Superoxide reactivates nitric oxide-inhibited catalase

초록

Catalase binds nitric oxide (NO) to generate ferricatalase-NO, an inhibited form of the enzyme. Superoxide (O<inf>2</inf>-) is also an inactivator of the enzyme. We found, however, that O<inf>2</inf>- efficiently converted the inhibited ferricatalase-NO to the active ferricatalase without producing detectable intermediates. The reaction slowed down when O<inf>2</inf>- was disproportionated to H<inf>2</inf>O<inf>2</inf> and O<inf>2</inf> by superoxide dismutase, but H<inf>2</inf>O<inf>2</inf> could displace the heme-bound NO slowly to regenerate ferricatalase. Reactivation was observed even under simultaneous generation of NO and O<inf>2</inf>-, suggesting that ferricatalase-NO reacts with O<inf>2</inf>- fast enough to compete with the rapid reaction of O<inf>2</inf>- and NO. Formation of peroxynitrite by the simultaneous generation of NO and O<inf>2</inf>- was only partially inhibited by ferricatalase, presumably due to slow binding of NO to catalase in comparison with the reaction of NO and O<inf>2</inf>-.

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