Indispensible amino acids

Mercer LP, Dodds SJ, Smith Dl Dispensable, indispensable, and conditionally indispensable amino acid ratios in the diet. In Absorption and Utilization of Amino Acids. Friedman M (editor). CRC Press, 1989. 

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. 

P. J. Reeds, Dispensable and Indispensable Amino Acids for humans. http //jn.nutrition.org J. Nutr. 2000, 130 (7), 1835S-1840S. PMID 10867060 http //www.ncbi.nlm..nih.gov (accessed 2 April 2008). 

A problem with tyrosine and cysteine is that they can be synthesised from other amino acids, phenylalanine and methionine, respectively. Since both phenylalanine and methionine are indispensable amino acids, if they are not present in the diet at or below minimal requirement, then there is not sufficient to synthesise tyrosine or cysteine and, therefore, these amino acids become indispensable, i.e. conditionally essential. 

An interesting possibihty, never investigated, is that the bear possesses the ability to synthesise both dispensable and non-dispensable amino acids de novo, from the ammonia released by the microorganisms. As far as is known, no mammal possesses the enzymes necessary to synthesise the indispensable amino acids. It would be interesting if another adaptation to such a long hibernation is development of enzymes necessary to synthesise indispensable amino acids. This possibility is supported by the... 

Among the various free amino acids reported in citrus juices (32), arginine is the only semi-indispensable amino acid that occurs in moderate amounts. The majority of amino acids in citrus are considered to be nonessential according to the classification... 

Histidine is an indispensable amino acid. Its requirement is easily demonstrated in young, growing arumals but is difficult to show in adults. Apparently, the signs of deficiency fail to materialize when adult animals are fed a histidine-free diet because of Nstidine stored in muscle in the form of a related compound, carnos-ine. The availability of camosine as a source of histidine varies among species. Human and rat muscle contain camosine, but mouse muscle does not. Fish muscle contains a methylated form of camosine, called anserine, that does not seem to be available to the fish as a source of histidine. 

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 indispensabJe (essenfioJ) amino acids. Those that may be present in the diet but can be omitted without threatening life are called dispensable (norte5sc dw0 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. 

The concerns of the nutritionist regarding protein extend well beyond protein content and protein density. A major concern in human and anima nutrition is the quality of the protein. The term protein quality refers to the ability of a particular protein to provide a balanced pattern of indispensable amino acids. A hi -quality protein contains high concentrations of the indispensable amino acids. A low-quality protein is deficient or lacking in one or more of these amino acids. Although beans and eggs are similar, as far as protein density is concerned, the quality of egg protein is considerably better than that of protein from beans. 

The chemical score is determined as follows. The contents of the indispensable amino adds of egg albumin (ovalbumin), which is used as a reference protein, and the food protein in question arc determined. Egg albumin is considered to be ideal and nutritionally complete The test requires several steps. The protein must be purified, hydrolyzed to the constituent amino acids, and then subjected to analysis using an amino acid analyzer. The values for each of the indispensable amino acids for ovalbumin and the test protein are then listed as shown in Table 8.14. The contents of the various amino acids in the two proteins are then compared. The amino acid in the test protein that is present in the low cst level, on a percentage basis, can be found by examining the table. This amino acid is called the limiting amino acid of the protein. The value of the percentage is the chemical score. Table S.14 indicates that the quantity of lysine in oat protein is 51% that in egg protein. Hence, the chemical score of oat protein is 51,... 

Gietzen DW, Rogers QR. 2006. Nutritional homeostasis and indispensable amino acid sensing A new solution to an old puzzle. Trends Neurosci 29 91-99. (Brief review of the sensing of lAA deficiency from an evolutionary perspective)... 

Gietzen DW, Ross CM, Hao S, Sharp fVY 2004. Phosphorylation of eIF2a is involved in the signaling of indispensable amino acid deficiency in the anterior piriform cortex of the brain in rats. J Nutr 134 717-723. (First demonstration of the phosphorylation of eIF2a in the APC in response to lAA deficiency)... 

Koehnle TJ, Russell MC, Morin AS, Erecius L, Gietzen DW. 2004a. Diets deficient in indispensable amino acids rapidly decrease the concentration of the limiting amino add in the anterior piriform cortex of rats. J Nutr 134 ... 

Sharp JW, Magrum LJ, Gietzen DW. 2002. Role of MAP kinase in signaling indispensable amino acid deficiency in the brain. Brain Res Mol Brain Res 105 11-18. 

Since L-tryptophan is an indispensable amino acid, its role as a substrate for protein synthesis is necessarily of much importance. However, L-tryp-tophan has other roles that are vital in maintaining normal physiological functions. For example, L-tryptophan has been reported to play a unique role in regulating protein synthesis, particularly in the liver, but also in other organs in a manner unrelated to its function as a precursor amino acid. This action of L-tryptophan is reviewed in this chapter. 

The plasma concentration of LNAA competes with tryptophan for uptake into the brain. The extent of uptake and net utilization influences levels in blood. Like tryptophan, tyrosine and phenylalanine are mainly metabolized in the liver.199 However, the branched-chain amino acids (BCAA) are taken up and metabolized mainly by skeletal muscle and little by the liver.200 Thus, following a meal, the BCAA rise more in peripheral blood than the other LNAA and other indispensable amino acid levels that are influenced by liver metabolism. The BCAA, therefore, have the dominating effect of the LNAA as a group on brain tryptophan uptake. 

In addition to being an indispensable amino acid that is present in proteins of animals and humans and that becomes incorporated into proteins during protein synthesis, tryptophan itself has been found to have a regulatory effect on protein synthesis. Tryptophan can stimulate hepatic protein synthesis. Although the mechanism for this regulatory action appears to be complex, it is apparent that this action involves a specific nuclear envelope receptor to which it binds, followed by enhanced nucleocytoplasmic translocation of mRNA, and subsequent increased cytoplasmic protein synthesis. 

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