Human cytochromes P450: problems and prospects.

Cytochromes P450 are a superfamily of haem-containing monooxygenases. In mammals, two general classes of P450s exist: six families involved in steroid and bile acid biosynthetic pathways of metabolism; four families containing numerous individual P450s, mainly responsible for metabolism of foreign compounds. Many of the latter P450s, particularly those in the CYP2 family, exhibit a large degree of inter- and intra-species variability in regulation and catalytic activities. From a practical standpoint, these variabilities suggest the need for careful characterization of P450 catalytic activities and determination of P450 expression levels in humans. Human P450-based in vitro systems are being developed to evaluate drug and carcinogen metabolism.