Thiamin-responsive maple-syrup-urine disease: decreased affinity of the mutant branched-chain alpha-keto acid dehydrogenase for alpha-ketoisovalerate and thiamin pyrophosphate.

1. Comparisons of the activity and kinetics of the branched-chain 2-oxo acid dehydrogenase in cultured skin fibroblasts from normal and classical maple-syrup-urine-disease (MSUD) subjects provide a kinetic explanation for the enzyme defect. 2. In the intact cell assays, normal fibroblasts demonstrated hyperbolic kinetics with 3-methyl-2-oxo[1-14C]butyrate as a substrate. Intact fibroblasts from four classical MSUD patients showed no decarboxylation over a substrate concentration range of 0.25 to 5.0 mM, and thiamin (4 mM) was without effect. 3. The overall reaction of the multienzyme complex was efficiently reconstituted by using a disrupted-cell system. Normals again showed typical hyperbolic kinetics at the 2-oxo acid concentrations of 0.1 to 5 mM. The Vmax. and apparent Km values were 0.10 +/- 0.02 m-unit/mg of protein and 0.05-0.1 mM respectively, with 3-methyl-2-oxobutyrate. In contrast, classical MSUD patients exhibited sigmoidal kinetics (Hill coefficient, 2.5) with activity approaching 40-60% of the normal value at 5 mM substrate. The K0.5 values from the Hill plots for MSUD patients were 4-7 mM. 4. The E1 (branched-chain 2-oxo acid decarboxylase) component of the multienzyme complex was measured in disrupted-particulate preparations. Normals again showed hyperbolic kinetics with the 2-oxo acid, whereas MSUD preparations exhibited sigmoidal kinetics with the activity of E1 strictly dependent on substrate concentration. Apparent Km or K0.5 were 0.1 and 1.0 mM for normal and MSUD subjects respectively. 5. Measurements of E2 (dihydrolipoyl transacylase) and E3 (dihydrolipoyl dehydrogenase) in MSUD preparations showed them to be in the normal range. 6. The above data suggest a defect in the E1 step of branched-chain 2-oxo acid dehydrogenase in classical MSUD patients.

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Deficiency of the E1β subunit in the branched-chain α-keto acid dehydrogenase complex due to a single base substitution to the intron 5, resulting in two alternatively spliced mRNAs in patient with maple syrup urine disease

Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease ◽

10.1016/0925-4439(94)90013-2 ◽

1994 ◽

Vol 1225 (3) ◽

pp. 317-325 ◽

Cited By ~ 13

Author(s):

Y HAYASHIDA ◽

H MITSUBUCHI ◽

Y INDO ◽

K OHTA ◽

F ENDO ◽

...

Keyword(s):

Maple Syrup Urine Disease ◽

Base Substitution ◽

Keto Acid ◽

Maple Syrup ◽

Single Base ◽

Alternatively Spliced ◽

Urine Disease ◽

Branched Chain ◽

Acid Dehydrogenase Complex ◽

Dehydrogenase Complex

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Branched Chain Amino Acids Induce Apoptosis in Neural Cells without Mitochondrial Membrane Depolarization or Cytochromec Release: Implications for Neurological Impairment Associated with Maple Syrup Urine Disease

Molecular Biology of the Cell ◽

10.1091/mbc.11.5.1919 ◽

2000 ◽

Vol 11 (5) ◽

pp. 1919-1932 ◽

Cited By ~ 82

Author(s):

Philippe Jouvet ◽

Pierre Rustin ◽

Deanna L. Taylor ◽

Jennifer M. Pocock ◽

Ursula Felderhoff-Mueser ◽

...

Keyword(s):

Amino Acids ◽

Cytochrome C ◽

Maple Syrup Urine Disease ◽

Mitochondrial Membrane ◽

Branched Chain Amino Acids ◽

Keto Acid ◽

Maple Syrup ◽

Mitochondrial Membrane Depolarization ◽

Urine Disease ◽

Branched Chain

Maple syrup urine disease (MSUD) is an inborn error of metabolism caused by a deficiency in branched chain α-keto acid dehydrogenase that can result in neurodegenerative sequelae in human infants. In the present study, increased concentrations of MSUD metabolites, in particular α-keto isocaproic acid, specifically induced apoptosis in glial and neuronal cells in culture. Apoptosis was associated with a reduction in cell respiration but without impairment of respiratory chain function, without early changes in mitochondrial membrane potential and without cytochrome c release into the cytosol. Significantly, α-keto isocaproic acid also triggered neuronal apoptosis in vivo after intracerebral injection into the developing rat brain. These findings suggest that MSUD neurodegeneration may result, at least in part, from an accumulation of branched chain amino acids and their α-keto acid derivatives that trigger apoptosis through a cytochrome c-independent pathway.

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