How hot can mitochondria be? Incubation at temperatures above 43 °C induces the degradation of respiratory complexes and supercomplexes in intact cells and isolated mitochondria

•We have examined by Blue-Native electrophoresis, both in intact cells and in isolated organelles, the stability of respiratory complexes and supercomplexes at different temperatures to obtain information about their tolerance to heat stress.•Upon incubation at temperatures above 43 °C and after relatively short periods, respiratory complexes, and especially complex I and its supercomplexes, are unstable even when the respiratory activity is inhibited, both in intact cells and isolated mitochondria.•These results support the conclusion that high temperatures (>43 °C) cause damage to mitochondrial structure and function and question the proposal that these organelles can physiologically work at close to 50 °C. Mitochondrial function generates an important fraction of the heat that contributes to cellular and organismal temperature maintenance, but the actual values of this parameter reached in the organelles is a matter of debate. The studies addressing this issue have reported divergent results: from detecting in the organelles the same temperature as the cell average or the incubation temperature, to increasing differences of up to 10 degrees above the incubation value. Theoretical calculations based on physical laws exclude the possibility of relevant temperature gradients between mitochondria and their surroundings. These facts have given rise to a conundrum or paradox about hot mitochondria. We have examined by Blue-Native electrophoresis, both in intact cells and in isolated organelles, the stability of respiratory complexes and supercomplexes at different temperatures to obtain information about their tolerance to heat stress. We observe that, upon incubation at values above 43 °C and after relatively short periods, respiratory complexes, and especially complex I and its supercomplexes, are unstable even when the respiratory activity is inhibited. These results support the conclusion that high temperatures (>43 °C) cause damage to mitochondrial structure and function and question the proposal that these organelles can physiologically work at close to 50 °C.