Human Cytochrome P450 2E1: Functional Comparison to Cytochrome 2A13 and 2A6

Abstract

The cytochrome P450 (CYP) superfamily of enzymes plays the predominant role in human phase I xenobiotic metabolism. The CYP2 family, in particular, is known for it extensive Phase I metabolism of a majority of the xenobiotic compounds [1]. The goal of this project is to determine the structural foundation for the substrate selectivities of the CYP2A6 and CYP2A13 enzymes versus CYP2E1. Because these enzymes metabolize both common, as well as unique, small molecule substrates, it is likely that only few key residue-substrate interactions are responsible for those metabolic capabilities that differ between them. Amino acid residues in regions of the CYP2E1 protein likely to contact ligands and that differ between CYP2E1 and the CYP2A enzymes were examined by site-directed mutagenesis. The resulting mutated CYP2E1 proteins were characterized for their ability to hydroxylate the reportedly selective CYP2E1 substrates p-nitrophenol (pNP) [2] and chlorzoxazone (CZN) [3], but none showed significant differences in activity from the CYP2E1 wild type enzyme. However, in contrast to previous literature reports [4], both CYP2A6 and CYP2A13 were observed to metabolize both CYP2E1 substrates pNP and CZN with catalytic efficiencies equal to or greater than CYP2E1. These unexpected activities of the CYP2A enzymes with CYP2E1 substrates demonstrate that the human CYP2A and CYP2E enzymes are more functionally similar than previously believed. References 1 Rendic, S. and Di Carlo, F. J. (1997) Human cytochrome P450 enzymes: a status report summarizing their reactions, substrates, inducers, and inhibitors. Drug Metab Rev 29, 413-580 2 Koop, D. R., Laethem, C. L. and Tierney, D. J. (1989) The utility of p-nitrophenol hydroxylation in P450IIE1 analysis. Drug Metab Rev 20, 541-551 3 Peter, R., Bocker, R., Beaune, P. H., Iwasaki, M., Guengerich, F. P. and Yang, C. S. (1990) Hydroxylation of chlorzoxazone as a specific probe for human liver cytochrome P-450IIE1. Chem Res Toxicol 3, 566-573 4 Zerilli, A., Ratanasavanh, D., Lucas, D., Goasduff, T., Dreano, Y., Menard, C., Picart, D. and Berthou, F. (1997) Both cytochromes P450 2E1 and 3A are involved in the O-hydroxylation of p-nitrophenol, a catalytic activity known to be specific for P450 2E1. Chem Res Toxicol 10, 1205-1212