Post-irradiation inactivation of the sulfhydryl enzyme malate synthase.

The sulfhydryl enzyme malate synthase was inactivated in air-saturated aqueous solution by X-irradiation (2 kGy). Changes of activity were registered up to 60 h after irradiation. Effects of specific additives (formate, superoxide dismutase, catalase), added before and/or after irradiation, revealed the role of the deleterious radical and non-radical species responsible for the radiation damage: inactivation during irradiation is mainly due to the action of OH, to a minor extent to O(2) and H2O2; post-irradiation inactivation is mainly caused by H2O2. A partial restoration of enzyme activity by dithiothreitol, added after irradiation, was found for all systems investigated; repairs were significant even when they were initiated 60 h after irradiation.