Maple Syrup Urine Disease: Analysis of Branched Chain Ketoacid Decarboxylation in Cultured Fibroblasts

Abstract

Extract: Kinetic data are presented for the decarboxylation of branched chain α-ketoacids (BCKA) by intact human fibroblasts. Cultured cells of normal individuals and nine patients with different clinical pictures of maple syrup urine disease (MSUD) are studied with both α-ketoisocaproic acid (2-oxo-4-methylpentanoic acid (KIC)) and α-ketoisovaleric acid (2-oxo-3-methylbutanoic acid (KIVA)) as substrates. One normal cell strain and one patient cell strain is analyzed with α-keto-β-methyl-n-valeric acid (2-oxo-3-methylpentanoic acid (MEVA)) as a substrate. A biphasic degradation kinetic for each BCKA is obtained for normal control subjects. The component with higher substrate affinity is affected in MSUD: for KIC the normally hyperbolic substrate curve is changed to sigmoid shape, for KIVA and MEVA as substrates this component is not detectable at all. Considering qualitative aspects of the BCKA decarboxylation kinetics intact fibroblasts yield the same results as our recent studies with the decarboxylase moieties of partially purified kidney BCKA dehydrogenase of normal individuals and one patient with classic MSUD (27). The decarboxylation velocities for normal and patient fibroblasts with one exception differ widely at low but not at high substrate concentrations of BCKA. To get meaningful data on the residual substrate degradation activities with intact fibroblasts of different phenotypes of MSUD physiologically low substrate concentrations are recquired in the assay. Speculation: Kinetic studies of the decarboxylation reaction for branched chain α-ketoacids with fibroblasts of patients suffering from maple syrup urine disease could yield information about the type of biochemicalabnormality in MSUD.