The uncoupling effect of free fatty acids on oxidative phosphorylation in mitochondria has been known for more than 35 years. The mechanism of action, however, remains controversial. In this report the physicochemical basis of uncoupling was elucidated by studying the effect of free fatty acids on the proton permeability and membrane potential of proteoliposomes containing reconstituted cytochrome c oxidase (COX). A threshold membrane potential of about 125 mV was identified for fatty acid-induced proton permeability. Only above this potential do free fatty acids translocate protons across the biological membrane. The data explain the controversial effects of long-chain fatty acids on oxidative phosphorylation as well as their role on non-shivering thermogenesis in larger mammals.
