Fig. 4

Schematic illustration of the bypass mechanism of NV118 and methylene blue in metformin-induced mitochondrial dysfunction. a At high concentrations, metformin induces inhibition of complex I (CI) and the mitochondrial glycerophosphate dehydrogenase (mGPD). As a result, ATP generation at complex V (CV/ATP-synthase) is decreased causing increased glycolysis to compensate for the reduced mitochondrial ATP production. Pyruvate metabolism is shifted towards lactate generation to meet the increased NAD⁺ demand, which is accompanied by intracellular and extracellular acidification. b Cell-permeable succinate prodrugs, such as NV118, can permeate the cell membrane independent of active transporters. Through intracellular metabolism, the succinate core is released and made available for oxidation by complex II (CII). Oxidation of succinate at CII restores downstream electron flow and mitochondrial respiration which is linked to phosphorylation (ATP generation). c The redox agent methylene blue (MB) is reduced by NAD(P)H-dependent dehydrogenases. The reduced form of methylene blue (MBH₂, leucomethylene blue) then donates the electrons to cytochrome C (labeled C), recycling MB. Electron donation by MBH₂ to cytochrome C restores downstream electron flow and mitochondrial respiration which is not coupled to phosphorylation. ADP: adenosine diphosphate, ATP: adenosine triphosphate, CIII: complex III, CIV: complex IV, cGPD: cytosolic glycerophosphate dehydrogenase, DHAP: dihydroxyacetone phosphate, G3P: glycerol-3-phosphate, NAD⁺/NADH: nicotinamide adenine dinucleotide, NADP⁺/NADPH: nicotinamide adenine dinucleotide phosphate, Q: Quinone, TCA: tricarboxylic acid