Methionine dietary requirement

Humans have a limited capacity to synthesize amino acids de novo, but extensive interconversions can occur. Those amino acids which cannot be formed within the body and must be supplied by the diet are called essential . Members of this group, which includes the branched chain amino acids leucine and valine, and also methionine and phenylalanine, are all dietary requirements. Such essential amino acids may be chemically converted, mainly in the liver, into the non-essential amino acids. The term non-essential does not equate with not biochemically important but simply means they are not strict dietary components. 

Excessive heat can cause destruction of amino acid residues. The amino acid most susceptible to direct heat destruction is cystine. Although not an essential amino acid, cystine does have a sparing effect on the dietary requirement for methionine. As a result, cystine destruction can be nutritionally important. In addition, many vegetable proteins are limiting in the sulfur amino acids. Cystine destruction would be particularly harmful for these proteins. 

In addition to their importance as essential amino acids for humans, the quantitative determination of cysteine and methionine seems to be growing in importance in the animal feed industry. The dietary requirements for the sulfur amino acids tend to be very high in many animals. This is presumably due to the magnitude of hair/feather growth and the fact that the structural proteins that comprise hair/feathers often have high cyst(e)ine content. 

A. Pernicious anemia occurs when the stomach does not produce adequate intrinsic factor for absorption of vitamin B12, which is required for the conversion of methylmalonyl CoA to succinyl CoA and homocysteine to methionine. A vitamin B12 deficiency results in the excretion of methylmalonic acid and an increased dietary requirement for methionine. The methyl group transferred from vitamin B12 to homocysteine to form methionine comes from 5 -methyl tetrahydrofolate, which accumulates in a vitamin B12 deficiency, causing a decrease in folate levels and symptoms of folate deficiency, including increased levels of FIGLU and decreased purine biosynthesis. 

Of the 20 amino acids in proteins, the body can readily synthesize eight if an appropriate nitrogen source is available. Two others can be synthesized from other amino acids of the diet tyrosine from phenylalanine and cysteine from methionine. The rest must be provided in the diet (Chapter 17), since the body can synthesize none or an insufficient amount. The dietary requirement depends on several factors. Beside essential amino acids, the diet should provide the nitrogen required for synthesis of the nonessential amino acids. 

Homocysteine provides the sulfur atom for the synthesis of cysteine (see Chapter 39). In this case, homocysteine reacts with serine to form cystathionine, which is cleaved, yielding cysteine and a-ketobutyrate. The first reaction in this sequence is inhibited by cysteine. Thus, methionine, via homocysteine, is not used for cysteine synthesis unless the levels of cysteine in the body are lower than required for its metabolic functions. An adequate dietary supply of cysteine, therefore, can spare (or reduce) the dietary requirement for methionine. 

Animals also derive methyl groups from dietary choline, which can partially substitute for the methionine nutritional requirement. An oxidation product of choline, betaine, is the actual methyl donor to homocysteine. This probably represents a salvage pathway for methyl groups in the catabolism of choline, but it can be of considerable importance if the capacity for de novo methyl synthesis is limited. 

The requirements for the essential amino acids are further complicated by the finding that two non-essential amino acids can only be synthesized in the body if two of the essential amino acids are present in sufficient amounts. Tyrosine is formed directly from phenylalanine so that the requirement for phenylalanine is less when tyrosine is present than when it is absent firom the diet. Similarly the sulphur that is required for the synthesis of cysteine can only be obtained fi-om methionine so that the dietary requirements for the sulphur-containing amino acids should be considered together. If cysteine is present in ample amounts the requirement for methionine will be minimal, but if cysteine is in short supply more methionine is needed. 

Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid ptecutsots (cysteine from methionine and tyrosine from phenylalanine). The dietary intakes of cysteine and tytosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can stiU be maintained. Howevet, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable they ate synthesized from common metabolic intetmediates (pyruvate, ox-... 

Two essential enzymatic reactions in humans require vitamin B12 (Figure 33-2). In one, methylcobalamin serves as an intermediate in the transfer of a methyl group from /V5-methyltetrahydrofolate to homocysteine, forming methionine (Figure 33-2A Figure 33-3, section 1). Without vitamin B12, conversion of the major dietary and storage folate, N5-... 

Our results have been recapitulated with other proteins of varying nutritional value to S. exigua and H. zea they include soy protein, tomato foliar protein, corn gluten and zein. In all cases, more than 2.5% dietary protein was required to alleviate antinutritional effects, because these proteins are less nutritious than casein (Table III). The ability of a protein to alleviate the toxicity of o-quinones is proportional to its nutritional value to the insect (Table III). The proteins ability to function as an alkylatable sink (alleviation of antinutritive effects) is correlated with the relative amounts of alkylatable amino acids (e.g., lysine, cysteine, histidine, methionine Felton and Duffey, unpublished data). 

The liver plays a central role in the synthesis of nearly all circulating proteins. Plasma contains 60-80 g/L of protein and this is turned over at a rate of approximately 250 g/day. A variety of proteins are constructed in the liver using amino acids (Aa) as their basic building blocks. Amino acids are categorised as essential and non-essential , the former being a requirement of dietary intake as they cannot be constructed in vivo, whereas the latter can be synthesised hepatically. The essential amino acids are further categorised as branched-chain amino acids (BCAA leucine, valine, isoleucine) or aromatic amino acids (AAA phenylalanine, tyrosine, methionine) according to their structure. Table... 

In many animals, dietary deprivation of choline leads to liver dysfunction and growth retardation, and some patients maintained on choline-free total parenteral nutrition develop liver damage that resolves when choline is provided, suggesting that endogenous synthesis may be inadequate to meet requirements (Zeisel, 2000). There is inadequate information to permit the setting of reference intakes, but the Acceptable Intake for adults is 550 mg (for men) or 425 mg (for women) per day (Institute of Medicine, 1998). In experimental animals choline deficiency is exacerbated by deficiency of methionine, folic acid, or vitamin B12, which impairs the capacity for de novo synthesis. 

The influence of cysteine on the methionine requirement is demonstrated by the following study. The subject was a student who was awarded a Ph.U. after conducting the study on himself. The subject consumed diets that were complete except that the concentration of methionine was varied (Table 8.12), The dietary amino acids were supplied in the form of pure amino acids, rather than as protein, to allow full control over the levels of amino adds supplied by the diet. Cysteine was supplied as cy.stine. Cystine is a dimer of cysteine, in which the two cysteine residues are connected via a disulfide bond (R——R). Cystine is readily converted to cysteine in the body. The methionine requirement was assessed by determining the conditions that supported a zero or slightly positive nitrogen balance. 

Methionine, USP. An adequate diet should provide the methionine ncccs.sary for normal metabolism in the human.. Viethionine is considered an essential amino acid in humans. It is the precursor in the biosynthesis of -adcnosylnie(hio-niiic, which is an important methylating coenzyme involved In a variety of methylations (e.g.. N-me(hyla(ion of norepinephrine to form epinephrine and O-methylation of catecholamines catalyzed by ca(cchul-CI-mcthyl(iansfcra.ses). Adenosylmcthionine also participates in the methylation of pho.sphatidylcthanolaininc to form phosphatidylcholine, but this pathway is not efficient enough to provide all of the choline required hy higher animals hence, adequate dietary availability of choline is ncces.sary. ... 

CAS 62-49-7. (CH3)3N(OH)CH2CH2OH. Member of the vitamin B complex. Essential in the diet of rats, rabbits, chickens, and dogs. In humans it is required for lecithin formation and can replace methionine in the diet. There is no evidence of disease in humans caused by choline deficiency. It is a dietary factor important in furnishing free methyl groups for transmethylation has a lipotropic function. 

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. 

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