Cori cycle physiology: which enzyme is central to interconverting lactate and pyruvate between muscle and liver during the Cori cycle?

Introduction / Context:
The Cori cycle coordinates energy metabolism between muscle and liver. Muscle produces lactate during intense activity; the liver converts lactate back to glucose that can be returned to muscle. This question targets the key enzyme enabling the lactate–pyruvate interconversion that underpins the cycle.

Given Data / Assumptions:

• Muscle operating anaerobically generates lactate from pyruvate.
• Liver performs gluconeogenesis using lactate as a substrate.
• Redox balance between NADH and NAD+ is critical in both tissues.

Concept / Approach:
Lactate dehydrogenase (LDH) catalyzes the reversible reaction: pyruvate + NADH + H+ ↔ lactate + NAD+. In muscle, LDH reduces pyruvate to lactate to regenerate NAD+ for glycolysis; in liver, LDH oxidizes lactate to pyruvate, which then enters gluconeogenesis.

Step-by-Step Solution:
1) Identify the transformation required by the cycle: lactate ↔ pyruvate.2) The enzyme that catalyzes this interconversion is LDH.3) In the liver, pyruvate proceeds to glucose via gluconeogenic enzymes; glucose returns to muscle through the bloodstream.

Verification / Alternative check:
Physiology texts consistently depict LDH at both ends of the cycle: lactate formation in muscle and lactate utilization in liver.

Why Other Options Are Wrong:
Glucokinase phosphorylates glucose in liver; G6PD functions in the pentose phosphate pathway; glucose isomerase interconverts glucose/fructose in industrial contexts; pyruvate kinase converts phosphoenolpyruvate to pyruvate in glycolysis but does not handle lactate.

Common Pitfalls:
Confusing LDH with pyruvate dehydrogenase; PDH converts pyruvate to acetyl-CoA, not lactate.

Final Answer:
Lactate dehydrogenase