INVOLVEMENT OF THIOL-GROUPS IN THE IMPAIRMENT OF CARDIAC SARCOPLASMICRETICULAR PHOSPHOLIPASE-D ACTIVITY BY OXIDANTS

Considerable phospholipase D (PLD) activity is localized in myocardial sarcoplasmic reticular (SR) membranes, where it may take part in the regulation of Ca2+ movements. In this study, we examined thiol group d ependence as a possible regulatory mechanism for SR PLD. SR membranes isolated from rat heart were exposed to four types of thiol group modi fiers, which all induced a decrease in SR PLD activity that was preven ted by dithiothreitol. Furthermore, since abnormalities in thiol statu s and Ca2+ homeostasis are characteristic for the myocardial cell dama ge induced by oxidative stress, we also studied the effects of oxidant s on the SR PLD activity. The enzyme was not affected by xanthine-xant hine oxidase, but was depressed by hydrogen peroxide and by hypochloro us acid. These inhibitory effects were prevented by catalase as well a s by methionine and dithiothreitol, respectively. Furthermore, reduced glutathione protected against the hydrogen peroxide-induced depressio n, whereas oxidized glutathione inhibited SR PLD. The results indicate that SR PLD activity is inhibited by nonradical oxidants, hydrogen pe roxide and hypochlorous acid, through reversible modification of assoc iated thiol groups. Thus, the enzyme may be controlled by the glutathi one redox status of the cardiac cell.