Enzymic- and thiol-mediated activation of halogen-substituted diaziridinylbenzoquinones: Redox transitions of the semiquinone and semiquinone-thioether species

Abstract

Activation of 2,5-diaziridinyl-1,4-benzoquinones bearing halogen (Cl, Br, or F) substituents at C 3 and C 6 by NADPH-cytochrome P 450 reductase and glutathione nucleophilic substitution was examined in terms of free radical production and DNA strand scission. A semiquinone species was observed by direct ESR in aerobic conditions during: (a) NADPH-cytochrome P 450 reductase-catalyzed reduction of the above quinones. (b) The interaction of these quinones with GSH entailing primarily reactivity of halogen substituents toward sulfur substitution. (c) NADPH-cytochrome P 450 reductase-catalyzed activation of products resulting from the quinone/GSH interaction. The semiquinone ESR signal observed during enzymic catalysis was suppressed by superoxide dismutase and was not affected by catalase. ESR studies in conjunction with the spin trapping technique on the autoxidation of the semiquinones formed by the above reaction pathways indicated the formation of superoxide radicals. In addition, thiyl radicals were formed during the reactions following glutathione nucleophilic substitution of the above quinones. The ESR signals of both superoxide and thiyl radicals were abolished by superoxide dismutase. No hydroxyl radicals were formed in solution during the redox transitions of these halogen-containing diaziridinylbenzoquinones. Bioreductive activation of these compounds via NADPH-cytochrome P 450 reductase or sulfur nucleophilic substitution was associated with the formation of DNA strand breaks. This process was substantially inhibited (74–86%) by superoxide dismutase and to a lesser extent (23–31%) by catalase. It is suggested that DNA strand breakage proceeds in a manner entailing a semiquinone-dependent reduction of metal-ligands bound at the DNA surface and leading to site-specific, hydroxyl radical production.