Nutritional control of branched chain alpha-ketoacid dehydrogenase in rat hepatocytes.

Branched chain alpha-ketoacid dehydrogenase (EC 1.2.4.4) complex, the rate-limiting enzyme of branched chain amino acid catabolism in most tissues, is subject to regulation by covalent modification, with phosphorylation inactivating and dephosphorylation activating the complex. The enzyme complex from liver of chow-fed rats is mainly in the active form but that from liver of rats fed a low-protein diet is mainly in the inactive form. Isolated hepatocytes were used to identify factors that affect interconversion of branched chain alpha-ketoacid dehydrogenase. The enzyme present in hepatocytes of rats fed a low-protein diet appears much more responsive to regulation by covalent modification than the branched chain alpha-ketoacid dehydrogenase present in hepatocytes of normal chow-fed rats. alpha-Chloroisocaproate, a specific inhibitor of the kinase responsible for phosphorylation and inactivation of the complex, greatly stimulates oxidation of alpha-keto[1-14C]isovalerate by hepatocytes prepared from rats fed a low-protein diet but not from normal chow-fed rats. Oxidizable substrates are also much more effective inhibitors of branched chain alpha-ketoacid oxidation with hepatocytes from rats fed a low-protein diet than from normal chow-fed rats. Activity measurements with cell-free extracts suggest that changes in flux through the dehydrogenase with intact hepatocytes prepared from rats fed a low-protein diet are explained in large part by changes in the proportion of the enzyme in the active, dephosphorylated form. Regulation of liver branched chain alpha-ketoacid dehydrogenase by covalent modification functions to conserve branched chain amino acids for protein synthesis during periods of restricted dietary protein intake.