PubMed1986
Ascorbate reversibly inhibits catalase, and this inhibition is enhanced and rendered irreversible by the prior addition of copper(II)-bishistidine. In the absence of copper, the inhibition was prevented and reversed by ethanol, but not by superoxide dismutase, benzoate, mannitol, thiourea, desferrioxamine, or DETAPAC. In the presence of the copper complex mannitol, benzoate, and superoxide dismutase still had no effect, but thiourea, desferrioxamine, DETAPAC, or additional histidine decreased the extent of inactivation to that seen in the absence of copper. In the presence of copper, ethanol protected at [ascorbate] less than 1 mM, but was ineffective at [ascorbate] greater than 2 mM, even in the absence of oxygen. Although in the absence of copper, complete removal of oxygen provided full protection against inactivation by ascorbate, this protection was not seen if the catalase was briefly preincubated with H2O2 prior to flushing with nitrogen, or if copper was present. In fact, if copper was present, inactivation was enhanced by the removal of oxygen. Increasing the concentration of oxygen from ambient to 100% slowed the inactivation, whether or not copper was present. It is concluded that the initial reversible inactivation involves reaction with H2O2 to form compound I, followed by one electron reduction of compound I to compound II. In the presence of added copper, the initial (reversible) inactivation allows H2O2 to accumulate sufficiently to permit irreversible inactivation. Since in the presence of copper oxygen is not required, and neither the reversible nor the irreversible inactivation was prevented by conventional scavengers of active forms of oxygen, the inactivation is likely mediated by semidehydroascorbate, and/or it may involve site-specific generation of the damaging intermediates.