Purification, resolution, and reconstitution of rat liver branched-chain alpha-keto acid dehydrogenase complex

Branched-chain alpha-keto acid dehydrogenase (BCKADH) was solubilized as an enzyme complex from rat liver mitochondria by sonic treatment. Dehydrogenase (E1) and dihydrolipoyltransacylase (E2) components of the complex were purified in an associated form and resolved into individual components in the presence of 1 M NaCl, while lipoamide dehydrogenase (E3) component was dissociated from the complex during purification. Analysis by gel electrophoresis in dodecyl sulfate revealed the E1 comprised two different subunits with apparent molecular weights of 36,000 and 45,500, presumably in an equal molar ratio, while E2 consisted of a single subunit with an apparent molecular weight of 51,000. The BCKADH complex was reconstituted by combining E1, E2, and E3, and the formation of the complex was confirmed by analysis by sucrose density gradient centrifugation. The reconstituted enzyme complex oxidized not only alpha-ketoisovalerate (KIV), alpha-ketoisocaproate (KIC), and alpha-keto-beta-methylvalerate (KMV), but also pyruvate and alpha-ketoglutarate. Apparent Km values were 10-12 microM for the branched-chain alpha-keto acids, 2.2 mM for pyruvate, and 2.5 mM for alpha-ketoglutarate.