Multiple forms of cytochrome P450 in the microsomal monooxygenase system

The microsomal monooxygenase system is characterized by its broad substrate specificity which includes endogenous substrates as well as lipophilic drugs and chemicals. From in vitro investigations it was known that the relative reactivities and the pattern of products varied greatly with species, sex, age, diet or pretreatment with drugs of the animal. The suggestion that this was possibly due to a variety of cytochrome P450 enzymes rather than a single monooxygenase was recently confirmed by the isolation of several cytochrome P450 species with different although overlapping substrate specificities. In view of the consequences of a genetic and environment-dependent pattern on monooxygenases for drug metabolism and drug-mediated toxicity the methods of a quantitative assessment of the various forms are discussed.