Regio- and stereoselectivity of various forms of purified cytochrome P-450 in the metabolism of benzo[a]pyrene and (-)trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene as shown by product formation and binding to DNA

Highly purified cytochromes P-450LM2 and P-450LM4 and partially purified P-450LM1, P-450LM3b, and P-450LM7 from rabbit liver microsomes exhibit different catalytic activities in the metabolism of benzo[a]pyrene (BzP) and (-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene [(-)trans-7,8-diol] in a reconstituted enzyme system. The two highly purified cytochromes also exhibit differences in the activation of BzP and (-)trans-7,8-diol to intermediates that bind to DNA, as well as in the stereoselective conversion of (-)trans-7,8-diol to the highly mutagenic and carcinogenic diol-epoxides r-7,t-8-dihydroxy-t-9,10-oxy- 7,8,9,10-tetrahydrobenzo[a]pyrene (diol-epoxide I) and r-7,t-8- dihydroxy-c-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (diol-epoxide II). P-450LM2 is more active than P-450LM4 in the metabolism of BzP and in its conversion to products that bind to DNA. In contrast, P-450LM4 is more active than P-450LM2 in the metabolism of (-)trans-7,8-diol and in its conversion to products that bind to DNA. The ratio of activity (percent substrate metabolized) with BzP relative to that with (-)trans-7,8-diol is 21 for P-450LM2 and 0.3 for P-450LM4; P-450LM1, P-450LM3b, and P-450LM7 gave intermediate ratios. Marked stereoselectivity in the oxygenation of the (-)trans-7,8-diol to the highly mutagenic and putatively carcinogenic diol-epoxides I and II was observed with P-450LM4, whereas the other preparations showed less selectivity. The ratio of diol-epoxide I to diol-epoxide II ranges from 0.3 for P-450LM7 to 11 for P-450LM4. The substrate specificity and regio- and stereoselectivity of the different forms of cytochrome P-450 may regulate the balance between activation and detoxification pathways of BzP and therefore determine the susceptibility of individual tissues, strains, and species to the carcinogenic action of BzP.