One- and two-electron reduction of 2-methyl-1,4-naphthoquinone bioreductive alkylating agents: kinetic studies, free-radical production, thiol oxidation and DNA-strand-break formation

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Abstract
The one- and two-electron enzymic reduction of the bioreductive alkylating agents 2-methylmethoxynaphthoquinone (quinone I) and 2-chloromethylnaphthoquinone (quinone II) was studied with purified NADPH-cytochrome P-450 reductase and DT-diaphorase respectively, and characterized in terms of kinetic constants, oxyradical production, thiol oxidation and DNA-strand-break formation. The catalytic-centre activity values indicated that DT-diaphorase catalysed the reduction of quinone I far more efficiently than NADPH-cytochrome P-450 reductase, although the Km values of the two enzymes for this quinone were similar (1.2-3.0 microM). The one-electron-transfer flavoenzyme also catalysed the reduction of quinone II, but the behaviour of DT-diaphorase towards this quinone did not permit calculation of kinetic constants. A salient feature of the redox transitions caused by the one- and two-electron catalysis of these quinones was the different contributions of disproportionation and autoxidation reactions respectively. In the former case, about 26% of NADPH consumed was accounted for in terms of autoxidation (as H2O2 formation), whereas in the latter, the autoxidation component accounted for most (98%) of the NADPH consumed. This difference was abrogated by superoxide dismutase, which enhanced autoxidation during NADPH-cytochrome P-450 catalysis to a maximal value. E.s.r. analysis indicated the formation of superoxide radicals, the signal of which was suppressed by superoxide dismutase and unaffected by catalase. The one- and two-electron reduction of these quinones in the presence of GSH was accompanied by formation of thiyl radicals. Although superoxide dismutase suppressed the thiol radical e.s.r. signal in both instances, the enzyme enhanced GSSG accumulation during NADPH-cytochrome P-450 catalysis of quinone I, whereas it inhibited GSSG formation during reduction of the quinone by DT-diaphorase. One- and two-electron reduction of quinone I led to calf thymus DNA-strand-break formation, a process that (a) was substantially decreased in experiments performed with dialysed DNA and in the presence of desferal and (b) was partially sensitive to superoxide dismutase and/or catalase. These findings are rationalized in terms of the occurrence of metal ions ligated to DNA, protecting against the toxic effects of superoxide radicals generated during enzymic reduction of quinones.

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