Aspartate aminotransferase (AST) is released from hepatocytes and cardiac myocytes after cellular injury. AST requires pyridoxal-5'-phosphate (PLP) and catalyzes: Aspartate + alpha-ketoglutarate ↔ Oxaloacetate + Glutamate. In severe liver disease, AST activity in serum may be falsely low despite extensive hepatocyte necrosis. Which mechanism explains this paradox?

• A Advanced cirrhosis reduces hepatocyte mass so fewer cells can release AST
• B Severe hepatic failure causes profound vitamin B6 deficiency, reducing PLP-bound AST holoenzyme content in residual hepatocytes ✓
• C In terminal liver failure, catabolic enzymes degrade AST faster in the circulation
• D End-stage liver disease impairs hepatocyte AST gene transcription, reducing enzyme synthesis

Explanation

AST (like ALT) exists as an apoenzyme (inactive, without PLP) and a holoenzyme (active, PLP-bound). In severe liver disease, hepatic vitamin B6 stores are depleted because the liver is the major organ for pyridoxal phosphate synthesis and storage. With inadequate PLP, a greater proportion of AST in remaining hepatocytes exists as the apoenzyme (inactive form). When standard enzymatic assay measures AST activity in serum samples without added PLP, the apoenzyme contributes minimally. Consequently, measured AST activity may appear disproportionately low relative to actual hepatocyte destruction. Assays using PLP-supplemented substrates yield higher (more accurate) AST values in this setting. This is a well-recognized pitfall in interpreting liver enzymes in end-stage liver disease.

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

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Written and medically reviewed by the StethoPrep medical team.