Methionine requirement

DMSP is formed from L-methionine required processes, not necessarily in this order, are methylation, deamination, and decarboxylation. Several available pathways have been comprehensively reviewed.35... 

In addition to ACC, ACS produces 5 -methylthioadenosine (MTA), which is recycled through methionine cycle to methionine (see Scheme 1). Recycling of MTA back to methionine requires only the available ATP. A constant concentration of cellular methionine is maintained even when ethylene is rapidly synthesized or when the pool of free methionine is small. The methionine cycle involves the following subsequent intermediates MTA, 5-methylthioribose (MTR), 5-methylthioribose-1-phosphate (MTR-l-P), 2-keto-4-methylthiobutyrate (KMB), and then the recycled methionine. ... 

There are two major disposal pathways for homocysteine. Conversion to methionine requires folate and vitamin B12-derived cofactors. The formation of cysteine requires vitamin B6(pyridoxine). 

The ability to covalently attach 50 (and probably more) residues of methionine (and other essential amino acids) to proteins has important, practical implications. For example, a 1 5 mixture of c-methionyl casein (50 residues of methionine) and casein (5 residues of methionine), fed at 10% weight of the diet, would meet the methionine requirements of rats (23). Therefore the cost of modification and the carry over of products from side reactions would be reduced in the feed. 

The conversion of homocysteine to methionine requires 5-methyl-THF and the vitamin B12 coenzyme. 

First, the incorporation of L-[methyl-uC]methionine into CDP-vinelose was investigated with cells of a methionine-requiring mutant of A. vinelandii.16 The degradation of the radioactive CDP-vinelose by the procedure shown in Scheme 2 showed that about half of the radioactivity was present in the O-methyl group (isolated as chloro-methane), and the other half was in the C-methyl group (isolated as p-bromophenacyl acetate). 

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. 

Cysteine is similar in structure to serine, but has an SH group on the b-carbon, instead of an OH group as in serine. This amino acid is extremely important in protein structure with regard to forming disulfide bonds and potential chelation. Cysteine is not considered an essential amino acid in people because it can be formed from serine and methionine. However, since methionine is an essential amino acid, in some cases improved growth can be obtained by adding cysteine to the diet, because it will spare the amount of methionine required to form the cysteine. Thus, on low-methionine diets, cysteine can be beneficial toward growth. A number of proteins are low in sulfur amino acids and, therefore, this methionine-cysteine relationship may become important. 

Methionine, required for the synthesis of SAM, is obtained from the diet or produced from homocysteine, which accepts a methyl group from vitamin B12... 

DL-Selenomethionine was initially synthesized by Painter" via a sodium/liquid ammonia reduction of DL-selenohomocystine followed by an alkylation of the resulting sodium selenohomocysteinate with methyl iodide. Other syntheses have since been reported including synthetic pathways for the preparation of optically active material" " and isotopically labeled material "" . DL-Selenomethionine has a solubility in water at 30° and pH 7 of 0.108 M which is considerably less than that for L-methionine (0.386 M). After seven hours of hydrolysis under anaerobic conditions in 6 N HCl at 110 °C selenomethionine is completely decomposed (under the same conditions 96% of methionine remains). Chemically, selenomethionine appears to be more reactive than methionine . For example, with cyanogen bromide, selenomethionine completely reacts in 0.1 M HCl in fifteen minutes while methionine requires twenty-four hours for the same reaction. In both cases the end product is homoserine. Although not as marked, this difference in reactivity was also confirmed in the reaction with hydrogen peroxide . 

Scott, M.L., Holm, E.R. Reynolds, R.E. (1963). Studies on the protein and methionine requirements of young Bobwhite quail and young Ringnecked pheasants. Poult. Sci, 42,676-80. 

Example of accumulation analysis. Two groups of l-methionine-requiring (met") E. coli were isolated. Both could be supplemented with L-homocysteine, whereas only one could use L-cysteine in place of l-methionine for growth. One mutant accumulated a compound, which satisfied the growth requirement of the other. This compound was identified as L-cysta-thionine. 

The remaining essential amino acids should be provided in the proportions relative to lysine provided in Table 13.7 in Chapter 13. For example, the methionine requirements for lactation are 0.30 of that of lysine, resulting in a daily methionine requirement of 19.5 g/day or 2.6 g/kg diet. Conducting a similar calculation for all the essential amino acids, and assuming that the non-essential amino acids are typically 1.5 times this level, results in a minimum Heal digestible protein content of 120 g/kg, or a dietary protein level of approximately 143 g/kg diet (120/0.84). 

Methionine requirement in the presence of crystine. " Phenylalanine requirement in the presence of tyrosine. 

Kreis, W., and Goodenow, M., 1978, Methionine requirement and replacement by homocysteine in tissue cultures of selected rodent and human malignant and normal cells. Cancer Res. 38 2259-2262. 

Under special conditions this reaction may be a useful one. For example, Pigg et al. (1962) found that S-adenosylmethionine could satisfy the methionine requirement of all but one of a series of methionine auxo-trophs of S. cerevisiae. The one mutant which was unable to use S-adenosylmethionine lacked activity of S-adenosylmethionine-homocysteine methyltransferase. Presumably the other mutants were generating methionine according to Eq. (6). There would be no net requirement for homocysteine, since S-adenosylhomocysteine hydrolase (an enzyme present in both mammalian liver (de la Haba and Cantoni, 1959) and yeast (Duerre, 1962 Duerre and Schlenk, 1962) could generate this compound according to Eq. (7). 

The conversion of homocysteine to methionine requires a containing transmethylase. In the process THF is regenerated. Since methionine is available from other sources (diet, salvage amino acid pools) it is thought that the importance of this reaction lies not in the synthesis of the amino acid but in the regeneration of THF,... 

Rose, W.C., and Wixom, R.L., 1955, The amino acid requirements of man. XIII. The sparing effect of cystine of the methionine requirement, J. Biol. Chem., 216 763-773. 

Modelling methionine requirements in growing chicken by using the dietary methionine efficiency... 

Schwab, C.G, 1996. Rumen-protectedaminoaoidsfor dairy cattle Progress towards determining lysine and methionine requirements. Anim. Feed Sci. Technol. 59, 87-101. 

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