Cysteine methionine relationship

As already noted, MT has several sources such as lyase enzymes for L-methionine and S -methyl-L-cysteine. There are complex relationships between DMS, MT, and other VOSCs in the atmosphere, and in marine and terrestrial environments. The previously cited reviews should be consulted. 

Amino acid variants of IL-2 have been used to investigate the relationship between retention and protein structure in gradient RPLC.22 The protein contains three cysteine residues in its primary sequence at positions 58, 105, and 125. The two located at positions 58 and 105 are linked in a disulfide bridge in the native molecule. A series of variants in which the three cysteinyl residues were replaced with serines were compared. Substitution with serine at positions 58 or 105 forces the molecule to form an unnatural disulfide between positions 125 and 58 or 105. A methionine residue located at position 104 can also be oxidized to the sulfoxide... 

Carboxymethylcysteine metabolism, its implications on therapy in cystinuria and on the methionine-cysteine relationship. Proc. Soc. Exptl. Biol. Med., 35, 501 (1936). With E. Brand, B. Kassell, and G. F. Cahill. 

Figure 21-1. Structural and metabolic relationships between methionine, homocysteine, and cysteine. CBS, cystathionine b-synthase CTH, cystathionine y-lyase MAT, methionine adenosyltransferase MS, methionine synthase 5-MTHF, 5-methyltetrahydrofoIate MTs, methyl transferases PLR pyridoxal phosphate SAH, S-adenosylhomocysteine SAHH, SAH hydrolase THF, tetrahydrofolate.

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. 

Two closely related aromatic amino acids are phenylalanine and tyrosine. The metabolism of these two amino acids is of medical interest for two reasons. First, a large number of metabolic diseases is associated with the metabolism of these two amino acids second, a large number of important biological compounds other than protein are formed from these amino acids. Phenylalanine can be converted to tyrosine in a unidirectional, physiologically irreversible reaction. Phenylalanine is an essential amino acid that must be preformed in the diet, whereas tyrosine is not considered an essential amino acid because it can be formed from L-phenylalanine. However, the relationship is analogous to that previously indicated for cysteine and methionine the amount of phenylalanine required in the diet depends on the tyrosine content of the diet, that is, the lower the tyrosine content, the more phenylalanine required. This is referred to as a sparing effect that tyrosine has on the phenylalanine requirement. 

Requirements for essential amino acids in poultry and pigs have been devised, and some of these are presented in Appendix 2, Tables A2.9 and A2.10. However, there are considerable complications associated with defining amino add requirements because of interactions between the essential amino acids, between essential and non-essential amino acids, and between amino acids and other nutrients. In chicks, the requirement for glycine is increased by low concentrations of methionine, arginine or B vitamins. Similarly, one amino acid may be converted to another. For example, if cystine or its metabolically active form cysteine is deficient in the diet, it can be synthesised by the animal from methionine. The requirement for methionine is therefore partly dependent on the cystine (or cysteine) content of the diet, and the two amino acids are often considered together (i.e. the requirement is stated for cystine -I- methionine). It should be noted, however, that the two are not mutually interconvertible methionine is not synthesised from cystine and therefore part of the total requirement must always be met by methionine. Phenylalanine and tyrosine have a similar relationship, and in the chick glycine and serine are interconvertible. 

FUNCTIONS OF SULFUR. Sulfur has an important relationship with protein. It is a necessary component of the sulfur-containing amino acids methionine, cystine, and cysteine. Sulfur is present in keratin, the tough protein substance in the skin, nails, and hair and it appears to be necessary for the synthesis of collagen. 

The metabolic relationship of methionine and cysteine was clarified by the now classic in vivo studies of du Vigneaud and his co-workers 99). The work of Tarver and Schmidt 34), who showed earlier the transfer of methionine sulfur to cysteine, was confirmed and extended by these workers. The in vivo conversion of methionine to cysteine, a process often referred to as transsulfuration proceeds through an intermediate cystathionine, formed by a loss of H2O from cysteine and serine. The biologically active forms of cystathionine are ... 

Selenium is readily absorbed, especially in the duodenum but also in the caecum and colon. Seleno-amino acids are almost completely absorbed selenomethionine via the gut methionine transporter and selenocysteine probably via the cysteine transporter. Both selenite and selenate are >50% absorbed, selenite more readily so than selenate, and for these forms there is competition with sulphate transport. Selenite is more efficiently retained then selenate because part of the latter is rapidly excreted into the urine. Vitamins A, E, and C can modulate selenium absorption, and there is a complex relationship between selenium and vitamin E that has not been entirely elucidated for man. A combined deficiency of both nutrients can produce increases in oxidative damage markers (malondialdehyde, Ei isoprostanes, and breath hydrocarbons) and in pathological changes that are not seen with either deficiency alone. Inorganic Se is reduced to selenide by glutathione plus glutathione reductase and is then carried in the blood plasma, bound mainly to protein in the very low-density lipoprotein fraction. Selenomethionine is partly carried in the albumin fraction. 

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