Methionine sulfur oxidation mechanisms

Selenium is an essential trace element and an integral component of heme oxidase. It appears to augment the antioxidant action of vitamin E to protect membrane lipids from oxidation. The exact mechanism of this interaction is not known however, selenium compounds are found in the selenium analogs of the sulfur-containing amino acids, such as cysteine and methionine. Se-cysteine is found in the active sites of the enzyme glutathione peroxidase, which acts to use glutathione to reduce organic hydroperoxides. 

In general, sulfide radical cations would either deprotonate in the a-position to the sulfur, yielding a-(alkylthio)alkyl radicals, or engage in the one-electron oxidation of additional substrates. However, recent hypothesis and results have focused on a possible role of methionine sulfide radical cations in hydrogen abstraction reactions within the Alzheimer s disease yS-amyloid peptide (ySAP) [73]. These mechanisms will be discussed here in some detail. 

In addition to the products of lipid oxidation, methanethiol and dimethyl trisulfide were shown to contribute to the complex odor characteristic of soy protein products such as SPI and soy protein concentrates (Boatright Lei, 2000 Lei Boatright, 2001) and soymilk (Lozano et al., 2007) at concentrations comparable to hexanal. Since the threshold in water for methanethiol was reported at 0.02 ppb compared to hexanal at 4.5 ppb (MacLeod C Ames, 1988), these sulfur compounds are intense flavor notes in soy protein products. Lei and Boatright (2007) provided evidence that methanethiol is generated in aqueous slurries of SPI or defatted soy flake from methionine by a free radical mechanism involving manganese, sulfite, and... 

Synthesis of Sulfur Amino Acids. Of the many oxidation states of sulfur, only sulfite has been shown to be utilized by cell-free systems in the net synthesis of compounds with carbon-sulfur bonds, although mutant studies have indicated that more reduced forms can be incorporated. The formation of cysteinesulfinic acid from sulfite has been demonstrated in extracts of acetone-dried rabbit kidney it is possible that this reaction participates in the principal mechanism of sulfur incorporation. In many organisms that require preformed sulfur amino acids, cysteine may be formed from methionine. Only the sulfur of methionine is transferred to cysteine the carbon skeleton of cysteine is derived exclusively from serine. Transsulfuration appears to require the formation of homocysteine from methionine. Homocysteine and serine condense to form a thioether, cystathionine (V). Pyridoxal phosphate has been... 

The formation of homocysteine from methionine is thus an established fact. This homocysteine, in animals, and particularly in dogs, imder-goes a series of reactions, resulting in the oxidation of its sulfur and excretion of the latter in the form of sulfate or taurine (136). We are thus led to the study of the mechanisms of this oxidation. 

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