Transamination and oxidative decarboxylation rates of branched-chain 2-oxo acids in cultured human skin fibroblasts.

Transamination and oxidative decarboxylation of branched-chain L-amino acid derived 2-oxo acids in cultured human skin fibroblasts from normal subjects and from a patient with maple syrup urine disease (variant form) were comparatively studied in incubations with 1-14C-labeled substrates (1 mmol/liter). With normal cells, 14CO2 release ranged from about 11 to 3 nmol/90 min/mg of cell protein in the order 3-methyl-2-oxo[14C]butanoate greater than (S)-3-methyl-2-oxo[14C]pentanoate greater than 4-methyl-2-oxo[14C]pentanoate greater than (R)-3-methyl-2-oxo[14C]pentanoate. Formation of the corresponding branched-chain amino[14C] acids was substantially higher than 14CO2 production (around 10-fold) and similar with L-valine, L-isoleucine, and L-leucine. L-Allo-isoleucine production [from (R)-3-methyl-2-oxopentanoate] was significantly lower. With maple syrup urine disease fibroblasts, comparable transamination rates were observed. Related to the findings with normal cells, 14CO2 release from each substrate was differently reduced and apparent residual branched-chain 2-oxo acid dehydrogenase complex activity with 3-methyl-2-oxobutanoate, 4-methyl-2-oxopentanoate, (S)-, and (R)-3-methyl-2-oxopentanoate amounted to 12, 13, 22, and 50%, respectively.