KINETICS OF FORMATION OF (±)-Anti-7, 8-Dihydroxy-9α,10α -EPOXY-7, 8, 9, 10-Tetrahydrobenzo[a]pyrene ADDUCTS WITH MOUSE AND HUMAN HEMOGLOBIN

Abstract

Previous studies on the metabolic activation of polycyclic aromatic hydrocarbons have shown conclusively that activation to reactive electrophilic intermediates occurs via metabolism of the unsubstituted aromatic hydrocarbon to electrophilic diol-epoxide intermediates. These reactive electrophilic derivatives undergo reaction with various biological nucleophilic centers to form covalent modifications, which can be assessed by a variety of analytical techniques. While the formation of these adducts with various cellular nucleophiles has been clearly established, the relationships between relative rates of formation of various adducts in biological systems has not been significantly investigated. In the present study, we investigated the pharmacokinetics of reaction of the ultimate carcinogenic metabolite of benzo(a)pyrene, (±)-anti-7,8-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE), with both male C57BL/6 mouse red cell hemoglobin and human red cells, in vitro. Comparative pharmacokinetic studies using both mouse and human hemoglobin were carried out by 143 Downloaded by [York University Libraries] at 23:40 02 January 2015 144 S. R. Myers et al.incubation of the packed red cells together with solutions of (±)-anti-7, 8-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene. Similar rate constants for the binding of the epoxide to hemoglobin in both mouse and human were found, suggesting a similarity of amino acids that were adducted in both species. Overall binding of (±) -anti-7,8-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene to mouse hemoglobin was found to be slightly higher than that found using human hemoglobin, suggesting the potential of a facilitation of the binding of the epoxide with mouse protein or an additional reactive amino acid to which the epoxide was bound. In parallel studies, mice were injected intraperitoneally with benzo(a)pyrene to compare the kinetics of binding of this carcinogen to hemoglobin, in vivo. Blood samples were obtained at specific intervals, and the kinetics of binding to hemoglobin characterized. These results suggested that the binding to hemoglobin in the in vivo studies was slightly lower than that compared to the in vitro assessments. However the differences in adduct levels strongly suggests that other metabolic pathways are involved in the metabolic activation of benzo(a)pyrene as well as other related carcinogenic compounds.