Oxidation of aflatoxin B1 by bacterial recombinant human cytochrome P450 enzymes

Human cytochromes P450 (P450) 1A2 and P450 3A4 were expressed in Escherichia coli, purified, and used in reconstituted oxidation systems. The optimal system for P450 3A4 included a mixture of phospholipids, sodium cholate, cytochrome b5, GSH, and MgCl2. Relatively high catalytic activities were obtained with such a system for aflatoxin (AF) B1 3 alpha-hydroxylation and 8,9-epoxidation. P450 3A4 was more active than P450 1A2 in forming genotoxic AFB1 oxidation products. Analysis of the AFB1 products indicated that P450 3A4 formed AFQ1 and the exo-8,9-epoxide; P450 1A2 formed AFM1, a small amount of AFQ1, and both the exo- and endo-8,9-epoxides. The endo epoxide is essentially nongenotoxic in the umu test, as found previously in bacterial mutagenicity assays [Iyer, R. S., Coles, B. F., Raney, K. D., Thier, R., Guengerich, F. P., and Harris, T. M. (1994) J. Am. Chem. Soc. 116, 1603-1609]. 7,8-Benzoflavone (alpha-naphthoflavone, alpha NF) stimulated AFB1 (exo) 8,9-epoxidation and inhibited 3 alpha-hydroxylation in human liver microsomes and a reconstituted P450 3A4 system but was a potent inhibitor of all reactions catalyzed by P450 1A2. Plots of AFB1 3 alpha-hydroxylation and 8,9-epoxidation vs AFB1 concentration were sigmoidal in both human liver microsomes and the reconstituted P450 3A4 system. The results are consistent with the view that P450 3A4 is a major human liver P450 enzyme involved in AFB1 activation, although the in vivo situation may be more complex due to the presence of the enzyme in the gastrointestinal tract.