Abstract
(±)-trans-7,8-Dihydroxy-7,8-dihydrobenzo(a)pyrene (BP-7,8-diol), the proximate form of the carcinogen benzo(a)pyrene, is cooxidized during the oxidation of arachidonic acid to prostaglandins by prostaglandin endoperoxide synthetase (PES). This enzyme can oxidize BP-7,8-diol to the reactive intermediate (±)-7β,8α-dihydroxy-9α, 10α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene, which binds covalently to macromolecules, is mutagenic in bacterial test systems, and forms 7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BP-tetrol) isomers. We have examined the cooxidation of BP-7,8-diol in an intact cell culture system of C3H/10T½ clone 8 mouse embryo fibroblasts, in which both the mixed-function oxidase and PES systems are present. When BP-7,8-diol is incubated for 72 hr with approximately 106 confluent cells, high-performance liquid chromatography analysis of the organic extractable products reveals all four pairs of BP-tetrols, with those from (±)-7β,8α-dihydroxy-9α, 10α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene predominating. The addition of arachidonic acid (100 µm) produced a 2- to 3-fold increase in the formation of BP-tetrols from (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene, while the metabolism to BP-tetrols from (±)-7β,8α-dihydroxy-9β, 10β-epoxy-7,8,9,10 tetrahydrobenzo(a)pyrene was unchanged. The addition of the PES inhibitor indomethacin (100 µm) completely eliminated this stimulation. Cell transformation assays were carried out under the same conditions. The addition of arachidonic acid resulted in a 10-fold increase in foci formation, while indomethacin inhibited the increase in foci formation by 70%. These results suggest that cooxidation of BP-7,8-diol to reactive intermediates by PES can occur in an intact cell system if stimulated with arachidonic acid. In addition to mixed-function oxidase-dependent activation of carcinogens, the cooxidation of chemicals to reactive metabolites during prostaglandin biosynthesis may also play a role in carcinogenesis.
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