SPECIFICITY OF SUBSTRATE AND INHIBITOR PROBES FOR HUMAN CYTOCHROMES-P450 1A1 AND 1A2

Kinetic and inhibitor studies using cDNA-expressed enzymes and human l iver microsomes have characterized the specificity of a range of cytoc hrome P450 (CYP) 1 A substrate and inhibitor probes towards the two is oforms comprising this subfamily. Expressed CYP1A1 and CYP1A2 both cat alyzed the O-deethylation of phenacetin, although the apparent K(m) wa s about 4-fold lower for CYP1A2 (25 vs. 108 muM). Phenacetin 0-deethyl ation exhibited biphasic kinetics in human liver microsomes, and the a pparent K(m) for the high-affinity component (9 +/- 6 muM) was consist ent with the involvement of CYP1A2 in this reaction. The prototypic CY P1A xenobiotic inhibitor and substrate probes alpha-naphthoflavone, el lipticine, 7-ethoxycoumarin and 7-ethoxyresorufin all inhibited CYP1A1 - and CYP1A2-mediated phenacetin O-deethylation as well as the high-af finity component of human liver phenacetin O-deethylase activity. Alph a-Naphthoflavone and 7-ethoxycoumarin were, however, approximately 1 0 -fold more potent as inhibitors of CYP1A2 than CYP1A1. Other putative human CYP1A xenobiotic substrates and inhibitors, including caffeine, 5- and 8-methoxypsoralen, nifedipine, paraxanthine, propranolol and th eophylline similarly inhibited CYP1A1- and 1A2-catalyzed phenacetin 0- deethylation and the high-affinity human liver phenacetin 0-deethylase . In contrast, the monoclonal antibody MAb 1-7-1, raised against 3-met hylcholanthrene-inducible rat cytochromes 450, almost abolished CYP1A1 -mediated phenacetin O-deethylation, but had no effect on human liver microsomal- or CYP1A2-catalyzed phenacetin dealkylation. Together with previous data, the results indicate that the majority of human CYP1A xenobiotic inhibitor and substrate probes are nonspecific in their rec ognition of CYP1A1 and CYP1A2, although selectivity is apparent for so me compounds.