Sulfur-containing amino acids oxidation

The amino acid Cys contains a free sulfhydryl (SH) function that Is very nucleophilic, easily oxidized, and must be protected. A discussion of the several different blocking groups used to protect the sulfur is beyond the scope of this chapter, but the interested reader should consult those references whose main thrust is peptide synthesis (29,30). The amino acid Met does not contain a free SH but, rather, a methylthio ether, S-CHj, and as such, it most often is used... 

Reaction products of reactive aldehydes derived from oxidised lipids, such as acrolein, ( )-4-hydroxynon-2-enal and malondi-aldehyde, with lysine, arginine and other amino acids are described as examples in Section 4.7.5.6. These products, ALE (advanced lipoxidation end products), formed in vivo are markers of oxidative stress in the organism. Reaction mechanisms are discussed in Section 3.8.1.12.1. As the final reaction products, proteins and oxidised lipids also form dark insoluble macromolecular products that contain variable proportions of lipid and protein fractions. In particular, such products include protein oligomers, proteins with oxidised sulfur amino acids, proteins containing imine bonds (C=N) formed by reaction with aldehydes or hydroperoxides (they mostly arise from the -amino group of bound lysine) and... 

The Dim ester was developed for the protection of the carboxyl function during peptide synthesis. It is prepared by transesterification of amino acid methyl esters with 2-(hydroxymethyl)-l,3-dithiane and Al(/-PrO)3 (reflux, 4 h, 75°, 12 torr, 75% yield). It is removed by oxidation [H2O2, (NH4)2Mo04 pH 8, H2O, 60 min, 83% yield]. Since it must be removed by oxidation it is not compatible with.sulfur-containing amino acids such as cysteine and methionine. Its suitability for other, easily oxidized amino acids (e.g., tyrosine and tryptophan) must also be questioned. It is stable to CF3CO2H and HCl/ether. - ... 

Mammalian sulfite oxidase is the last enzyme in the pathway for degradation of sulfur-containing amino acids. Sulfite oxidase (SO) catalyzes the oxidation of sulfite (SO ) to sulfate (S04 ), using the heme-containing protein, cytochrome c, as electron acceptor ... 

Of the twenty amino acids that are normally found in proteins, only two contain sulfur, cysteine and methionine. Cysteine has long been recognized as being easily oxidized and this oxidation is associated with the loss of biological activity of many proteins. In recent years, it has been shown that methionine also shares these characteristics. Methionine was first isolated by Mueller19 and was one of the last amino acids discovered. Its structure was later proven to be y-methylthio-a-aminobutyric acid by Barger and Coyne20 who named the amino acid methionine as a contraction for its chemical name. 

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

Sulfur-containing amino acids, such as methionine and cystine, are probably the precursors of the mercaptans, sulfides, and disulfides.3 Dimethyl sulfide yields dimethyl sulfoxide and its oxidized product dimethyl... 

The alkali-soluble protein of the peel of lemons treated with hydrogen sulfide, sulfur dioxide, and sulfuric acid contained radioactive sulfur, but the fruit treated with hydrogen sulfide had a significantly lower per cent specific activity in the alkali-soluble protein fraction than did the sulfur dioxide or sulfuric acid treated fruits (Table VII). These results suggest that sulfur dioxide and sulfuric acid react with protein more directly, while hydrogen sulfide perhaps must be oxidized first, as indicated in Table III. It also appears (from Table VII) that the alkali-soluble protein may have been dismuted as the amounts isolated were less in both the hydrogen sulfide and sulfur dioxide treated fruit than in the incubated or nonincubated controls. Other evidence of dismutation has been obtained in experiments where incubation at 60° C. was accompanied by the production of free ammonia (18), and the recovery of free ammonia and six amino acids in the exudates of incubated and sulfur-dusted fruits (18). 

L.G. Shaidarova, S.A. Ziganshina, L.N. Tikhonova, and G.K. Budnikov, Electrocatalytic oxidation and flow-injection determination of sulfur-containing amino acids at graphite electrodes modified with a ruthenium hexacyanoferrate film. J. Anal. Chem. 58, 1144-1150 (2003). 

In contrast to nucleic acids, which can be repaired after oxidative damage by excision and insertion mechanisms (see Chapter 28), the repair of oxidized proteins does not occur except the oxidized sulfur-containing amino acid residues [22]. Instead, oxidized proteins are... 

Protons are mainly derived from two sources—free acids in the diet and sulfur-containing amino acids. Acids taken up with food— e.g., citric acid, ascorbic acid, and phosphoric acid—already release protons in the alkaline pH of the intestinal tract. More important for proton balance, however, are the amino acids methionine and cysteine, which arise from protein degradation in the cells. Their S atoms are oxidized in the liver to form sulfuric acid, which supplies protons by dissociation into sulfate. 

Other than water, protein is the major constituent of meat averaging nearly 21% in heef or chicken meat, with fat varying fiom 4.6 to 11.0% in beef and fiom 2.7 to 12.6% in chickoi. The principal radiolytic reactions of aqueous solutions of aliphatic amino acids are reductive deamination and decarboxylation. Alanine yields NH3, pyruvic add, acetaldehyde, propionic acid, CO2, H2, and ethylamine (6). Sulfur-containing amino adds are espedally sensitive to ionizing radiation. Cysteine can be oxidized to cystine by the hydroxyl radical or it can react with the hydrated electron and produce... 

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