Toxins, Poisons, and Venoms

FIELDS OF STUDY: Biochemistry

ABSTRACT

The basic properties of toxins, poisons, and venoms are described, and their various methods of operation are discussed. Because of their molecular structures and modes of action, all three are inimical to the biochemistry of living things.

The Nature of Toxins, Poisons, and Venoms

Biochemical systems such as living organisms are complex chemical factories in which tens of thousands of specific chemical reactions take place. Each reaction performs a function on either a specific material or a class of materials with similar structural features. Toxins, poisons, and venoms interfere with a system’s proper functioning in ways that range from minor to fatal.

To understand how toxins, poisons, and venoms affect the chemical functions of the body, think of them as the various fluids used to keep an automobile running smoothly. Using windshield-wiper fluid in the radiator instead of a water–ethylene glycol solution would have only a minor effect on the efficiency of the cooling system, analogous to the automobile "organism" having a mild fever. On the other hand, using wiper fluid in place of lubricating oil would quickly result in the destruction of the engine, as the water would eat the metal and allow the engine’s moving parts to grind against each other. Such "tissue damage" would quickly lead to the "death" of the automobile.

Toxins, poisons, and venoms are chemical compounds that have the same effects on living organisms as improper fluid replacements have on automobiles. They often come from other living organisms that produce these substances as part of their normal biochemical processes, either as metabolic waste products or as defensive or offensive mechanisms. The terms "toxin," "poison," and "venom" are often used interchangeably, but in fact they have distinct meanings. All three are indeed toxic to the targeted organism, and all may also be poisonous, but the source of the material determines the proper terminology.

Toxins

Toxins are bacterial waste products that act as poisons within the host organism. They may consist of normal biochemical materials that have been modified by metabolic processes, or they may be various proteins or peptides that interfere with the normal biochemical processes of the host. The human digestive tract, like the digestive tracts of all animals, is home to billions of bacterial flora that secrete compounds such as proteins and enzymes to break down food, making them essential to their host’s health and normal digestive functions. When abnormal bacteria invade the host, they generally prove toxic to the endemic bacterial colonies—and ultimately to the host. In some cases, the endemic bacteria may be almost completely eradicated, leaving the host in a poor state of health until the invading bacteria can be eliminated and the proper strains regained. In other cases, recovery is not possible, as the toxins produced by the invading bacteria and viruses prove too damaging to the surrounding tissues.

The most damaging toxins are those against which the immune system of the host organism has little or no resistance. Bacteria such E. coli and the dinoflagellate algae responsible for "red tides" in the oceans produce toxins that affect the functioning of the nervous system of the host, and they may prove fatal if sufficiently severe. Viruses such as Ebola cause the host organism’s own biochemical processes to produce compounds that actively destroy vital tissues, usually resulting in death.

Poisons

Poisons are chemical compounds and may be of animal, vegetable, or mineral origin. Given the precise nature of the biochemical systems within an organism, every chemical compound in existence can act as a poison at some point, whether by simply overloading normal biochemical pathways or by completely disrupting an organism’s biochemical processes.

Human biochemistry includes and is dependent on the presence of many different elements of the periodic table. The vast majority of a living organism is organic in nature, meaning it is composed of principally carbon-based materials. Much smaller quantities of other elements, such as iron and selenium, serve as central atoms for various essential materials, including hemoglobin, cofactors in the active sites of enzymes, and electrolytes in cellular fluids. If a biochemical system has too little of these cofactors and micronutrients, it will not function properly; however, too much will effectively poison the system, interfering with the properly balanced functioning of the different biochemical pathways. Many biochemical cycles function in equilibrium with one another. When too much of one component is present, other components maintaining the equilibrium can become saturated, and the system cannot function as it should.

Certain mineral components are not native to human physiology, and their presence in the human body usurps the place of the chemical species that are supposed to be present, preventing the correct compounds and ions from performing their proper systemic functions. Hemoglobin, for example, transports oxygen in the red blood cells by reversibly coordinating the O₂ molecule. The presence of carbon monoxide (CO) or the cyanide ion (CN⁻) usurps this function by binding irreversibly with the iron atom in hemoglobin, preventing it from transporting oxygen. Heavy metals such as lead, mercury, cadmium, and manganese poison the system in a similar way, though by different mechanisms. Lead and mercury poison the neurological system of an organism by interfering with the function of the neurotransmitter compounds that normally pass electrical signals from cell to cell, and they also are able to link to the protein molecules of cellular receptors and prevent cellular communication.

Poisons of animal origin are typically compounds secreted by various animals as defense mechanisms. In some cases, the poisons derive from the foods the animals consume. For example, caterpillars of the monarch butterfly feed exclusively on the milkweed plant, causing them carry a load of alkaloids that will poison and perhaps kill any birds that consumes them. Various toads are known to secrete compounds that are hallucinogenic when ingested, which might discourage other animals from eating them. Certain fish are known to secrete the compounds tetrodotoxin and saxitoxin, which interfere with the function of sodium-ion channels in membranes, effectively shutting down nerve function in a way that often results in death.

The largest group of poisons is organic compounds produced by plants, and the many pharmaceutical compounds designed to imitate them. These range from compounds as simple as oxalic acid (the bitter component of rhubarb) and nicotine (derived from tobacco and other alkaloid-producing plants) to complex molecules such as the aflatoxins (typically from molds found on peanuts) and ketophalloidin (from the death cap mushroom, Amanita phalloides). Alkaloids are a class of nitrogen-containing compounds that includes lysergic acid diethylamide (LSD), cocaine, caffeine, and theobromine (found in chocolate). Hundreds of thousands of such "natural product" compounds have been identified. The fact that essentially all natural compounds have physiological effects has led researchers to experiment with changing the molecular structures of such compounds in order to make them more potent and effective for pharmaceutical purposes. Thus, every pharmaceutical compound is a poison used to alleviate certain medical conditions, either by interfering with normal functions or by selectively poisoning an invading organism; painkillers are an example of the former, antibiotics of the latter. Every pharmaceutical compound is described with an LD₅₀ dose, which is the dosage that would be lethal for 50 percent of individuals of a certain body weight given a predetermined amount of the compound. Simply stated, the higher the LD₅₀ dose of a compound, the safer it is to use.

Venoms

Venoms are unique compounds produced specifically by animals for the purpose of killing or severely incapacitating other animals. Snake venoms are the most familiar, but many other biting animals also produce venoms. Venoms are specialized proteins that work against the central nervous system or other tissues. In the worst cases, the venom kills tissue, requiring its immediate removal to save the life of the bite victim. In other cases, the venom works slowly to incapacitate the bite victim for a long period of time. Even a seemingly innocuous envenomating bite or sting, as from a small bee, can have serious consequences if the bite victim is allergic to the venom. Because venoms are small, specialized proteins, they have complex and poorly understood modes of action.

Uses of Toxins, Poisons, and Venoms

The various properties and effects of toxins, poisons, and venoms can be applied to specific medical interventions. Botox, a preparation made from the botulinum toxin, is routinely used for cosmetic effects, but it is also effective in treating facial tics and other conditions of an impaired central nervous system. Toxins and venoms are used to prepare antitoxins and antivenins for front-line treatment of snake and other bites. Unfortunately, antivenins are generally creature specific, so to be of use, the antivenin specific to the venom in question must be available. Poisons of all kinds are studied intensively for pharmaceutical use. Though pharmaceuticals are not referred to as poisons, they all have poisonous thresholds for use.

PRINCIPAL TERMS

• cellular receptor: a protein within a cell that specific signaling molecules bind to in order to elicit a specific response from the cell.
• enzyme: a protein molecule that acts as a catalyst in biochemical reactions.
• neurotransmitter: a chemical species that carries electrochemical signals across the synapses between neurons in the nervous system.
• peptide: an organic compound composed of a relatively small number of amino acid molecules, held together by covalent bonds between a carbon atom of one molecule and a nitrogen atom of the next.
• protein: a biological polymer consisting of one or more long chains of amino acids linked by peptide bonds in a sequence specified by an organism’s DNA.

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