Enzyme

Figure 17.1 (a) Normal hydrolysis of acetocholine by the enzyme acetocholinesterase; (b) reaction of organophosphate pesticides with enzyme; (c) reaction of carbamate pesticides with enzyme.

reversibly. Both have similar effects, but recovery from carbamate poisoning is faster, making this a somewhat safer material to use. Another critical difference is that organo-phosphate exposure, being irreversible, is cumulative. A series of small exposures successively depletes the useful supply of acetylcholinesterase until a level is reached that causes detectable neurological effects. In fact, experiments with chickens have demonstrated that the cumulative dose of an organophosphate pesticide that produces an effect can be less than the single dose that would produce the same effect.

Membrane Changes A common target of industrial pollutants is the plasma membrane. This is because many industrial chemicals, especially the solvents, are lipophilic. Once absorbed into the organism, they preferentially partition into the lipid bilayer, changing its physicochemical properties. These properties include its fluidity, permeability, and physicochemical interactions with embedded proteins. The effect on proteins affects their function, which includes acting as receptors for normal biochemical functions or as agents in active transport of ions and other substances across the plasma membrane. These actions are the presumed cause of narcosis, one of the major symptoms of solvent toxicity. In extreme cases, exposure to solvents can leach lipids out of the membrane, or cause the membrane to rupture, destroying the cell or organelle.

Irritants Some of the more severe modes of action lead to irritation (damage to cells of a tissue). These modes include oxidation, severe pH changes, dehydration, or precipitation or denaturing of proteins. Dehydration can be caused by concentrated salt solutions. Proteins can be precipitated or denatured not only by changes in pH or ionic strength, but also by organics such as aldehydes or ketones. Examples of strong oxidizers are ozone and chlorine.

Free Radicals Oxidizers, ionizing radiation, and metabolic transformations of some organic pollutants, can result in the formation of free radicals. Free radicals are compounds with unpaired electrons. They are very reactive and electrophilic and therefore tend to oxidize compounds. However, the reactions are nonspecific; that is, the radicals tend to react with almost any compound with which they come into contact. Some of the simpler radicals are the superoxide, peroxide, or hydroxyl free radicals:

HO2 H2O2 OH

The dot symbolizes the unpaired electron. Some toxins themselves react to become free radicals, such as in the biotransformation of carbon tetrachloride:

CCl4 ) CCl3

When radicals react with cellular compounds, they may either oxidize them, or they may transform them into radicals, such as lipid or DNA free radicals, which undergo further reactions. Several enzymes and the dietary antioxidants vitamin E, vitamin C, and glu-tathione act to protect the cell by destroying radicals.

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