The overall objective is to establish the mechanisms for regulation of branched-chain amino acid catabolism. Catabolism of these essential amino acids (leucine, valine and isoleucine) is altered by various physiological conditions: diabetes, fasting, obesity and trauma. They also appear to affect various biological processes: protein synthesis and degradation, insulin and prolactin release, and possibly the synthesis of certain neurotransmitters. Branched-chain amino acid catabolism is generally regulated by the activity of an intramitochondrial multienzyme complex, branched-chain alpha-ketoacid dehydrogenase (BCKDH). The inherited lack or diminished activity of this complex is called Maple Syrup Urine Disease. The activity of this complex is regulated by product inhibition and by a reversible phosphorylation (inactivation) mechanism. The degree of phosphorylation or inactivation of the complex is tissue-specific and appears to be regulated, at least partially, by the amount of dietary protein and the branched-chain alpha-ketoacids. There are two phosphorylation sites of which one appears to directly regulate inactivation. Studies will be conducted to: (1) isolate the specific phosphatase and kinase, characterize their specificity for different phosphorylation sites, and identify possible physiological regulators; (2) determine the reaction mechanism and the influence of phosphorylation on the kinetic parameters of BCKDH; (3) determine regulation of activity and phosphorylation of the different phosphorylation sites of BCKDH in intact mitochondria and the influence of the insulin factor from plasma membranes on this regulation; (4) determine the BCKDH activity and phosphorylation state in several tissues and identify possible physiological signals that alter these parameter; (5) determine the influence of various physiological conditions on the transcription of mRNA coding for the different subunits of BCKDH and possible regulation of the transport and subsequent processing of the translation product of the mRNA; and (6) determine the influence of dietary protein and developmental state on the activity and regulation of BCKDH in the fetus and newborn rat. These studies should establish a greater understanding of the regulation of branched-chain amino acid catabolism and possibly elucidate the mechanism of altered catabolism seen with various physiological conditions.
| Paxton, R; Sievert, L M (1991) An improved assay for pyruvate dehydrogenase in liver and heart. Biochem J 277 ( Pt 2):547-51 |
| Paxton, R; Harris, R A; Sener, A et al. (1988) Branched chain alpha-ketoacid dehydrogenase and pyruvate dehydrogenase activity in isolated rat pancreatic islets. Horm Metab Res 20:317-22 |
| Paxton, R (1988) Branched-chain alpha-keto acid dehydrogenase and its kinase from rabbit liver and heart. Methods Enzymol 166:313-20 |
