Succinate accumulates during
ischemia, and its oxidation at reperfusion drives injury. The mechanism of ischemic
succinate accumulation is controversial and is proposed to involve reversal of mitochondrial complex II. Herein, using stable-
isotope-resolved metabolomics, we demonstrate that complex II reversal is possible in hypoxic mitochondria but is not the primary
succinate source in hypoxic cardiomyocytes or ischemic hearts. Rather, in these intact systems
succinate primarily originates from canonical Krebs cycle activity, partly supported by
aminotransferase anaplerosis and glycolysis from
glycogen. Augmentation of canonical Krebs cycle activity with dimethyl-α-ketoglutarate both increases ischemic
succinate accumulation and drives substrate-level phosphorylation by
succinyl-CoA synthetase, improving ischemic energetics. Although two-thirds of ischemic
succinate accumulation is extracellular, the remaining one-third is metabolized during early reperfusion, wherein acute complex II inhibition is protective. These results highlight a bifunctional role for
succinate: its complex-II-independent accumulation being beneficial in
ischemia and its complex-II-dependent oxidation being detrimental at reperfusion.