Which enzyme catalyzes the conversion of lactate to pyruvate in the Cori cycle?

Explanation

## Lactate-to-Pyruvate Conversion in the Cori Cycle **Key Point:** Lactate dehydrogenase (LDH) catalyzes the reversible conversion of lactate ↔ pyruvate using NAD^+^ / NADH as a cofactor. ### Enzyme Mechanism LDH is a bidirectional enzyme: - **In muscle (anaerobic conditions):** Pyruvate → Lactate (uses NADH, regenerates NAD^+^) - **In liver (Cori cycle):** Lactate → Pyruvate (uses NAD^+^, regenerates NADH) ### The Cori Cycle Overview ```mermaid flowchart LR A[Muscle: Glucose]:::outcome --> B[Glycolysis]:::action B --> C[Pyruvate]:::outcome C --> D[Lactate via LDH]:::action D --> E[Blood lactate]:::outcome E --> F[Liver]:::outcome F --> G[Lactate → Pyruvate via LDH]:::action G --> H[Gluconeogenesis]:::action H --> I[Glucose]:::outcome I --> A ``` **High-Yield:** LDH is the **only enzyme** that catalyzes lactate ↔ pyruvate interconversion. It is NAD^+^-dependent and reversible. **Clinical Pearl:** Elevated serum LDH and lactate occur in tissue hypoxia, sepsis, and malignancy — conditions where anaerobic metabolism increases lactate production. ### Why LDH, Not Other Enzymes? - **Lactate oxidase:** Does not exist in human metabolism; lactate is not oxidized directly to CO₂ - **Pyruvate kinase:** Catalyzes PEP → pyruvate (glycolysis), not lactate → pyruvate - **Pyruvate dehydrogenase:** Converts pyruvate → Acetyl-CoA (enters TCA cycle), not lactate metabolism [cite:Lehninger Principles of Biochemistry Ch 14]