Catabolism of the branched-chain amino acids (BCAAs: leucine, isoleucine, and valine) is regulated by the branched-chain alpha-ketoacid dehydrogenase (BCKDH) complex, which in turn is regulated by phosphorylation catalyzed by BCKDH kinase (BDK). Thiamine pyrophosphate (TPP) is required as a coenzyme for the E1 component of the BCKDH complex and can also bring about activation of the complex by inhibiting BDK. The present study shows that free Ca2+ in the physiological range greatly increases the sensitivity of BDK to inhibition by TPP (IC50 of 2.5 mu M in the presence of 1 mu M free Ca2+). This novel mechanism may be responsible for the stimulation of BCAA oxidation by conditions that increase mitochondrial free Ca2+ levels, e.g. in skeletal muscle during exercise. (C) 2018 Elsevier Inc. All rights reserved.