Modeling of Branched Chain Amino Acid Metabolism

Spitzer, John

Abstract

The branched chain amino acids play a role in the regulation of protein turnover, and carbohydrate and muscle energy metabolism. Physiological and pathological conditions such as food deprivation diabetes, trauma, and clofibrate treatment increase oxidation of the branched chain amino acids in muscle. The potential importance of these observations is underscored by clinical results which show that branched chain amino acid infusin in patients suffering from trauma or burns markedly reduces negative nitrogen balance. Branched chain ketoacid dehydrogenase (BCKD) is thought to be a major rate-controlling step in the degradation of the branched chain amino acids. However, the extent to which it actually determines flux through this pathway probably varies depending upon the metabolic state and the tissue involved. The physiological modulators of branched chain amino acid metabolism are also not known. Thus the overall goal of this proposal is to create of a model for the regulation of BCKD activity via allosteric modulators and changes in phosphorylation state and to evaluate its role in the regulation of branched chain amino acid degradation. The major innovations in this proposal are the use of metabolic control theory and computer simulation techniques as a means of both formulating and testing hypotheses. a mathematical model of the regulation of BCKD is developed and then metabolic control theory is applied to evaluate the regulatory significance of various modulators and potential of this enzyme to control flux in various tissues. In order to carryout this plan, the potential modulators and kinetic characteristics of BCKD and its interconverting enzymes, BCKD phosphatase and BCKD kinase must be determined. Computer simulation and metabolic control theory will be used to model the system to predict the behavior of BCKD complex in the presence of various modulators. The model will be tested using mitochondria incubated in media in which the particular modulator concentrations are controlled. This system will also be used to examine the control of flux by BCKD. Using computer simulation to make quantitative predictions along with experimental verification of model parameters, it will be possible to verify even subtle regulatory mechanisms in complex (realistic) metabolic pathways in the cell. These experiments will result in a quantitative model of regulation which will be able to identify the rate-controlling steps and major regulatory modulators in the tissues which degrade branched chain amino acids and how they are altered by trauma and disease.