Effect of oxidants on Na,K,ATPase and its reversal

Ischemia-reperfusion heart cell injury may be mediated, at least in part, through the generation of oxy radicals. Therefore, mechanisms of action of two oxidants on a membrane model, partially purified Na,K,ATPase, were investigated. Effects of H2O2, an oxygen intermediate postulated to play a primary role in reperfusion injury, on the function of the enzyme were time-dependent and potentiated by Fe ions. The inhibition of enzyme activity was prevented by chelators, but not by hydroxyl radical scavengers. The results support the view that the possible mode of enzyme modification involves H2O2-derived, Fe ion-catalyzed, localized ("site-specific") hydroxyl radical formation. The action of hypochlorous acid (HOCl), a powerful oxidant postulated to be produced by activated neutrophils, was quantitatively similar to that of H2O2 plus Fe ions in causing enzyme dysfunction. This is partly because relatively large doses of oxidants were required, due to the presence of physiological anti-oxidant defense mechanisms in the membrane. Although a combination of deferoxamine (Fe ion chelator) and dithiothreitol (DTT) (sulfhydryl reducing agent) was most effective in preventing the enzyme modification, once enzyme inactivation by oxidants is in progress, deferoxamine plus DTT could only arrest further deterioration of the enzyme function. Therefore, the oxidant-induced change in membrane dysfunction advances with time; the advance can be stalled, but the enzyme activity cannot be restored to normal.