Mitochondria Are Physiologically Maintained At Close To 50 C
In endothermic species, heat released as a product of metabolism ensures stable internal temperature throughout the organism, despite varying environmental conditions. Mitochondria are major actors in this thermogenic process. Part of the energy released by the oxidation of respiratory substrates drives ATP synthesis and metabolite transport, while a noticeable proportion is released as heat. Using a temperature-sensitive fluorescent probe targeted to mitochondria, we measured mitochondrial temperature in situ under different physiological conditions. At a constant external temperature of 38 {degrees}C, mitochondria were more than 10 {degrees}C warmer when the respiratory chain was fully functional, both in HEK293cells and primary skin fibroblasts. This differential was abolished in cells lacking mitochondrial DNA or by respiratory inhibitors, but preserved or enhanced by expressing thermogenic enzymes such as the alternative oxidase or the uncoupling protein 1. The activity of various RC enzymes was maximal at, or slightly above, 50 {degrees}C. Our study prompts a re-examination of the literature on mitochondria, taking account of the inferred high temperature.