Oxidation of reduced nicotinamide hypoxanthine dinucleotide phosphate by intact rat liver mitochondria

1. 1. While intact rat liver mitochondria oxidize external NADPH at an extremely low rate (0.1-0.4 nmole per mg protein per min), reduced nicotinamide hypoxanthine dinucleotide phosphate (NHDPH) is relatively rapidly oxidized (12-13 nmoles per mg protein per min). 2. 2. After sonication, the rate of mitochondrial NHDPH oxidation is increased by 2-fold, and the NADPH oxidation rate is double that of NHDPH. 3. 3. NADPH and NHDPH oxidation is neither coupled to ATP synthesis nor inhibited by rotenone or CN^-, but is inhibited under anaerobic conditions. These data indicate that, although molecular oxygen is the ultimate electron acceptor, the respiratory chain is not functional in the oxidation reaction. 4. 4. The NADPH oxidase activity resides in the submitochondrial particle fraction isolated from sonicated mitochondrial preparations. 5. 5. These results are interpreted to indicate the presence of an NADPH oxidase system in rat liver mitochondria which is bound to the inner surface of the inner membrane and independent of the respiratory chain. Further, it is concluded that the inner membrane may be significantly more permeable to NHDPH than to NADPH.