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Fatty Acid-dependent Benzo(a)pyrene Oxidation in Colonic Mucosal Microsomes: Evidence for a Distinct Metabolic Pathway¹

Department of Medicine, Veterans Administration Medical Center, and University of Pittsburgh, Pittsburgh, Pennsylvania 15240

The present study examines and compares the ability of reduced nicotinamide adenine dinucleotide phosphate or fatty acids to increase the conversion of [³H]benzo(a)pyrene (BP) to protein-bound metabolites. The results demonstrate the presence of a pathway for fatty acid-dependent BP oxidation in colonic microsomes, which is distinct from reduced nicotinamide adenine dinucleotide phosphate-dependent cytochrome P-450 activity. The unsaturated fatty acids (arachidonate or linoleate, but not palmitate) increased [³H]BP metabolism approximately 2-fold. The product of fatty acid-induced increases in [³H]BP metabolism was tightly bound to protein and could not be extracted, indicating the generation of reactive BP metabolites during cooxidation. In contrast to fatty acids, no increase in protein-bound metabolites was observed with reduced nicotinamide adenine dinucleotide phosphate compared to that in the heated microsome control. Similar concentration response relationships were observed between arachidonate-induced increases in immunoreactive prostaglandin E (IPGE) synthesis and conversion of [³H]BP to protein-bound metabolites. Half-maximal effects of arachidonate on both parameters were observed at 10 µM with optimal effects expressed at 25 µM arachidonate. The prostaglandin endoperoxide synthetase inhibitor, indomethacin, suppressed basal and arachidonate-stimulated [³H]BP metabolism (35 to 65%) and inhibited microsomal IPGE synthesis (90 to 95%). In contrast, 2-diethylaminoethyl-2,2-diphenylvalerate (2 mM) or 7,8-benzoflavone (50 µM), cytochrome P-450 inhibitors, did not influence basal or fatty acid-stimulated [³H]BP metabolism. The ability of linoleic acid, which did not increase microsomal IPGE accumulation, to increase [³H]BP metabolism and the failure of indomethacin to completely suppress arachidonate-induced increases in [³H]BP metabolism suggest a pathway for fatty acid-induced [³H]BP metabolism independent of prostaglandin synthesis. A role for fatty acid hydroperoxides in colonic [³H]BP metabolism was supported by the ability of 5,8,11,14-eicosatetraynoic acid, an inhibitor of fatty acid oxidation, to suppress both arachidonate- and linoleate-induced increases in [³H]BP metabolism. These results support the existence of a distinct fatty acid-responsive pathway for [³H]BP metabolism in colonic microsomes. The relative importance of this pathway and the cytochrome P-450 pathway for local colonic metabolism of this and other procarcinogens requires further study.

¹ This work was supported by USPHS Grant CA 31680 from the National Cancer Institute through the Large Bowel Cancer Project.

² To whom requests for reprints should be addressed, at the Veterans Administration Medical Center, University Drive C, Pittsburgh, Pa. 15240.

Received 4/ 5/82. Accepted 10/ 6/82.