Characteristics of mitochondrial proton leak and control of oxidative phosphorylation in the major oxygen-consuming tissues of the rat.

Maintenance of an electrochemical proton gradient across the mitochondrial inner membrane against the significant proton permeability of the membrane accounts for 25-30% of resting oxygen consumption in hepatocytes. It has been proposed that proton leak could be a significant contributor to resting metabolic rate in mammals if it were present in other tissues. Mitochondria were isolated from the major oxygen-consuming tissues (liver, kidney, brain and skeletal muscle) of the rat. In each tissue, the mitochondria showed significant proton leak with the same characteristic non-linear dependence on membrane potential. Liver and kidney mitochondria showed similar membrane proton permeability per mg of mitochondrial protein; brain and muscle permeabilities were greater when expressed in this way. Differences in the kinetic response of the substrate oxidation and phosphorylating systems to membrane potential were observed. The substrate oxidation system was more active in kidney, brain and skeletal muscle mitochondria than in liver mitochondria per mg of mitochondrial protein. Liver and kidney phosphorylating systems were less active than brain and skeletal muscle per mg of mitochondrial protein. The control of oxidative phosphorylation was also assessed. The distribution of control in mitochondria isolated from the four tissue types was found to be similar.