Fluorescence study of DNA-binding metabolites of benzo(a)pyrene formed in hepatocytes isolated from...

Hepatocytes and liver microsomes isolated from 3-methylcholanthrene-treated rats metabolize benzo(a)pyrene to products that bind to endogenous DNA and exogenously added calf thymus DNA, respectively. By using a sensitive fluorescence technique, it has been possible to characterize the major DNA-binding metabolite in hepatocytes as being produced by further metabolism of 9-hydroxybenzo(a)pyrene. In microsomes, two products binding to calf thymus DNA were recovered, a major species formed by activation of 9-hydroxybenzo(a)pyrene and a minor fraction formed by further metabolism of 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene. The available evidence indicates that the ultimate products responsible for binding to DNA were identical to 9-hydroxybenzo(a)pyrene 4,5-oxide and 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide, respectively. Our data further suggest that metabolic activation of 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene and 9-hydroxybenzo(a)pyrene results in quite different DNA:metabolite complexes. The former product(s) seems to be strongly associated with hydrophobic regions in DNA, whereas the latter metabolite(s) appears to be more exposed to the exterior.