Abstract
The slow, cyanide-insensitive oxidation of NADPH by oxygen catalyzed by rabbit liver microsomes is greatly stimulated by K3, 1, 4- or 1, 2-naphthoquinone. The K3-stimu-lated oxidation, but not the slow oxidation in the absence of K3, is considerably activated by high phosphate buffer concentrations. The K3 concentration giving half maximal stimu-lation of NADPH oxidation is 3.2 μM. The NADPH oxidation in the presence of K3 is accompanied by oxygen consumption; the reduction product of oxygen seems to be mainly hydrogen peroxide. Although it appears likely that in this oxidation K3 functions as a catalyst by undergoing reduc-tion to K3H2 by NADPH followed by direct or enzymatic reoxidation of the quinol by oxygen, this possibility has been excluded by several observations. It is concluded that the catalytic mechanism does not involve the formation of K3H2. Solubiliza-tion of microsomes with steapsin and subse-quent fractionation with ammonium sulfate yields a soluble preparation which shows K3-dependent NADPH oxidase activity but is practically incapable of oxidizing NADPH in the absence of K3. It seems that liver microsomes contain a K3-requiring NADPH oxidase in addition to the system responsible for the slow oxidation of NADPH in the absence of added cofactor.
