Can NAD(P)⁺ transhydrogenase (NNT) mediate a physiologically meaningful increase in energy expenditure by mitochondria during H₂O₂ removal?

Smith et al. () studied skeletal muscle mitochondria (SMM) from different inbred mouse strains (C57BL/6NJ versus C57BL/6J; NNT function is absent in the latter) and concluded that NNT activity mediates meaningful increases in respiration when NADPH-dependent mitochondrial H₂O₂ removal is stimulated. To be valid, this contention requires the NNT reaction stoichiometry to be ∼200 H⁺ translocated per NAD(P) hydride transferred based on expected stoichiometries in mitochondrial respiration (at least 6H⁺:1O) and some of the authors’ reported values (). The authors argued that NNT’s stoichiometry may be altered when the enzyme is generating NADPH (), but a ratio of 200 H⁺ per hydride transfer challenges recent advances on the structure of NNT (, ), which provide strong evidence these two molecular events are tightly coupled with a ratio of 1:1. Smith et al. () assert NADPH-dependent H₂O₂ removal can be linked to respiration by adding auranofin plus bis-chloroethylnitrosourea to mitochondria. However, these compounds caused similar decreases (∼35%, see Fig. 4D ()) in O₂ consumption in both mouse strains, indicating suppression of respiration irrespective of NNT flux and making conclusions based on these inhibitors ambiguous. Finally, previous studies comparing mitochondria from mice with or without functional NNT (using genetically closer controls) found no evidence of NNT-mediated flux sufficient to be measured as increased respiration (, ). While the findings of Smith et al. () are attractive, we contend more direct evidence is needed before the role of redox circuits through NNT can be definitively understood in the context of energy expenditure.