Properties of the oxidation of exogenous NADH and NADPH by plant mitochondria. Evidence against a phosphatase or a nicotinamide nucleotide transhydrogenase being responsible for NADPH oxidation

(1) The optimum pH for the oxidation of exogenous NADH by mitochondria from both Jerusalem artichoke (Helianthus tuberosus) tubers and Arum maculatum spadices was 7.0–7.1. NADPH oxidation had a lower optimum pH of 6.6 in Arum and 6.0 in Jerusalem artichoke mitochondria. In both types of mitochondria the rates of NADH and NADPH oxidation were identical below pH 6.0–5.5. (2) It is shown conclusively that neither a phosphatase converting NADPH to NADH nor a nicotinamide nucleotide transhydrogenase was involved in the oxidation of NADPH by these mitochondria. (3) Palmitoyl-CoA, an inhibitor of transhydrogenase activity in mammalian mitochondria, inhibits both NADH and NADPH oxidation by plant mitochondria with a K_i of about 10 μM. (4) It is concluded that the known properties of NAD(P)H oxidation are best explained by assuming the presence of a second dehydrogenase specific for NADPH. At low pH, electron flow from the two dehydrogenases to oxygen shares a common rate-limiting step.