Modulation of cytochrome c-mediated extramitochondrial NADH oxidation by contact site density.

Data presented in previous reports suggest that in rat liver mitochondria a "bi-trans-membrane" electron transport pathway is present which promotes the transfer of reducing equivalents directly from cytosolic NADH to molecular oxygen inside the mitochondria. Here we show that the oxidation of external NADH is stimulated by atractylate + ADP and greatly inhibited by glycerol. These two conditions have been documented to promote the increase and the decrease respectively of the frequency of "contact sites" between the two mitochondrial membranes. NADH oxidation is not affected at all by glycerol and atractylate + ADP when TMPD and endogenous cytochrome c are utilized as electron carriers. The results obtained are consistent with the proposal that the bi-trans-membrane electron transport chain might be localized at the level of respiratory contact sites having the function of promoting the oxidation of the surplus amount of cytosolic NADH. This electron transport pathway has been suggested to play a decisive role in the early stages of apoptosis