Dehydrogenase and transhydrogenase properties of the soluble NADH dehydrogenase of bovine heart mitochondria.

The soluble NADH dehydrogenase of low molecular weight, isolated from complex I (NADH:ubiquinone oxidoreductase, EC 1.6.5.3) of the respiratory chain, has been shown to have NADPH dehydrogenase and NADPH leads to NAD transhydrogenase activities. Both activities are greatly increased in the presence of added guanidine-HCl and at pH values less than 6.5. The chromophores of the soluble enzyme (flavin and iron--sulfur centers) are reduced by NADH and NADPH to the same extent. The latter reduction is extremely slow, and is considerably stimulated in the presence of guanidine-HCl. The soluble dehydrogenase has little or no NADH leads to NADP and NADPH leads to NADP transhydrogenase activity. The former reaction is known to be energy-linked in submitochondrial particles; the latter was shown in the present studies also to be energy-linked. In view of the above and earlier results, possible mechanisms for dehydrogenation and transhydrogenation (nonenergy-linked and energy-linked) involving reduced and oxidized NAD and NADP are proposed.