ENZYMATIC-MEDIATED AND THIOL-MEDIATED ACTIVATION OF HALOGEN-SUBSTITUTED DIAZIRIDINYLBENZOQUIONES - REDOX TRANSITIONS OF THE SEMIQUINONE ANDSEMIQUINONE-THIOETHER SPECIES

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 reduct ase and glutathione nucleophilic substitution was examined in terms of free radical production and DNA strand scission. A semiquinone specie s was observed by direct ESR in aerobic conditions during: (a) NADPH-c ytochrome P-450 reductase-catalyzed reduction of the above quinones. ( b) The interaction of these quinones with GSH entailing primarily reac tivity of halogen substituents toward sulfur substitution. (c) NADPH-c ytochrome P-450 reductase-catalyzed activation of products resulting f rom the quinone/GSH interaction. The semiquinone ESR signal observed d uring enzymic catalysis was suppressed by superoxide dismutase and was not affected by catalase. ESR studies in conjunction with the spin tr apping technique on the autoxidation of the semiquinones formed by the above reaction pathways indicated the formation of superoxide radical s. In addition, thiyl radicals were formed during the reactions follow ing glutathione nucleophilic substitution of the above quinones. The E SR signals of both superoxide and thiyl radicals were abolished by sup eroxide dismutase. No hydroxyl radicals were formed in solution during the redox transitions of these halogen-containing diaziridinylbenzoqu inones. Bioreductive activation of these compounds via NADPH-cytochrom e P-450 reductase or sulfur nucleophilic substitution was associated w ith 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 i n a manner entailing a semiquinone-dependent reduction of metal-ligand s bound at the DNA surface and leading to site-specific, hydroxyl radi cal production.