Organisms are exposed to a large number of xenobiotics (compounds that are foreign to the organism but can be metabolized by it) that penetrate through the skin and respiratory system. In addition, specific groups of foreign substances are delivered to the human organism by the gastrointestinal tract. These include drugs, natural food constituents, and chemical pollutants found in food and drinking water. Organisms deal with these substances, which are usually lipophilic, by means of enzymatic biotransformation systems that defend against the toxic action of xenobiotics. Excre-

tion of the xenobiotic is the simplest means of detoxification; however, this can be accomplished only in the case of relatively polar, hydrophilic compounds able to pass through the kidneys. Therefore, a lipophilic xenobiotic must undergo some kind of biotransformation to be converted into a relatively hydrophilic substance.