L-Alloisoleucine is a branched-chain amino acid and diastereomer of L-isoleucine.1 It is formed from L-isoleucine by transamination.2 Plasma L-alloisoleucine levels are increased in patients with maple syrup urine disease (MSUD), an inborn error of metabolism characterized by a deficiency in the branched-chain α-ketoacid dehydrogenase complex, the complex that catalyzes the degradation of branched-chain amino acids, leading to the accumulation of branched-chain amino acids in the plasma and urine, a maple syrup-like odor in the urine, and neurological impairments.1
1.Schadewaldt, P., Bodner-Leidecker, A., Hammen, H.W., et al.Significance of ?-alloisoleucine in plasma for diagnosis of maple syrup urine diseaseClin. Chem.45(10)1734-1740(1999) 2.Schadewaldt, P., Bodner-Leidecker, A., Hammen, H.W., et al.Formation of L-alloisoleucine in vivo: An L-[13C]isoleucine study in manPediatr. Res.47(2)271-277(2000)