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Update Information on Human P450s

Today much of the information on the characteristics of human P450s presented here is almost taken for granted, but the acquisition of this information was not particularly easy. By 1980, several rabbit and rat P450s had been purified from liver microsomes, but their catalytic activities were only understood in the context of some model substrates and not drugs or carcinogens. Some studies with human samples had been done, primarily with placenta and liver biopsy and autopsy samples, although in retrospect these studies were relatively descriptive (1). In 1975, Kaschnitz and Coon (2) were able to separate the P450, NADPHP450 reductase, and lipid components of human liver microsomes and show that the general features of the system resembled those of the animal liver P450s. In the mid-1980s most of the major human P450s were isolated from liver microsomes on the basis of catalytic activities toward particular drugs, e.g., 1A2 (3), 2D6 (3–6), 3A4 (7), 2C9 (isolated on basis of S-mephenytoin and 40tolbutamide methyl hydroxylation) (8,9), and 2A6 (10). These preparations were utilized directly in the characterization of some reactions, although the limited amount of material was an obstacle. Inhibitory polyclonal (and monoclonal) antibodies raised to these preparations could be added to human liver microsomes, and the fraction of inhibition of a particular catalytic activity could be used as an estimate of the fraction of the activity attributable to that particular P450 (11–13). Later these antibodies proved to be useful as probes in the cloning of cDNAs for