Nutritionally nonessential amino acids

Humans can synthesize 12 of the 20 common amino acids from the amphiboHc intermediates of glycolysis and of the citric acid cycle (Table 28-1). While nutritionally nonessenrial, these 12 amino acids are not nonessential. AH 20 amino acids are biologically essential. Of the 12 nutritionally nonessential amino acids, nine are formed from amphibolic intermediates and three (cysteine, tyrosine and hydroxylysine) from nutritionally essential amino acids. Identification of the twelve amino acids that humans can synthesize rested primarily on data derived from feeding diets in which purified amino acids replaced protein. This chapter considers only the biosynthesis of the twelve amino acids that are synthesized in human tissues, not the other eight that are synthesized by plants. 

NUTRITIONALLY NONESSENTIAL AMINO ACIDS HAVE SHORT BIOSYNTHETIC PATHWAYS... 

In Parenteral and Enteral Nutrition. Amino acid transfusion has been widely used since early times to maintain basic nitrogen metaboHsm when proteinaceous food caimot be eaten. It was very difficult to prepare a pyrogen-free transfusion from protein hydrolysates. Since the advances in L-amino acid production, the crystalline L-amino acids have been used and the problem of pyrogen in transfusion has been solved. The formulation of amino acid transfusion has been extensively investigated, and a solution or mixture in which the ratio between essential and nonessential amino acid is 1 1, has been widespread clinically. Special amino acid mixtures (eg, branched chain amino acids-enriched solution) have been developed for the treatment of several diseases (93). 

The diet must provide those amino acids which the body cannot synthesize (essential amino acids, EAA) and nitrogen in the form of nonessential amino acids (NEA). Both EAA and NEA are required for biosynthesis of proteins and other nitrogen-containing compounds necessary for homeostasis or growth. Thus, the total nitrogen content of a specific food must be considered to be nutritionally significant. 

The literature on concentrated sweet potato protein is sparse. Amino acid patterns for sweet potato protein isolates have been reported by three groups (16, 45, 46). One report showed that when compared to the FAO standard (47), no amino acids were limiting. The other reports showed total sulfur amino acids and lysine to be limiting (Table III). The patterns indicate a nutritionally well balanced protein. The improvement in nutritional quality, when compared to amino acid patterns from whole sweet potato, is due to the fact that whole sweet potatoes contain substantial amounts of NPN, which consists mainly of nonessential amino acids. This effectively dilutes the EAA and lowers the amino acid score. 

Severe negative nitrogen balance may occasionally have to be corrected by hyperalimentation or total parenteral nutrition (TPN). Intravenous solutions used in TPN contain essential and nonessential amino acids, plus a source of calories in the form of fat and carbohydrate. They "spare" the administered amino acids and allow them to be used for tissue repair. The TPN fluid must also contain all other nutritional factors required for life, including essential fatty acids, vitamins, and minerals. Severe metal and essential fatty acid deficiencies have been observed in situations in which such inclusions had not been made. 

In Table 1.5 the classical amino acids are segregated according to their necessity in the diet. Those that are required to maintain life are called indispensable (essential) amino acids. Those that may be present in the diet but can be omitted without threatening life are called dispensable (nonessential) amino acids. The proteins of the most value, from a nutritional point of view, are the ones that contain all the indispensable amino acids as well as a variety of dispensable amino acids. 

Transthyretin levels are often used as an indicator of protein status because of its relatively short half-life, a high tryptophan content, a high proportion of essential-to-nonessential amino acids, and small pool size. However, it is a negative APR. Levels fall in inflammation and malignancy and in cirrhosis of the liver and protein-losing diseases of the gut or kidneys. Therefore a sensitive acute phase reactant, such as CRP, should always be assayed along with TTR if levels are to be used to estimate nutritional status. History and physical examination are also important aspects of such evaluations. ... 

Protein is an essential nutrient for human growth, development, and homeostasis. The nutritive value of dietary proteins depends on its amino acid composition and digestibility. Dietary proteins supply essential amino acids, which are not synthesized in the body. Nonessential amino acids can be synthesized from appropriate precursor substances (Chapter 17). In human adults, essential amino acids are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine. Histidine (and possibly arginine) appears to also be required for support of normal growth in children. In the absence from the diet of an essential amino acid, cellular protein synthesis does not occur. The diet must contain these amino acids in the proper proportions. Thus, quality and quantity of dietary protein consumption and adequate intake of energy (carbohydrates and lipids) are essential. Protein constitutes about 10-15% of the average total energy intake. 

Amino acids can be divided into two major nutritional classes. Essential amino acids are those that cannot be synthesized by the body and are required in the diet. Nonessential amino acids are those amino acids that can be synthesized by the body and need not be included in the diet. Table 19.2 lists the essential and nonessential amino acids. 

Although gelatin is 98-99% protein by dry weight, it has less nutritional value than many other complete protein sources. Gelatin is unusually high in the nonessential amino acids glycine and proline (i.e., those produced by the human body) while lacking certain essential amino acids (i.e., those not produced by the human body). It contains no tryptophan and is deficient in isoleucine, threonine, and methionine. 

On the basis of whole-animal nutritional studies, 14 amino acids are conventionally considered as essential for cultured cells arginine, cysteine, cystine, glutamine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, tyrosine, and valine. Conversely, the naturally occurring nonessential amino acids include alanine, serine, asparagine, proline, glycine, aspartic acid, and glutamic acid. Nutritional requirements for amino acids vary, both quantitatively and qualitatively, with cell type, culture condition, and genetic modification. 

The importance of the basal medium in culturing normal cells was demonstrated by Dr. Richard Ham (Ham and McKeehan, 1979). Dr. Ham s laboratory developed a number of different types of culture media, in which the concentrations of the individual nutritional components have been optimized to support the growth of specific types of normal, differentiated cells. The most widely used is nutrient mixture FI2, which contains nonessential, as well as essential, amino acids, a number of lipids, and trace elements (Table 5). 

Not all proteins are nutritionally equivalent. Mote of some than of others is needed to maintain nittogen balance because different proteins contain different amounts of the various amino acids. The body s requirement is for specific amino acids in the correct proportions to replace the body proteins. The amino acids can be divided into two groups essential and nonessential. There are nine essential or indispensable amino acids, which cannot be synthesized in the body histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. If one of these is lacking or inadequate, then—regardless of the total intake of protein—it will not be possible to maintain nitrogen balance since there will not be enough of that amino acid for protein synthesis. 

Standard CAA products contain a balanced profile of essential, semiessential, and nonessential L-amino acids and are designed for patients with normal organ function and nutritional requirements. Standard CAA... 

Finally, they may be classified on the basis of whether they are nonessential, essential (exogenous), and conditionally essential amino acids. Food and tissue proteins contain 20 amino acids of nutritional importance. Nine of these amino acids (histidine (His), He, Leu, Lys, methionine (Met), Phe, Thr, Trp, and Val) cannot be synthesized by the body and they are therefore essential or indispensable nutrients that must be... 

Extra attention was devoted to proteins and enzymes, as they have been the traditional "bread and butter" of the nutritional biochemist. The amino acids that form proteins can be classified in several ways (1) relatively hydrophilic or lipophilic, (2) dispensable or indispensable (nonessential or essential), (3) glycogenic or ketogenic, and, finally, (4) classical or modified. Only a minority of the amino acids in proteins are modified after incorporation into the polypeptide chain. 

---