A patient on valproate for epilepsy develops thrombocytopenia and elevated serum ammonia without liver failure. Which mechanism best explains the hyperammonemia?

• A Valproate inhibits arginase in the urea cycle directly through competitive inhibition
• B Valproate-induced hepatotoxicity reduces hepatic uptake of ammonia from portal blood
• C Valproate causes thiamine deficiency, impairing pyruvate dehydrogenase and diverting amino acids toward ammoniagenesis
• D Valproate inhibits carbamoyl phosphate synthetase-1 and causes secondary carnitine deficiency, impairing the urea cycle ✓

Explanation

Valproate-induced hyperammonemia without overt hepatotoxicity occurs through two synergistic mechanisms. First, valproate inhibits N-acetylglutamate synthase (NAGS) and carbamoyl phosphate synthetase-1 (CPS1), the first enzyme of the urea cycle, thereby impairing ammonia detoxification. Second, valproate and its metabolites are metabolized by hepatic mitochondrial beta-oxidation and sequester CoA and carnitine, inducing a functional carnitine deficiency that further impairs fatty acid oxidation and mitochondrial energy metabolism needed for the urea cycle. L-carnitine supplementation can ameliorate valproate-induced hyperammonemia. Thrombocytopenia is a dose-dependent effect of valproate on megakaryopoiesis.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.