Figure 1. Substrate utilization by cardiac mitochondria.

Pyruvate enters the Krebs cycle by pyruvate dehydrogenase (PDH), producing acetyl-CoA (Ac-CoA). Octanoyl-carnitine undergoes β-oxidation and produces acetyl-CoA, NADH and FADH₂. Malate enters mitochondria through oxoglutarate/malate carrier (OMC) and produces mainly NADH by mitochondrial malate dehydrogenase (mMDH), while glycerol-3 phosphate (G3P) produces FADH₂ by the mitochondrial glycerol-phosphate dehydrogenase (mGDH). Succinate produces mainly FADH₂ that enters the respiratory chain through complex-II. NADH is reoxidized in the respiratory chain at complex-I and FADH₂ at complex-II.