Bovine liver catalase suffered a biphasic inactivation when exposed to NH2Cl. A rapid and irreversible phase of activity loss was followed by a much slower and reversible inactivation. Removal of tightly bound NADPH from the enzyme decreased the extent of the rapid phase; whereas addition of NADPH augmented it. The catalase from Aspergillus niger, which does not contain bound NADPH, was not nearly as sensitive toward NH2Cl as was the liver enzyme and was not sensitized by added NADPH. NADPH is oxidized by NH2Cl, as evidenced by loss of the 340-nm absorption band, but the product of this oxidation was not NADP+. The rapid inactivation of liver catalase by NH2Cl was accompanied by some bleaching of the bands in the visible, while the slow inactivation was coincident with the appearance of a new band at 570 nm. A tentative explanation for these observations is proposed.
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