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J Biol Chem. 1979 Jun 25;254(12):5522-6.

Purification and characterization of branched chain alpha-ketoacid dehydrogenase from bovine liver mitochondria.


Branched chain alpha-ketoacid dehydrogenase (EC was solubilized and purified from bovine liver mitochondria for the first time. Decarboxylation of alpha-ketoisovalerate, alpha-keto-beta-methylvalerate, and alpha-ketoisocaproate was catalyzed by this multienzyme complex and this activity was co-purified for each substrate. Three enzymatic functions were contained in the complex including decarboxylation of the above ketoacids, transacylation of their simple acid derivatives, and reduction of NAD+ as an overall reaction. Product stoichiometry of these three reactions was 1 CO2:1 acyl-CoA:1 NADH. Activity depended upon the addition of thiamin pyrophosphate, CoASH, and NAD+ which were dissociable cofactors. Physically, two active forms of the enzyme complex were found: a 275,000-dalton unit and a 2 x 10(6)-dalton component. Both showed a characteristic flavin spectra and catalyzed all functions of the complex, implying that 10 small units aggregated into the larger unit. The soluble complex as visualized by electron microscopy had a diameter ranging from 12 to 24 nm corresponding to a molecular weight of 2 x 10(6). The size of the native membrane-bound component remains to be determined.

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