Energy-linked mitochondrial transhydrogenation from NADPH to NADP analogs

The mitochondrial energy-linked transhydrogenase enzyme catalyzes hydride ion transfer between NAD and HADP, of which the reaction NADH leads to NADP is slow in the absence of energy and is accelerated 10-fold or more when the mitochondrial membrane is energized by ATP hydrolysis or respiration. The enzyme is a proton pump and effects proton translocation coupled to hydride ion transfer from NADPH to NAD (Earle, S.R., and Fisher, R.R. (1980) J. Biol Chem. 255, 827-830). The present studies have shown that submitochondrial particles also catalyze transhydrogenation from NADPH to two NADP analogs, namely 3-acetylpyridine adenine dinucleotide phosphate (AcPyADP) and thionicotinamide adenine dinucleotide phosphate (thioNADP). Both reaction rates are greatly accelerated when the system is energized by ATP hydrolysis (inhibitable by uncouplers or rutamycin) or succinate oxidation (inhibitable by uncouplers or antimycin A). As in the case of NAD(H) in equilibrium with NADP(H) reactions, the transhydrogenations from NADPH to AcPyADP and thioNADP are inhibited by treatment of submitochondrial particles with trypsin or the arginyl residue modifier, butanedione. The Km values of the above substrates and the Vmax values under energy-linked conditions have been determined. The finding that the mitochondrial energy-linked transhydrogenase enzyme catalyzes transhydrogenation from NADPH to NADP analogs has revealed features regarding substrate site specificities and the effect of substrates on the directionality of proton translocation by the enzyme.