Isozymes of lactate dehydrogenase (LDH) are tetramers of H and M subunits. LDH-1 (HHHH) predominates in cardiac muscle and erythrocytes. How does the biochemical property of LDH-1 differ from LDH-5 (MMMM, liver/skeletal muscle), making LDH-1 better suited to aerobic tissue?

• A LDH-1 has lower Km for pyruvate (inhibited by pyruvate at high concentrations), favouring lactate oxidation in aerobic conditions; LDH-5 has high affinity for pyruvate, converting it efficiently to lactate in anaerobic tissue ✓
• B LDH-1 has higher Km for pyruvate, favouring lactate oxidation (lactate → pyruvate) in well-oxygenated tissue
• C LDH-1 is not inhibited by pyruvate, allowing unrestricted lactate production in cardiac muscle
• D LDH-1 uses NADP+ while LDH-5 uses NAD+, allowing differential coupling to metabolic pathways

Explanation

LDH-1 (cardiac/brain) has high affinity for its substrates but is significantly inhibited by high pyruvate concentrations (substrate inhibition). In well-oxygenated aerobic tissue where pyruvate is rapidly entering the TCA cycle (low cytoplasmic pyruvate), LDH-1 functions in the reverse direction (lactate → pyruvate), using lactate (from circulation or anaerobic metabolism) as a fuel. LDH-5 (liver/skeletal muscle) has lower affinity for pyruvate and is not inhibited by it, efficiently converting high pyruvate concentrations to lactate during intense anaerobic exercise. Elevated LDH-1 (cardiac form) in serum with LDH-1 > LDH-2 ('flipped' ratio) is historically diagnostic of myocardial infarction.

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

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