Alanine and inter-organ relationships in branched-chain amino and 2-oxo acid metabolism |
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| Authors: | T. Norman Palmer Margaret A. Caldecourt Keith Snell Mary C. Sugden |
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| Affiliation: | (1) Department of Biochemistry, Charing Cross and Westminster Medical School, Fulham Palace Road, W6 8RF London, UK;(2) Department of Biochemistry, University of Surrey, GU2 5XH Guildford, Surrey, UK;(3) Department of Chemical Pathology, London Hospital Medical College, Turner Street, E1 2AD London, UK |
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| Abstract: | Branched-chain amino acid metabolism in skeletal muscte promotes the production of alanine, an important precursor in hepatic gluconeogenesis. There is controversy concerning the origin of the carbon skeleton of alanine produced in muscle, specifically whether it is derived from carbohydrate via glycolysis (the glucose-alanine cycle) or from amino acid precursors (viz. glutamate, valine, isoleucine, methionine, aspartate, asparagine) via a pathway involving phosphoenolpyruvate (PEP) carboxykinase and pyruvate kinase, or NADP-malate dehydrogenase (malic enzyme). The relevant literature is reviewed and it is concluded that neogenic flux from amino acids is unlikely to be of major quantitative importance for provision of the carbon skeleton of alanine either in vitro or in vivo. Evidence is presented that branched-chain amino acid oxidation in muscle is incomplete and that the branched-chain 2-oxo acids and the products of their partial oxidation (including glutamine) are released. The role of these metabolites is discussed in the context of fuel homeostasis in starvation. |
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