisonous substances produced by bacteria. Examples include cholera toxin from Vibrio cholera, tetanus toxin from Clostridium tetani, botulism toxin from Clostridium botulinum, and anthrax toxin from Bacillus anthracis.

Bacterial toxins can damage the cell wall of the host (eg., alpha toxin of Clostridium perfringens), stop the manufacture of protein in host cells or degrade the proteins (eg., exotoxin A of Pseudomonas aeruginosa or the protein degrading toxins of Clostridium botulinum), or stimulate an immune response in the host that is so strong as to damage the host (et., three different toxins of Staphylococcus aureus resulting in toxic shock syndrome) (Lerner and Lerner 2004).

Bacterial toxins are classified as either exotoxins or endotoxins. An exotoxin is a soluble protein excreted by a microorganism, including bacteria, fungi, algae, and protozoa. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. Endotoxins are potentially toxic natural compounds found inside pathogens such as bacteria. Classically, an endotoxin is a toxin that, unlike an exotoxin, is not secreted in soluble form, but is a structural component in bacteria that is released mainly when bacteria are lysed. Exotoxins also may be released if the cell is lysed.

Exotoxins are susceptible to antibodies produced by the immune system, but many exotoxins are so toxic that they may be fatal to the host before the immune system has a chance to mount defenses against it (Nester, 2007).

Types of exotoxins

Exotoxins can be categorized by their mode of action on target cells.

• Type I toxins: Toxins that act from the cell surface. Type I toxins bind to a receptor on the cell surface and stimulate intracellular signaling pathways. For example, "superantigens" produced by the strains of Staphylococcus aureus and Streptococcus pyogenes cause toxic shock syndrome.
• Type II toxins: Membrane damaging toxins. These toxins are designed primarily to disrupt the cellular membrane. Many type II exotoxins have hemolysin activity, which causes red blood cells to lyse in vitro.
• Type III toxins: Intracellular toxins. Intracellular toxins must be able to gain access to the cytoplasm of the target cell to exert their effects. Some bacteria deliver toxins directly from their cytoplasm to the cytoplasm of the target cell through a needle-like structure. The effector proteins injected by the type III secretion apparatus of Yersinia into target cells are one example. Another well-known group of intracellular toxins is the AB toxins. The 'B'-subunit attaches to target regions on cell membranes, the 'A'-subunit enters through the membrane and possesses enzymatic function that affects internal cellular bio-mechanisms. The structure of these toxins allows for the development of specific vaccines and treatments. Certain compounds can be attached to the B unit, which is not generally harmful, which the body learns to recognize, and which elicits an immune response. This allows the body to detect the harmful toxin if it is encountered later, and to eliminate it before it can cause harm to the host. Toxins of this type include cholera toxin, pertussis toxin, Shiga toxin and heat-labile enterotoxin from E. coli.
• Toxins that damage the extracellular matrix. These toxins allow the further spread of bacteria and consequently deeper tissue infections. Examples are hyaluronidase and collagenase.

Endotoxin examples

The prototypical examples of endotoxin are lipopolysaccharide (LPS) or lipo-oligo-saccharide (LOS) found in the outer membrane of various Gram-negative bacteria. The term LPS is often used exchangeably with endotoxin, owing to its historical discovery. In the 1800s, it became understood that bacteria could secrete toxins into their environment, which became broadly known as "exotoxin". The term endotoxin came from the discovery that portions of Gram-negative bacteria itself can cause toxicity, hence the name endotoxin. Studies of endotoxin over the next 50 years revealed that the effects of "endotoxin" was in fact due to lipopolysaccharide.

There are, however, endotoxins other than LPS:

• For example, delta endotoxin of Bacillus thuringiensis makes crystal-like inclusion bodies next to the endospore inside the bacteria. It is toxic to larvae of insects feeding on plants, but is harmless to humans (as we do not possess the enzymes and receptors necessary for its processing followed by toxicity).
• The only gram positive bacteria that produces endotoxin is Listeria monocytogenes.

LPS consist of a polysaccharide (sugar) chain and a lipid moiety, known as lipid A, which is responsible for the toxic effects. The polysaccharide chain is highly variable amongst different bacteria. Humans are able to produce antibodies to endotoxins after exposure but these are generally directed at the polysaccharide chain and do not protect against a wide variety of endotoxins.

Dinoflagellates

Dinoflagellates can produce toxic substances of danger to humans. For instance, mussels should be avoided along the west coast of the United States during the warmer months. This is usually caused by dinoflagellates creating elevated levels of toxins in the water that do not harm the mussels, but if consumed by humans can bring on illness. Usually the United States government monitors the levels of toxins throughout the year at fishing sites.

Fungi

Two species of mold—Aspergillus flavus and Aspergillus parasiticus—produce aflatoxin, which can contaminate potatoes afflicted by the mold (Lerner and Lerner 2004). This can lead to serious and even fatal illness.

Plants

Many plants produce toxins designed to protect against insects and other animal consumers, or fungi.

The roots of the tobacco plant produce a substance called nicotine, which is stored mainly in the leaves. Nicotine is a powerful poison and seems to benefit the plant by protecting it from insects, working by attacking the junctions between the insects' nerve cells (Stuart 2004). Tobacco leaves are sometimes soaked or boiled and the water sprayed on other plants as an organic insecticide. Nicotine is also a deadly poison to humans. Two to four drops (pure nicotine is an oily liquid) are a fatal dose for an adult. Smoking and chewing tobacco results in a much smaller dose; however, people have died as a result of mistaking wild tobacco for an edible herb and boiling and eating a large quantity (IPCS 2006).

Animals

Some of the more well known types of biotoxins include:

• Hemotoxins target and destroy red blood cells, and are transmitted through the bloodstream. Organisms that possess hemotoxins include:
  • Pit Vipers, such as rattlesnakes.
• Necrotoxins cause necrosis (i.e., death) in the cells they encounter and destroy all types of tissue. Necrotoxins spread through the bloodstream, but infect all tissues. In humans, skin and muscle tissues are most sensitive to necrotoxins. Organisms that possess necrotoxins include:
  • The brown recluse or "fiddle back" spider.
  • Necrotizing fasciitis (the "flesh eating" bacteria)
• Neurotoxins primarily affect the nervous systems of animals. Organisms that possess neurotoxins include:
  • The Black Widow and other widow spiders.
  • Most scorpions.
  • The box jellyfish.
  • Elapid snakes.
  • The Cone Snail.

Plant Toxins

Ricin is found in the castor bean plant.

Non-technical usage

When used non-technically, the term "toxin" is often applied to any toxic substances. Toxic substances not of biological origin are more properly termed poisons. Many non-technical and lifestyle journalists also follow this usage to refer to toxic substances in general, though some specialist journalists at publishers such as BBC and The Guardian maintain the distinction that toxins are only those produced by living organisms.

In the context of alternative medicine the term is often used nonspecifically to refer to any substance claimed to cause ill health, ranging anywhere from trace amounts of pesticides to common food items like refined sugar or additives like artificial sweeteners and MSG.[1] These claims are often made without a clear scientific basis or conclusive evidence[2], and as such have little to no acceptance in mainstream medicine.

The term is also used commonly in pop psychology to describe things that have an adverse effect on psychological health, such as a "toxic relationship," "toxic work environment" or "toxic shame."

Terminology

(Gk. τοξικόν toxikon "(poison) for use on arrows,")

In the context of biology, poisons, or atters are substances that can cause damage, illness, or death to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism. Paracelsus, the father of toxicology, once wrote: "Everything is poison, there is poison in everything. Only the dose makes a thing not a poison".

In medicine (particularly veterinary) and in zoology, a poison is often distinguished from a toxin and a venom. Toxins are poisons produced via some biological function in nature, and venoms are usually defined as biologic toxins that are injected by a bite or sting to cause their effect, while other poisons are generally defined as substances which are absorbed through epithelial linings such as the skin or gut.

Some poisons are also toxins, usually referring to naturally produced substances, such as the bacterial proteins that cause tetanus and botulism. A distinction between the two terms is not always observed, even among scientists.

Animal toxins that are delivered subcutaneously (e.g. by sting or bite) are also called venom. In normal usage, a poisonous organism is one that is harmful to consume, but a venomous organism uses poison to defend itself while still alive. A single organism can be both venomous and poisonous.

The derivative forms "toxic" and "poisonous" are synonymous.

Within chemistry and physics, a poison is a substance that obstructs or inhibits a reaction, for example by binding to a catalyst.

The phrase "poison" is often used colloquially to describe any harmful substance, particularly corrosive substances, carcinogens, mutagens, teratogens and harmful pollutants, and to exaggerate the dangers of chemicals. The legal definition of "poison" is stricter.

See also

• Bioaerosol
• Exotoxin

References

ISBN links support NWE through referral fees

• Textbook of Bacteriology
• Sofer, G.; Hagel, L. (1997). Handbook of Process Chromatography: A guide to Optimization, Scale-up, and Validation. Academic Press, 158-161. ISBN 0-12-654266-X

References

• Lerner, K. L., and B. W. Lerner. 2004. Encylcopedia of espionage, intelligence, and security. Detroit: Thomson/Gale. ISBN 0787675466.

External links

• Society of Toxicology
• The Journal of Venomous Animals and Toxins including Tropical Diseases
• Cycad toxicity
• Corrections and clarifications, The Guardian, 30 May, 2005.

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