Oxidation of reduced nicotinamide hypoxanthine dinucleotide by intact rat liver mitochondria

1. The rate of NADH oxidation catalyzed by intact rat liver mitochondria is greatly stimulated in the presence of oxidized nicotinamide hypoxanthine dinucleotide (NHD⁺). 2. Mitochondrial oxidation of external NHDH is from 20- to 40-fold more rapid than that of NADH, although these coenzymes are oxidized at similar rates by sonicated mitochondria. 3. NADH and NADPH inhibit, while NADP⁺ stimulates NHDH oxidation. 4. NHDH oxidation is inhibited by rotenone and CN^-. 5. NHDH oxidation is coupled to the phosphorylation of ADP to ATP, yielding P:2e^- ratios approaching 3. 6. These studies indicate that external NHDH is oxidized by the intramito-chondrial respiratory chain NADH dehydrogenase and that the inner mitochondrial membrane is significantly more permeable to NHDH than to NADH. Mammalian liver mitochondria have been reported to catalyze the enzymatic deamination of NAD(H) to NHD(H) [Buniatian, H. C. (1970) in Handbook of Neurochemistry (Lajtha, A., ed.), Vol. 3, pp. 399-411, Plenum Press, London and New York; Movcessian, S. G. and Manassian, R. F. (1967) in Problems of Brain Biochemistry, Vol. 3, pp. 53-66, Academic Press, Yerevan], suggesting a metabolic function for the deaminated analogue. It is concluded that this deamination reaction may be operative in a mechanism for the oxidation of cytoplasmic NADH by the respiratory chain.