The Effect of the Exogenous NADH Dehydrogenase of Heart Mitochondria on the Transmembranous Proton Movement

Heart mitochondria can be made to oxidize extramitochondrial NADH via the exogenous NADH dehydrogenase. Oxidation of extramitochondrial NADH was found to be associated with the disappearance of H+from the suspension medium. Our studies on the possible pathway through which H+may disappear from the extramitochondrial space were focused on (i) an unspecific transmembranous H+leakage along the electrochemical H+gradient following peroxidative membrane alteration, (ii) stimulation of a controlled H+reconduction through the H+channel of the ATP synthase, and (iii) stimulation of the Na+/H+counterporter by Ca2+release. Our experiments revealed that none of these H+pathways was involved in the observed alkalinization of the extramitochondrial space during respiration of external NADH. The latter effect was inhibited when oxidation of external NADH via the respiratory chain was blocked and could be turned into the opposite when artificial e−acceptors of the exogenous NADH dehydrogenase were used to reactivate NADH consumption. Stoichiometric analysis of H+disappearance and O2consumption revealed that reducing equivalents of external NADH were transferred to oxygen via cytochrome oxidase and H+from the suspension was used to release water.