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The Biochemistry and Physiology of Protein and Amino Acid Metabolism, with Reference to Protein Nutrition

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The Biochemistry and Physiology of Protein and Amino Acid Metabolism, with Reference to Protein Nutrition Protein Metabolism During Infancy, edited by Niels C. R. Raiha. Nestle Nutrition Workshop Series, Vol. 33. Nestec Ltd., Vevey/ Raven Press, Ltd., New York © 1994. The Biochemistry and Physiology of Protein and Amino Acid Metabolism, with Reference to Protein Nutrition Vernon R. Young, Antoine E. El-Khoury, Melchor Sanchez, and Leticia Castillo Laboratory of Human Nutrition, School of Science and Clinical Research Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA All of the relevant topics that could be included under the title of this chapter would cover an extraordinary range of biological knowledge; more than is possible to include in an article of restricted length. Because of this and because of our own limited area of understanding, we have adopted the strategy of covering a few selected areas without attempting to be comprehensive; further elaboration of some of these areas will be made in later chapters. The emphasis to be given here is toward physiol- ogy rather than biochemistry, since our own interests relate to the flows of metabo- lites through pathways and the impact of nutritional and other factors that affect these, as well as the in vivo mechanisms involved. A particular biochemical focus would involve giving more attention to the detailed cellular and subcellular pathways responsible for the formation of proteins and amino acids, their movement to particu- lar sites, and their interconversion and degradation, together with details of the molec- ular mechanisms by which these processes take place. Our objective is to attempt to provide a better understanding of the metabolic basis of the protein and amino acid requirements under various pathophysiological conditions. Since amino acids are the currency of the nitrogen and protein economy of the host, this introductory chapter begins with a statement about them, particularly those of quantitative dietary and nutritional significance. We shall then discuss the major metabolic systems responsible for the utilization of dietary amino acids and the main- tenance of body protein and amino acid homeostasis. THE NUTRITIONALLY RELEVANT AMINO ACIDS There are hundreds of amino acids in nature (1) but only approximately 20 of these appear in proteins via charging by cognate tRNA with subsequent recognition of a codon on the mRNA, as for example a UGA codon which was identified recently 1 2 PROTEIN AND AMINO ACID METABOLISM for the uncommon amino acid, selenocysteine, which is made from serine on a unique tRNA (2). Other amino acids, such as hydroxyproline, hydroxylysine, NT-methylhis- tidine, and the rare amino acids 3-hydroxyaspartic acid and 3-hydroxyasparagine, to mention a few examples, are also present in proteins due to post-translational modification of specific amino acid residues. It is, however, these 20 amino acids, together with a few others not in peptide-bound form, such as ornithine, citrulline, and taurine, that are of physiologic and quantitative importance in the nitrogen econ- omy and nutrition of the mammalian organism. Three amino acids, tryptophan, lysine, and histidine, had been shown to be indis- pensable components of the diet in the growing rat by 1932; once an abundant supply of threonine became possible all of the dietary amino acids that were necessary for rat growth could be identified, and in 1948 Rose and his co-workers (e.g., see 3) had determined that these were valine, leucine, isoleucine, methionine, threonine, lysine, phenylalanine, tryptophan, histidine, and arginine. Further, by the middle of the twentieth century evidence had been obtained showing that except for histidine and arginine, these same amino acids were essential for establishment and preservation of body nitrogen equilibrium in human adults. A "final" classification of the dietary amino acids, according to their role in the maintenance of nitrogen equilibrium in healthy young men, was made by Rose et al. (3) and this is reproduced in Table 1 under the columns headed "1954." Research subsequent to the impressive series of metabolic balance studies by Rose and his co-workers has indicated that this earlier nutritional classification of amino acids is no longer satisfactory. Thus histidine is now considered to be an essential (indispensable) amino acid (4), and the amino acids glycine, cystine, tyrosine, proline, and arginine, together with glutamine and taurine, are more appropriately defined as "conditionally indispensable." In particular, active TABLE 1. Changing view of the role of amino acids in human nutrition Conditionally Indispensable (essential) indispensable Dispensable (non-essential) 1954a Present* Present" 1954s Present" Valine Valine Glycine Glycine Isoleucine Isoleucine Cystine Cystine Leucine Leucine Glutamine Glutamic acid Glutamic acid Lysine Lysine Tyrosine Tyrosine Methionine Methionine Proline Proline Phenylalanine Phenylalanine Arginine Arginine Threonine Threonine Taurine Alanine Alanine Tryptophan Tryptophan Serine Serine Histidine Aspartic acid Aspartic acid Histidine Asparagine Hydroxyproline Citrulline • Based on "Final classification" by Rose et al. (3). * A current interpretation. PROTEIN AND AMINO ACID METABOLISM 3 areas of current research in amino acid metabolism and nutrition focus heavily on the role played by these various amino acids in the maintenance of organ and body protein homeostasis and function under various pathophysiological conditions. For example, an exciting and relatively recent development in this context is the role of arginine as a precursor of nitric oxide, which functions in cellular communication, signal transduction, and in defense against invading microorganisms (5). The non- essential (dispensable) and conditionally indispensable amino acids probably will continue to attract increasing research attention, particularly in terms of the potential therapeutic benefits to be achieved by manipulating their intakes and/or tissue avail- ability in different disease states. Amino Acid Functions The nutritionally relevant amino acids (Table 1) serve multiple, diverse functions. Some of these are listed in Table 2, together with those functions met by the proteins TABLE 2. Some functions of amino acids and their products3 Function Example Amino acids Substrates for protein synthesis Those for which there is a codon Regulators of protein turnover Leucine, arginine Regulators of enzyme activity Arginine and NAG synthetase; Phe and (allosteric) PAH activation Precursor of signal transducer Arginine and nitric oxide Methylation reactions Methionine Neurotransmitter Tryptophan (serotonin); glutamate Ion fluxes Taurine; glutamate Precursor of "physiologic" molecules Arg (creatinine); glut(NH2)-purines Transport of nitrogen Alanine; glut(NH2) Oxidation-reduction properties Cystine; glutathione Precursor of conditionally indispensable Methionine (cys); phe (tyr) amino acids Gluconeogenic substrate and fuel Alanine; serine; glut(NH2) Proteins Enzymatic catalysis BCKADH Transport B-12 binding proteins; ceruloplasmin; apolipoproteins Messenger/signals Insulin; growth hormone Movement Kinesin; actin Structure Collagens; elastin Storage/sequestration Ferritin; metallothionein Immunity Antibodies; TNF; interleukins Growth; differentiation; gene EGF; IGFs; transcription factors expression * PAH, phenylalanine hydroxylase; BCKADH, branched-chain ketoacid dehydrogenase; TNF, tumor necrosis factor; NAG, /v-acetylglutamate. 4 PROTEIN AND AMINO ACID METABOLISM which are elaborated from the amino acid pools. The point we should make is that in the course of carrying out their physiologic and functional roles, the proteins and amino acids turn over and part of their nitrogen and carbon is lost via the excretory pathways, including CO: in expired air and urea and ammonia in urine. To maintain an adequate body protein and amino acid status, these losses must be balanced by an appropriate dietary supply of (a) a utilizable source of nitrogen to support cell and organ function and (b) the indispensable amino acids, and under specific states the "conditionally indispensable" amino acids, to replace those that are lost during their daily metabolic transactions and/or deposited in new tissues during growth, repletion, and/or secretion via milk during lactation. It is relevant, therefore, to con- sider the nature of the systems that underlie these dietary needs and the changes in these systems when physiological and/or pathological factors intervene. THE MAJOR SYSTEMS The principal metabolic systems responsible for the maintenance of body protein and amino acid homeostasis are shown in Fig. 1. They are: protein synthesis, protein degradation, amino acid oxidation and urea production, and amino acid synthesis, in the case of the nutritionally dispensable or conditionally indispensable amino acids. Changes in the rates of these systems lead to an adjustment in nitrogen balance and retention, the direction and extent of which depend on the factor(s) responsible for initiating an effect. For purposes of this brief introductory review, the biochemical and physiological processes that might be highlighted are schematically outlined in Fig. 2; other than gene transcription, which will not be discussed in detail here, these events include the translational phase of protein synthesis, post-translational events, particularly protein degradation, and subsequent intra- and intercellular transport of amino acids, amino acid oxidation, the production of urea, and the formation of conditionally indispensable and dispensable amino
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