Snapshot A 23-year-old man is running a marathon. In the last mile of the race he experiences an intense burning in his muscles. Once he completes the race and rests, the burning slowly subsides. He asks his physician the next day why this may have occurred and what he can do to mitigate it. His physician explains that a training regimen can increase his ability to perform aerobic metabolism, and thus decrease the conversion of pyruvate to lactate (via lactate dehydrogenase) which caused the burning in his muscles that he experienced. Overview Pathways pyruvate → lactate catalyzed by lactate dehydrogenase (LDH) reversible generated in anaerobic glycolysis allows conversion of NADH → NAD+ in the liver, LDH converts lactate to pyruvate for gluconeogenesis or for metabolism to acetyl-CoA Cori cycle shifts energy generation from periphery to the liver pyruvate → acetyl-CoA catalyzed by pyruvate dehydrogenase (PDH) irreversible acetyl-CoA enters the citric cycle pyruvate → oxaloacetate catalyzed by pyruvate carboxylase (PC) irreversible oxaloacetate can replenish the citric acid cycle substrate for gluconeogenesis pyruvate → alanine catalyzed by alanine transaminase (ALT) reversible alanine carries amino groups to the liver from muscle in the liver, ALT converts alanine to pyruvate for gluconeogenesis