Inhibition and reactivation of Mn-catalase. Implications for valence changes at the active site manganese

The Mn-catalase of Lactobacillus plantarum was inactivated when exposed to NH2OH plus H2O2, an effect which was not reversed by dialysis. N-Methylhydroxylamine was approximately 1% as effective as was NH2OH, while O-methylhydroxylamine was not detectably active in this regard. Approximately 40% of the lost activity could be restored by dithionite or by O-2, whereas other reductants such as ethanol, ascorbate, or nitrite were without effect. Oxidants such as persulfate and ferricyanide also failed to reactivate the enzyme. The active enzyme was inactivated, to an apparent limit of 50%, by an enzymic or photochemical flux of O-2 and this was entirely prevented by superoxide dismutase. The catalytic cycle of the enzyme is thought to involve the trivalent and pentavalent forms of the active site Mn; while inactivation by H2O2 + NH2OH appears to be due to conversion to the quadrivalent state. Partial bleaching of the enzyme by H2O2 and the nearly complete bleaching caused by NH2OH + H2O2 are in accord with this interpretation. The enzyme was unaffected by 2.0 mM EDTA, thiourea, o-phenanthroline, alpha, alpha'-dipyridyl, 8-hydroxyquinoline, diethyldithiocarbamate, thiourea, hydrazine, phenylhydrazine, isoniazid, semicarbazide, sulfite, nitrite, or sulfide, all at pH 7.0.