Environmental chemicals are both activated and detoxified by phase I and phase II enzymes. The principal enzymes involved in phase I reactions are the cytochrome P-450s. The phase II enzymes include hydrolase and the conjugative enzymes such as glucuronyltransferases, glutathione transferases, N-acetyltransferase, and sulfotransferase. Although other phase I and phase II enzymes exist, the present review is limited to these enzymes. Once thought to be a single enzyme, multiple cytochrome P-450 enzymes have been purified and characterized from many different species across the evolutionary tree. The application of molecular biology techniques to this field has identified more than 150 cytochrome P-450 genes to date. At least 20-30 cytochrome P-450 enzymes appear to exist in each mammalian species, and many polymorphisms in these enzymes are being identified. The cytochrome P-450 enzymes can now be expressed in recombinant form using cDNA expression systems. The phase II conjugative enzymes add a hydrophilic moiety such as sulfate, glucuronide, or acetate to compounds, which increases their water solubility and facilitates their excretion. However, conjugates of a number of compounds also result in more reactive electrophilic species, which appear to be the ultimate carcinogens. Many of these phase II enzymes also represent families of enzymes, and polymorphisms can affect the ability of these enzymes to metabolize chemicals. Whenever possible, we have reviewed knowledge of the human enzymes involved in particular pathways.
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