Identification of the mitochondrial branched chain aminotransferase as a branched chain alpha-keto acid transport protein.

Conditions were developed which optimized reconstitution of branched chain alpha-keto acid transport activity, which was measured as alpha-ketoisocaproate (KIC) transport, and pyruvate transport activity. Reconstitutable KIC transport activity was about 40-fold higher in heart than in liver mitochondrial extracts and 40-fold higher than heart pyruvate transport activity but only about 7-fold higher than liver pyruvate transport activity. A purification procedure was developed for the branched chain alpha-keto acid and pyruvate transport proteins which resulted in partial purification of the proteins from rat heart mitochondria. Pyruvate transport activity appeared to be associated with a 32.5-kDa protein, whereas KIC transport activity appeared to be associated with two proteins around 41 kDa. As shown by immunoblotting and immunoaffinity chromatography, these proteins were recognized by an antiserum raised against purified rat heart mitochondrial branched chain aminotransferase (BCATm). Procedures used to extract BCATm from mitochondria effectively solubilized KIC transport activity, and both transaminase and transport activities in the sonicate supernatant were immunoprecipitated by BCATm antiserum. The tissue distribution of reconstitutable KIC transport activity was identical with the tissue distribution of BCATm. On the other hand, the distribution of reconstitutable pyruvate transport activity was distinct from that of KIC transport and branched chain aminotransferase activities, and pyruvate transport activity could not be immunoprecipitated by BCATm antiserum. When incorporated into phospholipid vesicles, purified BCATm exhibited branched chain alpha-keto acid transport activity but did not transport pyruvate. Transport was saturable, and Km values for KIC and alpha-ketoisovalerate uptake were 10 and 25 microM, respectively. Substrate competition experiments indicated that KIC transport could be inhibited substantially by branched chain alpha-keto acids and their derivatives but not by substrates for the pyruvate transporter. Transport was also inhibited by several aromatic carboxylic acid derivatives and alpha-ketoglutarate, but not by N-butylmalonate and succinate. Studies with covalent protein-modifying reagents indicated that transport was inhibited by sulfhydryl reagents, the histidine reagent diethyl pyrocarbonate, and the tyrosine reagent N-acetylimidazole. When BCATm was incorporated into phospholipid vesicles, pyridoxal 5-phosphate was an inhibitor of transport (75% inhibition at 10 mM) but had little effect on aminotransferase activity. The data indicate BCATm is a bifunctional protein catalyzing branched chain amino acid transamination and branched chain alpha-keto acid transport. The transport properties of BCATm suggest that this protein may be the branched chain alpha-keto acid transporter that was originally identified and characterized kinetically in isolated rat heart mitochondria (Hutson, S.M., and Rannels, S.L. (1985) J. Biol. Chem. 260, 14189-14193).