Analysis of maple syrup urine disease in cell culture: use of substrates.

Branched-chain 2-oxo acid dehydrogenase activity in human skin fibroblasts against L-leucine, L-valine, L-isoleucine and derived 2-oxo acids was compared in incubations with 1 mmol/l of 1-14C-labelled substrate. The results suggested that the amino acids are the more suitable substrates for an estimation of decarboxylation activity in intact cells. In control cell lines (n = 12), 14CO2 release from amino acids was highest for valine and least for leucine. In a representative number of fibroblast strains of patients with different forms of MSUD (n = 11; residual decarboxylation activity 2-60% of the controls), 14CO2 release from the different amino acids was reduced to a similar degree. Additional measurement of 2-oxo[1-14C]acid release suggested that substrate supply to the branched-chain 2-oxo acid dehydrogenase complex was not rate limiting in the cell lines under investigation.