An 18-month-old boy with repeated episodes of vomiting, lethargy, irritability, acidosis, and hypoglycemia from the age of 2 months was proved to have a variant form of maple syrup urine disease (MSUD). Amino acid load tests for leucine, isoleucine, valine, and phenylalanine were performed and compared to similar tests in a child with classic MSUD; maximum serum concentrations during tolerance studies in our patient were 24, 16, 26, and 18 mg/dl, respectively. All amino acid challenges were tolerated well except for valine. Within one hour after valine administration the child developed symptoms of lethargy, vomiting, ataxia, irritability, and hypoglycemia. Cortisol, growth hormone, insulin, and glucagon response to hypoglycemia were normal. Filbroblasts studies on cultured cells from our patient demonstrated abnormally low rates of 14CO2 production from both 14C-labeled leucine and 14C-labeled valine. Decarboxylation with valine as substrate was significantly less than the decarboxylation with leucine. A proteinrestricted diet was started but he continued to have occasional symptomatic episodes of hyperaminoacidemia and at the age of 4 years died during one of these episodes. Results of neuropathologic studies differed from those reported in the literature for classic MSUD disease.
Evidence for a catabolic role of glucagon during an amino acid load.
