Abstract
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 NADPH dehydrogenase and NADPH .fwdarw. NAD transhydrogenase activities. Both activities are greatly increased in the presence of added guanidine .cntdot. HCl and at pH values <6.5. The chromophores of the soluble enzyme (flavin and Fe-S 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 .cntdot. HCl. The soluble dehydrogenase has little or no NADH .fwdarw. NADP and NADPH .fwdarw. NADP transhydrogenase activity. The former reaction is known to be energy-linked in submitochondrial particles; the latter was shown in the present studies to be energy-linked. Possible mechanisms for dehydrogenation and transhydrogenation (nonenergy-linked and energy-linked) involving reduced and oxidized NAD and NADP are proposed.

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