Maple syrup urine disease: Branched-chain amino acid concentrations and metabolism in cultured human lymphoblasts

The intracellular concentration of free leucine, isoleucine, and valine and their metabolism were studied in lymphoblast cultures established from peripheral blood of an individual with maple syrup urine disease (MSUD) and a control subject. Branched-chain -keto acid decarboxylase activity in the MSUD cells was 10% or less of the control value as measured by the ability of the cells to release ¹⁴CO₂ from the corresponding [1-¹⁴C]labeled branched-chain amino acid. The intracellular concentrations of free leucine and isoleucine were increased three-fold in MSUD lymphoblasts as compared to control cells. Free valine was present in only trace amounts of less than 0.1 mMin both cell lines. Exposure of normal and mutant cells to a 10 mMload of leucine, isoleucine, and valine resulted in a comparable concentration within cells after 24 hr. Concentrations returned to base values in normal cells 12 hr after removal of load, but leucine remained elevated in MSUD cells after 3 days. Leucine and its keto acid, -ketoisocaproic acid, added to the culture medium gave significant growth inhibition of MSUD lymphoblasts but not of normal cells, in the millimolar range. Isoleucine, valine, and their keto acids had no effect.This investigation was supported in part by Grants AM-13622, AM-05646, and GM-17702 from the United States Public Health Service, Veterans Administration Grant M.R.I.S. No. 3181 to Dr. Nathan Gochman, and grants from the National Foundation and the Kroc Foundation. S. D. S. is a Postdoctoral Research Fellow supported by United States Public Health Service Training Grant AM-05646.