Superoxide reactivates nitric oxide-inhibited catalase

Catalase binds nitric oxide (NO) to generate ferricatalase-NO, an inhibited form of the enzyme. Superoxide (O₂^-) is also an inactivator of the enzyme. We found, however, that O₂^- efficiently converted the inhibited ferricatalase-NO to the active ferricatalase without producing detectable intermediates. The reaction slowed down when O₂^- was disproportionated to H₂O₂ and O₂ by superoxide dismutase, but H₂O₂ could displace the heme-bound NO slowly to regenerate ferricatalase. Reactivation was observed even under simultaneous generation of NO and O₂^-, suggesting that ferricatalase-NO reacts with O₂^- fast enough to compete with the rapid reaction of O₂^- and NO. Formation of peroxynitrite by the simultaneous generation of NO and O₂^- was only partially inhibited by ferricatalase, presumably due to slow binding of NO to catalase in comparison with the reaction of NO and O₂^-.