Mechanism of aralkyl-DNA adduct formation from benzo[a]pyrene in vivo

Three different pathways have been proposed for the metabolic activation of the ubiquitous polycyclic aromatic hydrocarbon, benzo[a]pyrene (BP). The most widely accepted activation mechanism is based on ring oxidation to diol epoxides; the other two relatively less studied pathways involve radical cation formation and benzylic electrophilic ester formation arising from a chain of substitution reactions. The present study was undertaken to test for the existence of the latter mechanism in vivo. Female Sprague-Dawley weanling rats were injected subcutaneously with 320 mumol of BP/kg body weight, and the formation of DNA adducts was examined. 32P-Postlabeling analysis of the subcutaneous tissue DNA under newly developed chromatography conditions exhibited two sets of adduct profiles: one resulting from alkyl substitution and the other from ring oxidation. One major and two minor aralkyl-DNA adducts were detected. The relative adduct labeling (adducts/10(10) nucleotides) remained constant at around 2 during the first 5 days of treatment and then increased to 6.4 +/- 2.6 at day 7. The corresponding total values of the known ring oxidation (e.g., diol epoxide) adducts were 15-50 times higher. When animals were injected with 6-methyl-BP, 6-(hydroxymethyl)-BP, and 6-(acetoxymethyl)-BP, the known or proposed intermediates in the benzylic ester pathway, each of these and the parent compound showed chromatographically identical profiles of aralkyl-DNA adducts. Cochromatography in multiple solvents of these in vivo adducts with standards prepared by reaction of 6-(bromomethyl)-BP with individual nucleotides showed that the predominant in vivo aralkyl-DNA adduct was derived from guanine while the second major adduct was from adenine.(ABSTRACT TRUNCATED AT 250 WORDS)