Body mass dependence of H+ leak in mitochondria and its relevance to metabolic rate

THE standard metabolic rate of an animal is the rate of heat production under conditions that minimize known extra requirements for energy1,2. In tissues and cells from aerobic organisms, energy expenditure can conveniently be measured as oxygen consumption3,4. Measurements made using isolated rat hepatocytes have shown that a significant contribution to resting oxygen consumption (and hence heat production) is made by a futile cycle of proton pumping and proton leak across the mitochondrial inner membrane5. Two important factors affecting standard metabolic rate, thyroid status and phylogeny, also affect the proton permeability5–10. A third major factor affecting standard metabolic rate is body mass11,12. Here we show that proton leak decreases with increasing body mass in mammals. We suggest that differences in proton leak may partly explain the differences in standard metabolic rate between mammals of different mass.