Protein and Amino Acid Metabolism

CHAPTER 17 Protein and Amino Acid Metabolism In contrast to the case of lipids and carbohydrates, no spe- and myosin (Chapter 21) are released during catabolism cial storage forms of either the nitrogen or the amino acid of these proteins and are excreted in the urine. components of proteins exist. Dietary protein in excess Protein turnover is not completely efficient in the re- of the requirement is catabolized to provide energy and utilization of amino acids. Some are lost by oxidative ammonia, a toxic metabolite that is converted to urea in catabolism, while others are used in synthesis of non- the liver and excreted by the kidneys. All body proteins protein metabolites. For this reason, a dietary source of serve a specific function (e.g., structural, catalytic, trans- protein is needed to maintain adequate synthesis of pro- port, regulatory) and are potential sources of carbon for tein. During periods of growth, pregnancy, lactation, or energy production. recovery from illness, supplemental dietary protein is re- Proteins constantly undergo breakdown and synthesis. quired. These processes are affected by energy supply and During growth, even though there is net deposition of pro- hormonal factors. An overview of amino acid metabolism tein, the rates of synthesis and breakdown are increased. is presented in Figure 17-1. Total protein turnover in a well-fed, adult human is estimated at about 300 g/day, of which approximately 100 g is myofibrillar protein, 30 g is digestive enzymes, 20 g is 17.1 Essential and Nonessential Amino Acids small intestinal cell protein, and 15 g is hemoglobin. The remainder is accounted for by turnover of cellular pro- Plants and some bacteria synthesize all 20 amino acids teins of various other cells (e.g., hepatocytes, leukocytes, (see also Chapter 2). Humans (and other animals) can platelets) and oxidation of amino acids, and a small amount synthesize about half of them (the nonessential amino is lost as free amino acids in urine. Protein turnover rates acids) but require the other half to be supplied by the vary from tissue to tissue, and the relative tissue contri- diet (the essential amino acids). Diet must also pro- bution to total protein turnover is altered by aging, dis- vide a digestible source of nitrogen for synthesis of the ease, and changes in dietary protein intake. Several pro- nonessential amino acids. The eight essential amino acids teins (e.g., many hepatic enzymes) have short turnover are isoleucine, leucine lysine, methionine, phenylalanine, times (less than 1 hour), whereas others have much longer threonine, tryptophan, and valine. In infants, histidine (and turnover times (e.g., collagen > 1000 days). Turnover of possibly arginine) is required for optimal development myofibrillar protein can be estimated by measurement of and growth and is thus essential. In adults, histidine is 3-methylhistidine in the urine. Histidyl residues of actin nonessential, except in uremia. Under certain conditions, 331 332 CHAPTER 17 Protein and Amino Acid Metabolism balance exists when the amount of nitrogen lost from the body (as nitrogen metabolites excreted in urine and feces) exceeds that taken in. This state continues until the essential amino acid deficiency is corrected. Negative nitrogen balance also occurs in malabsorption syndromes, fever, trauma, cancer, and excessive production of catabolic hormones (e.g., hypercortisolism; see Chapter 32). When the dietary nitrogen intake equals nitrogen losses, the body is in nitrogen balance. In normal adults, anabolism equals catabolism. When nitrogen intake exceeds nitrogen losses, there is a positive nitrogen balance, with anabolism ex- ceeding catabolism. The body retains nitrogen as tissue protein, which is a characteristic of active growth and tissue repair (e.g., growth in children, pregnancy, recovery from an emaciating illness). Quality and Quantity of Dietary Protein Requirement FIGURE 17-1 Dietary protein provides organic nitrogen and the essen- Overall metabolism of proteins and amino acids. Body protein is maintained by the balance between the rates of protein synthesis and tial amino acids. The quantitative estimation of protein breakdown. These processes are influenced by hormones and energy requirement must take into account the quality of protein, supply. as determined by its essential amino acid composition. Dietary protein should provide all of the essential amino acids in the appropriate amounts. If the concentration of some nonessential amino acids may become essential. For one amino acid is significantly greater or less than that of example, when liver function is compromised by disease the others (in a protein or amino acid mixture), utilization or premature birth, cysteine and tyrosine become essential of the others may be depressed and will be reflected in because they cannot be formed from their usual precursors growth failure. (methionine and phenylalanine). A procedure for assessment of protein quality consists Glutamine, a nitrogen donor in the synthesis of purines of feeding growing rats various levels of the test protein and pyrimidines required for nucleic acid synthesis (Chap- and assessing the slope of regression lines relating growth ter 27), aids in growth, repair of tissues, and promotion rate and protein intake. Wheat protein is deficient in ly- of immune function. Enrichment of glutamine in enteral sine when compared with lactalbumin, which contains all and parenteral nutrition augments recovery of seriously of the essential amino acids in desirable concentrations. ill patients. Arginine may be considered as a semiessen- Wheat protein is therefore assessed to be 20% as effec- tial amino acid. It participates in a number of metabolic tive as an equivalent amount of lactalbumin. Similarly, pathways, namely, formation of urea and ornithine, crea- proteins from corn, which are also deficient in lysine, do tine and creatinine, spermine, agmatine and citrulline, and not support optimal growth. However, genetic selection nitric oxide (NO). The endothelial cells lining the blood and breeding programs have yielded strains of corn with vessels produce NO from arginine, which has a major role higher lysine content. Proteins of animal originmnamely, in vasodilator function (discussed later). Dietary arginine meats, eggs, milk, cheese, poultry, and fishmare of good supplementation improves coronary blood flow, reduces quality since they provide all of the essential amino acids. episodes of angina, and helps in patients with walking Gelatin, the protein derived from collagen, lacks trypto- pain due to claudication. phan and is of poor quality. In general, plant proteins are of poor quality because they lack one or more essential amino acids. The best quality plant proteins are found in Nitrogen Balance legumes and nuts. Therefore, the diet of a pure vegetarian For protein synthesis to occur, all 20 amino acids must requires careful planning to achieve a combination of pro- be present in sufficient quantities. Absence of any one teins that provide necessary amounts of all essential amino essential amino acid leads to cessation of protein synthe- acids. Combinations of complementary vegetable proteins sis, catabolism of unused amino acids, increased loss of include rice and black-eyed peas; whole wheat or parched nitrogen in urine, and reduced growth. Negative nitrogen crushed wheat (bulgur) with soybeans and sesame seeds; SECTION 17.1 Essential and Nonessential Amino Acids 333 cornmeal and kidney beans; and soybeans, peanuts, brown stay in the hospital. Thus, protein energy malnutrition can rice, and bulgur (Chapter 12). Deficiency of vitamin B12 cause morbidity, mortality and also has economic con- (Chapter 38) may occur in persons on a pure vegetarian sequences. Acute stressful physiological conditions such diet. Although plant proteins used singly do not provide as trauma, burn, or sepsis can also precipitate protein en- all of the essential amino acids, their inclusion in the diet ergy malnutrition due to hypermetabolism caused by the provides nonessential amino acids that would otherwise neuroendocrine system. Prompt diagnosis and appropriate have to be synthesized at the expense of the nitrogen of nutritional intervention is required in the management of the essential amino acids. The following estimates of daily patients with protein energy malnutrition. protein needs are for persons on a Western diet: adults, Measurements of the levels of serum proteins such as al- 0.8 g/kg; newborns, 2.2 g/kg; infants (0.5-1 year), 2 g/kg. bumin, transthyretin (also known as prealbumin), transfer- During pregnancy and lactation protein intake above the rin and retinol-binding protein are used as biochemical pa- normal adult level is recommended (Appendix IV). These rameters in the assessment of protein energy malnutrition protein requirements are valid only when energy needs (Table 17-1). An ideal protein marker should have rapid are adequately met from nonprotein sources. If intake of turnover and present in sufficiently high concentrations in carbohydrates and lipids is insufficient to meet the energy serum to be measured accurately. Transthyretin has these expenditure, dietary protein is utilized to meet the energy properties; it is a sensitive indicator of protein deficiency deficit and results in negative nitrogen balance. and is effective in assessing improvement with refeeding. Protein Energy Malnutrition Transport of Amino Acids into Cells Two disorders of protein energy nutrition that are wide- Intracellular

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