Branched‐chain L‐amino acid metabolism in classical maple syrup urine disease after orthotopic liver transplantation

We characterized the effect of orthotopic liver transplantation on the catabolism of branched‐chain L ‐amino acids in a female patient with classical form of maple syrup urine disease. Transplantation was performed at the age of 7.4 years due to a terminal liver failure triggered by a hepatitis A infection. Since then, the patient is on an unrestricted diet and plasma concentrations of branched‐chain L ‐amino and 2‐oxo acids are stable, yet at moderately increased levels (2‐ to 3‐fold of control). L ‐Alloisoleucine concentrations, however, remained remarkably elevated (>5‐fold of control). In vivo catabolism was investigated by measuring the metabolic L ‐alloisoleucine clearance and whole‐body leucine oxidation in the postabsorptive state. In an oral loading test with 580 μmol alloisoleucine per kg body wt, the L ‐alloisoleucine elimination rate constant (0.067 h −1 ) was in the normal range (0.069±0.012 h −1 , n =4). In an oral L ‐[1‐ 13 C]leucine load (38 μmol/kg body wt), 19.5% of the tracer dose applied was recovered in exhaled 13 CO 2 versus 18.9±3.6% in healthy subjects ( n =10). Thus, the patient exhibited obviously normal whole‐body catabolic rates although branched‐chain L ‐amino acid oxidation was confined to the liver transplant. Most likely, the enhanced substrate supply from extrahepatic sources led to an elevation of the plasma concentrations and thus induced a compensatory enhancement of the metabolic flux through the branched‐chain 2‐oxo acid dehydrogenase complex in the intact liver tissue.