Demonstration and possible function of NADH:NAD⁺ transhydrogenase from Ascaris muscle mitochondria

Mitochondria from the muscle of A. lumbricoides var. suis function anaerobically. NADH is generated in the intermembrane space as a consequence of the 'malic' enzyme reaction. It has been suggested that this reducing equivalent in the form of hydride ion, would be translocated across the inner membrane in order to mediate ATP generation via the fumarate reductase reaction. In accord with this suggestion, intact Ascaris mitochondria showed appreciable NADH oxidase activity. Sonication resulted in an approximately 2 fold increase in NADH oxidase activity, whereas 'malic' enzyme, fumarase, and NADH:NAD⁺ transhydrogenase activities increased approximately 7 to 14 fold, respectively. Phosphorylation capabilities and permeability toward pyridine nucleotides also indicated the intactness of the mitochondria. Ascaris mitochondria incubated anaerobically in the presence of fumarate, and [¹⁴C]NADH catalyzed a rapid reduction of the fumarate to succinate with the concomitant formation of equivalent quantities of extramitochondrial NAD⁺. However, very little isotope was recovered from the washed mitochondria, indicating the possibility of hydride ion translocation in the absence of nucleotide translocation. NADH:NAD⁺ transhydrogenase has been isolated from the muscle mitochondria of the intestinal mematode, Ascaris lumbricoides var. suis. The enzyme seems to have been solubilized from the mitochondrial membrane fraction by treatment with sodium deoxycholate followed by dialysis and subsequent adsorption by and elution from alumina Cγ. No NADPH:NAD⁺ transhydrogenase activity was detectable, making the Ascaris system unique over others reported. Activity was protected by L cysteine, reduced glutathione and dithioerythritol, but strongly inhibited by low concentrations of p chloromercuribenzoate or silver nitrate. The thionicotinamide derivative of NAD⁺ (thioNAD⁺) was employed to accept hydride ions from NADH in order to assay spectrophotometrically at 398 nm. Apparent Km values for thioNAD⁺ and NADH were 1 x 10^-4 M and 8 x 10^-6 M, respectively. That the physiological nucleotide, could act as hydride ion acceptor from NADH was indicated by the findings that NAD⁺ competitively inhibited the reduction of thioNAD⁺ when assayed at 398 nm. The additional finding of a noncompetitive inhibition between NAD⁺ and NADH suggested at least two binding sites on the enzyme, one for NADH and another common site for NAD⁺ and thioNAD⁺. More conclusive evidence indicating the participation of NAD⁺ as acceptor was obtained by incubation of the enzyme with NADH and [¹⁴C]NAD⁺ and demonstrating a rapid formation of [¹⁴C]NADH. These findings, in conjunction with those discussed above, suggest a physiological function of this enzyme in hydride ion translocation.