An infant born to a 25-year-old female develops vomiting and lethargy several days after birth. Physical examination reveals hypertonicity and muscle rigidity. The mother also notices an odor of burnt sugar in her diapers. Which of the following amino acids should most likely be restricted in this infant’s diet:
Ans. C. Leucine
The odor of urine in maple syrup urine disease (branched-chain ketonuria, or MSUD) suggests maple syrup, or burnt sugar. The biochemical defect in MSUD involves the α-ketoacid decarboxylase complex.Plasma and urinary levels of leucine, isoleucine, valine, and their α-keto acids and α-hydroxy acids (reduced α-keto acids) are elevated, but the urinary keto acids derive principally from leucine. Signs and symptoms of MSUD often include fatal ketoacidosis, neurological derangements, mental retardation, and a maple syrup odor of urine. The mechanism of toxicity is unknown. Early diagnosis by enzymatic analysis is essential to avoid brain damage and early mortality by replacing dietary protein by an amino acid mixture that lacks leucine, isoleucine, and valine. The molecular genetics of MSUD are heterogeneous. MSUD can result from mutations in the genes that encode E1α, E1β, E2, and E3. Based on the locus affected, genetic subtypes of MSUD are recognized. Type IA MSUD arises from mutations in the E1α gene, type IB in the E1β gene, type II in the E2 gene, and type III in the E3 gene.