A potent, heat-stable protein inhibitor of [branched-chain alpha-keto acid dehydrogenase]-phosphatase from bovine kidney mitochondria.

A heat- and acid-stable protein inhibitor of the [branched-chain alpha-keto acid dehydrogenase]-phosphatase was purified over 100,000-fold from extracts of bovine kidney mitochondria. The nearly homogeneous protein was recovered with a yield of 4-8%. The apparent molecular weight of the inhibitor is about 36,000. This protein is a noncompetitive inhibitor of the phosphatase, and the inhibitor constant (Ki) is about 0.13 nM. The inhibition was reversed 50% by about 1.3 mM Mg2+ and about 0.1 mM spermine. This protein inhibitor is different from the cytosolic protein phosphatase inhibitors 1 and 2.