REACTIONS OF O‐2, H2O2 AND OTHER OXIDANTS WITH SULFHYDRYL ENZYMES‡

Abstract

Abstract— The inactivation of a number of sulfhydryl enzymes by H2O2. OH and O‐2 is discussed. and the efficiencies of inactivation for the free radicals are compared with those of halide radicals. The enzymes with essential acyl‐binding SH groups (papain and GAPDH) exhibit a higher sensitivity to inactivation by O‐2 and H2O2 than those with “unactivated” SH groups (LDH and ADH). The sulfenic acid derivatives from the reactions of papain and GAPDH with H2O2 are repairable by RSH molecules, the rates of repair depending on the nature of the R group.Comparisons of rate constants show that competitive scavenging by RSH must be a very important mechanism for the protection of sensitive SH‐containing proteins against OH, and for papain in oxygen‐free solutions the efficiency of this protection increased in the order GSH < PenSH < CysSH < dithiothreitol. On the other hand, catalase and superoxide dismutase are more effective than RSH compounds in rendering protection against H2O2 and O‐2. In fact in oxygen‐containing systems the effects of RSH are highly complex, because the OH‐ and/or O‐2‐initiated chain oxidation of these molecules produces reactive species, which may inactivate essential SH groups. Experiments with papain showed that the yields and nature of the RSH‐derived species depended on the structure of the R group. Also, very large yields of papain Cys25SSR were produced from the reactions of the radicals and H2O2. In effect, it is apparent that under certain conditions the oxidation of “activated” SH groups of important target molecules could become coupled to the OH or O‐2 ‐initiated oxidation of RSH molecules. This should be taken into account in assessing the overall effectiveness of RSH compounds as cellular protectors.