Alterations in heart sarcolemmal Ca2+-ATPase and Ca2+-binding activities due to oxygen free radicals

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Abstract
Effects of oxygen free radicals on Ca2+/Mg2+ ATPase and ATP-independent Ca2+-binding activities were examined in rat heart sarcolemma. Membranes were incubated with different oxygen radical generating media such as xanthine + xanthine oxidase, hydrogen peroxide, and hydrogen peroxide + Fe2+. In the presence of xanthine + xanthine oxidase, Ca2+ ATPase activity was stimulated and this effect was prevented by the addition of superoxide dismutase. Hydrogen peroxide also showed a significant increase in Ca2+-ATPase activity in a dose-dependent manner and this effect was blocked by catalase. On the other hand, a combination of hydrogen peroxide + Fe2+ decreased Ca2+-ATPase activity; this depression was prevented by the addition of D-mannitol. The observed change in Ca2+-ATPase activity due to oxygen free radicals was associated with changes in Vmax, whereas Ka remained unaffected. Both xanthine + xanthine oxidase and hydrogen peroxide increased whereas, hydrogen peroxide + Fe2+ inhibited the ATP-independent Ca2+-binding activities. It is suggested that oxygen free radicals may influence Ca2+ movements in the cell by altering the Ca2+/Mg2+ ATPase and Ca2+-binding activities of the membrane and these effects may be oxygen-radical species specific.

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