Reaction of hematin with allylic fatty acid hydroperoxides: identification of products and implications for pathways of hydroperoxide-dependent epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene.

Reaction of 10-hydroperoxyoctadec-8-enoic acid (10-OOH-18:1) (50 microM) with hematin (0.5 microM) in sodium phosphate buffer containing Tween 20 (200 microM) generates 10-oxooctadec-8-enoic acid, 10-oxodec-8-enoic acid (10-oxo-10:1), and 10-hydroxyoctadec-8-enoic acid in relative yields of 79, 4, and 17%, respectively. The product profile and relative distribution are unaffected by 1 mM butylated hydroxyanisole. Approximately 5% of the hydroperoxide isomerizes from the 10- to the 8-position. 10-Oxo-10:1 most likely arises via beta-scission of an intermediate alkoxyl radical to the aldehyde and the n-octyl radical. To test this, 10-hydroperoxyoctadeca-8,12-dienoic acid was reacted with hematin under identical conditions. 10-Oxooctadeca-8,12-dienoic acid, 10-oxodec-8-enoic acid, and 10-hydroxyoctadeca-8,12-dienoic acid are formed in relative yields of 50, 45, and 5%, respectively. The product ratios are constant with time and hydroperoxide to catalyst ratio and unaffected by inclusion of phenolic antioxidants. The higher yield of 10-oxo-10:1 from 10-OOH-18:2 compared to 10-OOH-18:1 is due to the higher rate of beta-scission of the intermediate alkoxyl radical from the former to the resonance-stabilized octenyl radical. Two products of reaction of the 2-octenyl radical with O2, octenal and octenol, were detected in 10% yield relative to 10-oxo-10:1. Inclusion of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) led to epoxidation by both 10-OOH-18:1 and 10-OOH-18:2. Studies with isotopically labeled hydroperoxide or O2 indicated approximately 65% of the epoxide oxygen was derived from O2 and 35% from hydroperoxide oxygen, consistent with the involvement of peroxyl free radicals as the oxidizing agents. The available evidence indicates that hematin reduces the fatty acid hydroperoxides homolytically to alkoxyl radicals that are oxidized to ketones, reduced to alcohols, or undergo beta-scission to aldehydes. Carbon radicals generated during these reactions couple to O2, generating peroxyl free radicals that epoxidize BP-7,8-diol. The smaller percentage of epoxidation that results from hydroperoxide oxygen may arise from oxidation of the hydroperoxide group to peroxyl radicals or from heterolytic cleavage of the hydroperoxide to alcohol and an iron-oxo complex.