[Activation of the external pathway of NADH oxidation in mitochondria at decreased pH]

The rates of external and internal pathways of NADH oxidation in mitochondria of liver and skeletal muscle and skeletal muscle homogenate were studied at decreased pH and increased concentrations of Pi and CaCl2. The results obtained are indicative of an existence of an external pathway of NADH oxidation, which is insensitive to antimycin A and rotenone in skeletal muscle mitochondria. In the mitochondria of liver and skeletal muscle NADPH does not replace NADH in the external pathway; the NADH oxidation is sensitive to mersalyl and KCN. The decrease of pH of the incubation medium from 7.5 to 6.5 or the increase in Pi concentration up to 8-10 mM activates the external pathway and inhibits the internal pathway in liver mitochondria; the same effect is observed after addition of 10 microM CaCl2, pH 7.2. The external pathway of NADH oxidation in liver mitochondria is sensitive to the inhibitors of phospholipase A2 (nupercaine, EGTA) and the inhibitor of lipid peroxidation (ionole).