## Enzyme Catalyzing Lactate to Pyruvate Conversion **Key Point:** Lactate dehydrogenase (LDH) is a reversible enzyme that catalyzes the interconversion of lactate and pyruvate. The reaction is: Lactate + NAD^+^ ⇌ Pyruvate + NADH + H^+^ ### Role in Cori Cycle During the Cori cycle: 1. Lactate produced in muscle during anaerobic glycolysis is transported to the liver 2. LDH catalyzes oxidation of lactate to pyruvate in hepatic mitochondria 3. Pyruvate is then gluconeogenically converted to glucose 4. Glucose returns to muscle for re-utilization **High-Yield:** LDH is a cytoplasmic enzyme present in all tissues. The direction of the reaction depends on the NAD^+^/NADH ratio — in muscle (high NADH), lactate is formed; in liver (high NAD^+^), lactate is oxidized to pyruvate. **Clinical Pearl:** Elevated serum LDH indicates tissue damage or anaerobic metabolism. LDH is used clinically as a marker of myocardial infarction, hemolysis, and liver disease. ### Why LDH Is the Answer LDH is the only enzyme listed that directly catalyzes the reversible oxidation-reduction reaction converting lactate back to pyruvate. This is the critical step in hepatic lactate clearance during the Cori cycle. 
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