The yeast possesses two routes of electron transfer from exogenous NAD(P)H to oxygen. Electrons are transferred either to the classical cytochrome pathway at the level of ubiquinone through an NAD(P)H dehydrogenase, or to an alternative pathway at the level of cytochrome through another NAD(P)H dehydrogenase which is insensitive to antimycin A. Analyses of mitoplasts obtained by digitonin/osmotic shock treatment of mitochondria purified on a sucrose gradient indicated that the NADH and NADPH dehydrogenases serving the alternative route were located on the mitochondrial inner membrane. The dehydrogenases could be differentiated by their pH optima and their sensitivity to amytal, butanedione and mersalyl. No transhydrogenase activity occurred between the dehydrogenases, although NADH oxidation was inhibited by NADP and butanedione. Studies of the effect of NADP on NADH oxidation showed that the NADH:ubiquinone oxidoreductase had Michaelis-Menten kinetics and was inhibited by NADP, whereas the alternative NADH dehydrogenase had allosteric properties (NADH is a negative effector and is displaced from its regulatory site by NAD or NADP).


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