Cytochrome P450 enzymes: ubiquitous "receptors" for drugs

Most foreign compounds bind to one or more cytochrome P450 drug-metabolizing isozymes. These heme monooxygenases are most concentrated in the endoplasmic reticulum of liver cells but are present in virtually all biological membranes and in all cells. Some radioligands for known hormone receptors have been found to label, with comparable affinities, specific P450 enzymes. A characteristic feature of P450 enzymes is their broad and overlapping drug specificities, with affinity constants ranging over several orders of magnitude. Because fatty acid derivatives and steroids are endogenous substrates for the P450 enzymes, drugs may interfere with the generation of functional cellular lipids. The functional significance of high-affinity binding of drugs to the oxygenases may, on the one hand, be minimal and reflect extraneous or trivial drug-protein interactions. On the other hand, the drug-P450 union may in other cases mediate the major pharmacological response.