“Alternative” or integrated role of mitochondrial glycerol-3-phosphate dehydrogenase

Herpe et al. (1) investigate the role of mitochondrial glycerol-3phosphate dehydrogenase (mtGpDH; E. C. 1. 1. 5. 3) in Drosophila bioenergetics. The flavoprotein mtGpDH in the mitochondrial inner membrane is not a proton pump, comparable to Complex II (CII), electron-transferring flavoprotein dehydrogenase (ETFDH), and proline dehydrogenase (ProDH; Fig. 1). Like Complex I (CI), these enzymes transfer electrons to the Q-junction. mtGpDH, and similarly ProDH, play important physiological roles not only when CI is impaired but within the metabolic context integrating oxidative phosphorylation, glycolysis, lipid metabolism, and gluconeogenesis in various tissues of many species (2-5). Interpretation of the reported phenotypes is therefore complex, particularly given the high mortality (~45 %) in the mtGpDH-deficient group, since primary effects on bioenergetics are likely confounded by secondary mitochondrial dysfunction associated with organismal failure. Regarding the ATP-yield and the contribution of glycerophosphate to O 2 consumption, the enzymes CI, CII, and mtG-pDH must be distinguished from the NADH-, succinate-, and glycerophosphate-linked electron transfer pathways to oxygen (N, S, and Gp, respectively; Fig. 1). Respiration was measured with substrate combinations NS and NSGp, not Gp alone. The contribution of the Gp-pathway to NSGp respiration cannot be quantified by the flux control efficiency, j = (NSGp -NS) /NSGp, which expresses the relative stimulatory effect of Gp on NS-respiration (6). Herpe et al. (1) misinterpret j = 1. 0 (i. e. , NS = 0) as a doubling of oxygen consumption after Gp addition. In fact, doubling flux would yield j = 0. 5. This affects interpretation of their results. An NS-pathway capacity close to zero, indicated by j -values up to 0. 99 in controls (1), is incompatible with insect physiology (7-9). Expressing ATP yield as "1. 5 ATP per FADH 2 " compared to the N-pathway is misleading, because glycerophosphate and succinate-not tightly bound FADH 2 -are the sources of reducing equivalents, 2H + +e -, in the Gp-and S-electron transfer pathways (10), whereas NADH supplies electrons to CI in the N-pathway (Fig. 1). Inconsistencies arise in calculating ATP/O ratios from ATP production and oxygen consumption (Fig. 3 A andD). These reach implausibly high values (CTRL: 461; GPO1: 245) far outside the range of 1 to 2 presented in Fig. 3G. The discrepancy likely reflects a unit error in ATP production (µmol vs. nmol), while conversion of O 2 -to atomic O-equivalents requires doubling rather than using half the corresponding O 2 consumption rate. More importantly, the highest and lowest ATP/O values (CTRL group) are omitted from Fig. 3G and suggested to be outliers, which was not confirmed by reanalysis with a Grubbs' test. Excluding these values affects the statistical outcome. Notably, because more protons are translocated in the N-than the Gp-pathway, loss of mtGpDH-supported respiration should increase the relative contribution of the