Methionine sulphoxide formation

Aksnes A. Methionine sulphoxide formation, occurrence and biological availability. A review. Fiskeridirektoratets Skrifter, Serie Erncering, 2 (5) 125-153 (1984). 

Previously, we have examined the formation of amino acid hydroperoxides following exposure to different radical species [100]. We observed that valine was most easily oxidised, but leucine and lysine are also prone to this modification in free solution. Scheme 12 illustrates the mechanism for formation of valine hydroperoxide. However, tertiary structure becomes an important predictor in proteins, where the hydrophobic residues are protected from bulk aqueous radicals, and lysine hydroperoxides are most readily oxidised. Hydroperoxide yield is poor from Fenton-derived oxidants as they are rapidly broken down in the presence of metal ions [101]. Like methionine sulphoxide, hydroperoxides are also subject to repair, in this case via glutathione peroxidase. They can also be effectively reduced to hydroxides, a reaction supported by the addition of hydroxyl radical in the presence of oxygen. Extensive characterisation of the three isomeric forms of valine and leucine hydroxides has been undertaken by Fu et al. [102,103], and therefore will not be discussed further here. 

In 95% methanol, the oxidation of methionine to methionine sulphoxide by [AuClJ" is stereospecific. The absorbance at 320 nm shows that the reaction is biphasic, involving formation of an intermediate species ... 

The oxidative reactivities of silver(ii) and silver(m) have been compared. The first- and second-order terms in [Ag ] in various reactions have been assessed and an attempt has been made to resolve the apparent reverse of reactivity between the bi-and ter-valent silver ions, depending on the substrate. A reaction scheme for the oxidation of water has been presented and the data are consistent with a reactive dimeric form of Ag. The hydrolysis constant of Ag is also considered to be less than 31 mol . The properties of octaethylporphyrins show the Agi complex to be stable,and cyclic voltammetric studies show reversibility in the +3<- +2 redox steps. In the gold(m) (as AuCl4 ) oxidation of methionine, there is stereospecificity in the reaction, the product being the corresponding sulphoxide with no evidence for a sulphone. The reaction is biphasic, with an initial complex formation which is too rapid for study by conventional techniques followed by a slower intramolecular redox reaction leading to gold(i). The rate of this latter process is dependent on the methionine concentration. The oxidation also takes place when the substrate is part of a peptide chain. 

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