Polycyclic aromatic hydrocarbon (PAH) ortho-quinone conjugate chemistry: Kinetics of thiol addition to PAH ortho-quinones and structures of thioether adducts of naphthalene-1,2-dione

Abstract

Polycyclic aromatic hydrocarbon (PAH) o-quinones are products of an NADP + dependent oxidation of non-K-region trans-dihydrodiols catalyzed by dihydrodiol dehydrogenase (EC 1.3.1.20). Since these PAH o-quinones could be detoxified by non-enzymatic or enzymatic conjugation with cellular thiols, their reactivity with 2-mercaptoethanol, cysteine and glutathione (GSH) was examined by ion-pair reverse phase high pressure liquid chromatography (RP-HPLC). Second-order rate constants for the addition of these thiols to naphthalene-1,2-dione (NPQ) in water ranging from 4.9 × 10 3−1.1 × 10 4 min −1 M −1 and the reactions were complete within 10 min. When these reactions were conducted at near physiological pH (50 mM potassium phosphate buffer pH 7.0), the rate constants increased by 2-orders of magnitude. When benzo[ a]pyrene-7,8-dione (BPQ) was substituted in these reactions the second-order rate constants decreased by 2–3 orders of magnitude and the reactions took several hours to reach completion. The decrease in reactivity can be explained by the presence of the bay region in BPQ. Methylation influenced the reactivity of PAH o-quinones with GSH and the following order of reactivity was observed: 7,12-dimethyl-benz[ a]anthracene-3,4-dione (7,12-DMBAQ) ⪢ 12-methyl-BAQ, 7-methyl-BAQ and BAQ ⪢ BPQ. Of these quinones 7,12-dimethyl-BAQ was almost equi-reactive with NPQ. This suggests that methyl substitution in the bay and peri regions enhances reactivity with GSH. Using NPQ as a model for other PAH o-quinones, N-acetyl- l-cysteine, l-cysteine and GSH conjugates of NPQ were synthesized and characterized by [ 1H]- and [ 13C]NMR. Evidence for Michael type 1,4-addition products was obtained in which the resultant adduct could exist as either a catechol or o-quinone. By contrast, l-cysteine was able to form adducts via S- or N-attack and N-attack gave a purple p-iminoquinone. There was no evidence for the formation of bis- N-acetyl- l-cysteinyl-, bis-glutathionyl adducts or phenolic coupled products. The toxicity of thiol conjugates of NPQ remains to be explored.