Lactate dehydrogenase (LDH) consists of two subunit types: H (heart) and M (muscle). The LDH1 isoenzyme (H4) has higher affinity for lactate than LDH5 (M4). The CLINICAL implication of this difference is:

• A LDH1 is inhibited by pyruvate at high concentrations, so heart preferentially oxidises lactate to pyruvate for energy
• B LDH1 (H4) has lower Km for lactate and low Ki for pyruvate inhibition, making it ideal for cardiac muscle to oxidise lactate in aerobic conditions ✓
• C LDH5 is inhibited by pyruvate and converts lactate to pyruvate in anaerobic muscle
• D LDH5 (M4) is preferentially expressed in cardiac muscle for efficient pyruvate reduction

Explanation

The heart is aerobic and preferentially oxidises lactate taken from blood. LDH1 (H4) has higher affinity (lower Km) for lactate and is inhibited by excess pyruvate (product inhibition) — this prevents futile cycling and ensures net pyruvate formation in an oxygen-rich environment. LDH5 (M4) in skeletal muscle has lower affinity for lactate and is not inhibited by high pyruvate, favouring pyruvate → lactate direction during anaerobic glycolysis. LDH1 elevation is characteristic of myocardial infarction (and haemolysis).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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