Using a stereochemical probe as described by Marnett (Carcinogenesis (Lond.), 8: 1365–1373, 1987), we have investigated the mechanism of oxidation of (+)-[3H]BaP-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene {(+)-[3H]BaP-7,8-diol} in mouse epidermis in vivo. Groups of mice were topically treated with (+)-[3H]BaP-7,8-diol (60 nmol/mouse) and sacrificed at intervals from ½ to 8 h post treatment. (-)-Anti- and (+)-syn-7,8-[3H]dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BPDE) were formed as metabolites in a ratio of about 4 to 1, respectively, as determined by HPLC analysis of the hydrolysis products. Pretreatment of mice with indomethacin, an inhibitor of prostaglandin H synthase, did not alter the ratio of anti- to syn-BPDE-derived hydrolysis products. Pretreatment of mice with the cytochrome P-450 inducer, β-naphthoflavone, yielded twice the level of syn-[3H]BPDE in mouse skin at the ½-h survival point. However, this enhancing effect diminished over time. Coadministration of 1,2-dihydroxybenzene (catechol) with (+)-[3H]BaP-7,8-diol decreased the formation of (-)-anti-[3H]BPDE and also decreased lipid peroxidation, as measured by the extent of formation of thiobarbituric acid-reactive material in mouse epidermis.

Analysis of mouse epidermal DNA adducts 24 h after topical application of (+)-[3H]BaP-7,8-diol indicated that the major adduct is not formed from the major metabolite (-)-anti-BPDE. Acid hydrolysis of the major adduct resulted in the formation of a small amount of r-7,c-9,c-10,t-8-tetrahydroxy-7,8,9,10-tetrahydrobenzo(a)pyrene and two unidentified products different from 7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydro-BaP. Coadministered catechol suppressed the formation of this adduct by 30%. The present observation suggests that a peroxyl radical-mediated epoxidation pathway is involved in the oxidation of (+)-[3H]BaP-7,8-diol in mouse skin in vivo.

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Supported by Grant CA 43910 from the National Cancer Institute. This is Paper 128 of the series, “A Study of Chemical Carcinogenesis.”

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