Mitochondria are physiologically maintained at close to 50 °C

TLDR

In endothermic organisms, metabolic heat keeps internal temperature stable, and mitochondria, as key sites of respiration, generate a substantial portion of this heat. Our study prompts a critical re‑examination of the literature on mitochondria. We used a temperature‑sensitive fluorescent probe targeted to mitochondria to measure their temperature in situ under various physiological conditions. Mitochondrial temperature rose by over 10 °C above the ambient 38 °C when the respiratory chain was active, a difference lost with mtDNA depletion or respiratory inhibition but maintained or amplified by thermogenic enzymes, and the activity of respiratory chain enzymes peaked at or just above 50 °C. In view of their potential consequences, these observations need to be further validated and explored by independent methods.

Abstract

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, but a substantial 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 °C, mitochondria were more than 10 °C warmer when the respiratory chain (RC) was fully functional, both in human embryonic kidney (HEK) 293 cells and primary skin fibroblasts. This differential was abolished in cells depleted of mitochondrial DNA or treated with 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 °C. In view of their potential consequences, these observations need to be further validated and explored by independent methods. Our study prompts a critical re-examination of the literature on mitochondria.