Mitochondria are incompletely coupled, and during oxidative phosphorylation some of the
redox energy in substrates is lost as heat. Incomplete coupling is mostly due to a natural
leak of protons across the mitochondrial inner membrane. In rat hepatocytes the futile cycle
of proton pumping and proton leak is responsible for 20–25% of respiration; in perfused rat
muscle the value is 35–50%. Mitochondrial proton cycling is estimated to cause 20–25% of
basal metabolic rate in rats. Proton cycling is equally prominent in hepatocytes from several …