NADPH mediates the inactivation of bovine liver catalase by monochloroamine

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.