John B. Schenkman

The cytochrome P450s are ubiquitous proteins found in most phyla. Several thousand different forms have already been identified. The individual species have large numbers of cytochrome P450 forms. In humans, for example, 57 different forms of cytochrome P450 have been identified in the genome. The mouse genome contains 93 different forms. The different forms of cytochrome P450 can be grouped into families, based upon amino acid sequence identity. In mammals, families 1-3 are generally considered as 'xenobiotic-metabolizing' enzymes, oxidizing the greatest number of drugs and chemicals encountered in the environment, as well as a number of endobiotics like steroids destined for excretion or used in intermediary metabolism of compounds necessary for homeostasis. Most of the families of cytochrome P450 are involved, in one way or another, in steroidogenesis, vitamin metabolism or maintenance of homeostasis in the body. In recent studies, however, it has become apparent that a number of the xenobiotic metabolizing cytochrome P450s are present in the conceptus and the developing embryo and fetus at specific times after conception (Choudhary et al., 2004a; Choudhary et al., 2003; Choudhary et al., 2005), appearing to function as development-influencing gene products (Schenkman et al., 2003).