WHAT MAKES P450s WORK? SEARCHES FOR ANSWERS WITH KNOWN AND NEW P450s*

Abstract

Random mutagenesis has been developed as an approach for the study of human cytochrome P450 (P450) enzymes and their structure and function. Sensitive screening methods are critical for the success of this approach. We have developed one system that takes advantage of the ability of human P450 1A2 to activate heterocyclic amines to mutagenic products [A. Parikh, P. D. Josephy, and F. P. Guengerich, Biochemistry, 38, 5283–5289 (1999)]. Mutants with both attenuated and enhanced activity have been recovered and subjected to further kinetic analysis. For phenacetin O-deethylation, the E225I mutant had k_cat 6× > wild type; D320A had k_cat 1/10× < wild type (and K_m 15× > wild type). With all three P450s, the rate of first electron reduction was similar, and all had similar binding constants for phenacetin (∼15μM). All three forms yielded intermolecular, noncompetitive kinetic deuterium isotope effects of 1.5–2 [^DV and ^D(V/K)] for O-deethylation of [OCD₂CH₃]-phenacetin. All three forms of P450 1A2 also formed a minor product, the acetol (C-hydroxylation of the acetyl group). This reaction had a deuterium isotope effect of ∼14 with all three forms of the enzyme, and C—H bond breaking is the rate-determining step. Another approach to P450 2A6 involves the recent observation that this P450 can accumulate indigo [E. M. J. Gillam, A. M. A. Aguinaldo, L. M. Notley, D. Kim, R. G. Mundkowski, A. A. Volkov, F. H. Arnold, P. Soucek, J. T. DeVoss, and F. P. Guengerich, Biochem. Biophys. Res. Commun 265, 469–472 (1999)]. Current results indicate that this process involves the conversion of endogenous indole to indoxyl by the P450. The reaction may be used in assays of random mutants and has some potential applications in industry.

Key Words :

• Random mutagensis
• Cytochrome P450
• Indole
• Indigo
• Heterocyclic amines
• Phenacetin