Investigating the thermal sensitivity of key enzymes involved in the energetic metabolism of three insect species.

The metabolic responses of insects to high temperatures have been linked to their mitochondrial substrate oxidation capacity. However, the mechanism behind this mitochondrial flexibility is not well understood. Here, we used three insect species with different thermal tolerances (the honey bee, Apis mellifera; the fruit fly, Drosophila melanogaster; and the potato beetle, Leptinotarsa decemlineata) to characterize the thermal sensitivity of different metabolic enzymes. Specifically, we measured activity of enzymes involved in glycolysis (hexokinase, HK; pyruvate kinase, PK; and lactate dehydrogenase, LDH), pyruvate oxidation and the tricarboxylic acid cycle (pyruvate dehydrogenase, PDH; citrate synthase, CS; malate dehydrogenase, MDH; and aspartate aminotransferase, AAT), and the electron transport system (Complex I, CI; Complex II, CII; mitochondrial glycerol-3-phosphate dehydrogenase, mG3PDH; proline dehydrogenase, ProDH; and Complex IV, CIV), as well as that of ATP synthase (CV) at 18, 24, 30, 36, 42 and 45°C. Our results show that at high temperature, all three species have significantly increased activity of enzymes linked to FADH2 oxidation, specifically CII and mG3PDH. In fruit flies and honey bees, this coincides with a significant decrease of PDH and CS activity, respectively, that would limit NADH production. This is in line with the switch from NADH-linked substrates to FADH2-linked substrates previously observed with mitochondrial oxygen consumption. Thus, we demonstrate that even though the three insect species have a different metabolic regulation, a similar response to high temperature involving CII and mG3PDH is observed, denoting the importance of these proteins for thermal tolerance in insects.