Selective interactions of cytochromes P-450 with the hydroxymethyl derivatives of 7, 12-dimethylbenz[a]anthracene

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Abstract
Competition between a hydroxylated metabolite and the parent polycyclic aromatic hydrocarbon (PAH) for metabolism at cytochromes P-450 may result in the generation of hydroxylated dihydrodiol epoxides. The effectiveness of the competition between 7-hydroxymethyl-12-methylbenz[a]-anthracene (7HOMMBA) or 12-hydroxymethyl-7-methylbenz[a]anthracene (12HOMMBA) and 7,12-dimethylbenz[a]anthracene (DMBA) is highly dependent on the form(s) of cytochrome P-450 in the microsomes. The inhibitory effects of exogenously added 7HOMMBA or 12HOMMBA on DMBA metabolism were 30- to 50-fold greater in 3-methylcholanthrene (MC)-induced rat liver microsomes (Ki = 0.4 .mu.M) compared to either uninduced or phenobarbital (PB)-induced liver microsomes (Ki = 14 and 11 .mu.M, respectively). Similarly, product inhibition of total DMBA metabolism by metabolites generated in situ was significant only in MC-induced liver microsomes (Ki'' = 2.5 .mu.M). Metabolism of 7HOMMBA in these microsomes was strongly restricted by an unusual substrate inhibition derived from the inhibitory binding of a second molecule of 7HOMMBA. This same phenomenon was observed with reconstituted cytochrome P-450c but not with PB-induced or uninduced microsomes. Complex formation by binding of DMBA, 7HOMMBA, and 12HOMMBA to purified P-450c reconstituted in phospholipid micelles was determined by optical spectroscopy and fluorescence quenching. Binding affinities of both the 7HOMMBA and 12HOMMBA (Kd = 95 and 110 nM, respectively), were 2.5-fold higher compared to that of DMBA (Kd = 265 nM). These results provide a first demonstration that hydroxylation of a PAH can lead to preferential metabolism through an increased affinity for cytochrome P-450.