Oxidation and nitrosation in the nitrogen monoxide/superoxide system

Abstract

Based on the previous report of McCord and co-workers (Crow, J. P., Beckman, J. S., and McCord, J. M. (1995) Biochemistry 34, 3544-3552), the zinc dithiolate active site of alcohol dehydrogenase (ADH) has been studied as a target for cellular oxidants. In the nitrogen monoxide (.NO)/superoxide (O.2) system, an equimolar generation of both radicals under peroxynitrite (PN) formation led to rapid inactivation of ADH activity, whereas hydrogen peroxide and .NO alone reacted too slowly to be of physiological significance. 3-Morpholino sydnonimine inactivated the enzyme with an IC50 value of 250 nM; the corresponding values for PN, hydrogen peroxide, and .NO were 500 nM, 50 μM, and 200 μM. When superoxide was generated at low fluxes by xanthine oxidase, it was quite effective in ADH inactivation (IC50 (XO) ≈ 1 milli-unit/ml). All inactivations were accompanied by zinc release and disulfide formation, although no strict correlation was observed. From the two zinc thiolate centers, only the zinc Cys2 His center released the metal by oxidants. The zinc Cys4 center was also oxidized, but no second zinc atom could be found with 4-(2-pyridylazo)resorcinol (PAR) as a chelating agent except under denaturing conditions. Surprisingly, the oxidative actions of PN were abolished by a 2-3-fold excess of .NO under generation of a nitrosating species, probably dinitrogen trioxide. We conclude that in cellular systems, low fluxes of .NO and O.2generate peroxynitrite at levels effective for zinc thiolate oxidations, facilitated by the nucleophilic nature of the complexed thiolate group. With an excess of .NO, the PN actions are blocked, which may explain the antioxidant properties of .NO and the mechanism of cellular S-nitrosations.