Methionine sulfur oxidation

After exposure to an oxidant, the potential types of oxidation products in proteins and peptides can be extensive (Stadtman and Fevine, 2000). Cysteine and methionine undergo a variety of sulfur oxidation reactions to yield cysteine disulfides, methionine sulfoxide, methionine... 

Rauk A, Armstrong DA, Fairlie DP (2000) Is oxidative damage by (1-amyloid and prion peptides mediated by hydrogen atom transfer from glycine a-carbon to methionine sulfur within (1-sheets. J Am Chem Soc 122 9761-9767... 

The presence of sulfur is probably the result of sulfur oxides in the air combining with the silver. These oxides are present whenever fossil fuels are burned, and they readily attack silver. Researchers (5) have stated that sulfur in such metallic yarns is the result of the breakdown of the silk protein, fibroin. This occurrence, however, is unlikely because silk contains almost none of the two sulfur-containing amino acids, cystine and methionine (20). The exact source of the chlorine present in the sample is less easy to explain. It is present in most, though not all,... 

Potency relative to methionine enkephalin receptor affinities obtained by binding studies with 3H-naloxone analgesic carried out in rats following peptide injection into brain periaqueductal gray matter. b The terminal carboxyl group is reduced to an alcohol function. c The sulfur atom of methionine is oxidized to sulfoxide ... 

Finally, methionine and tyrosine are known to be sensitive to peroxidation. Methionine was oxidized to methionine sulfoxide in the presence of peroxidizing methyl linoleate (13) or peroxidizing oil (37), illustrating the ease of radical initiation on sulfur substituents. The radical destruction of tyrosine is known (38), but 1 am not aware of any studies that specifically subject tyrosine to peroxidized lipid. Extrapolation from other radical reactions of tyrosine indicates that the initial event is H-abstraction of the phenol to afford a phenoxy radical. 

The metalloproteins consist of a metal complex imbedded in and bonded to a protein net of covalently bonded amino acids. The most commonly studied systems are the myoglobins and cytochromes, which contain Fe(II) or Fe(III) in a porphyrin complex, or the copper blue proteins, which have Cu(II) or Cu(I) compiexed most often by histidine nitrogens and cysteine and methionine sulfurs from the protein. Metalloproteins can be oxidized or reduced by standard transition-metal complex reagents, and the latter usually are chosen to ensure outer-sphere electron transfer. This area has been the subject of numerous reviews. 

The sulfur of both l- and D-methionine is oxidized in the intact animal. However, it does not appear that the sulfur of methionine is directly susceptible to oxidation, as neither methionine sulfoxide 0 0... 

The oxidation of methionine sulfur in the living animal appears to take place in both the d- and Z-forms. Among the products of this oxidation, notably sulfate, thiosulfate, and taurine have been isolated. 

The oxidation of methionine sulfur to sulfate in humans, after ingestion of the Z-form was observed for the first time by Mueller (88). It has since been encountered in a number of other cases in rabbits, after ingestion or subcutaneous or intravenous injection of the [Pg.372]

The formation of thiosulfate after injection of dZ-methionine has been observed in rabbits (49) which were kept on an extremely low sulfur diet. Of the 378 mg. of methionine sulfur introduced, 0.6%. was recovered from the urine after a 48-hour interval. This amount, though very small, is nevertheless significant. It bears out the fact that thiosulfate forms an intermediary compound in the oxidation of sulfur to sulfate, as we shall see on page 400. 

Regarding the oxidation of methionine sulfur to taurine, the results of the experiments by Virtue and Doster-Virtue (134) with dogs have shown the existence of the oxidation reaction to be highly probable. The animals were first given a diet to deplete their taurine reserves a bile fistula operation was performed then ingestion of dZ-methiouine and cholic acid simultaneously was forced. This ingestion produces a decided increase in the quantity of excreted taurocholic acid. The absolute proof of the oxidation of sulfur of methionine to taurine has been furnished by Tarver and Schmidt... 

These authors fed methionine containing radioactive sulfur to dogs and isolated from the bile of these animals taurine which contained the same sulfur. They state that the oxidation of methionine sulfur to taurine also takes place in rats. 

OXIDATION MECIIANISM.S OF METHIONINE SULFUR 1. Does Direct Oxidation of Methionine Sulfur Exist ... 

Oxidation of Methionine Sulfur after Previous Transformation of This Compound... 

Despite the lack of facts concerning the direct oxidation of methionine sulfur, it seems nearly certain that this direct oxidation plays no role in sulfate formation or contributes only negligibly to the formation of taurine. On the contrary, it appears that the oxidation of methionine sulfur takes place after previous conversion of the methionine to molecules of other types. 

The importance of the demethylation on the oxidation of methionine to sulfate is borne out particularly by the work of Borek and Waelsch (22). These authors repeated quantitatively the studies by Borsook and Dubnoff and found that under their experimental conditions only 2% of the introduced methionine is utilized for methylation of glycocyamine. This 2% corresponds quite reasonably to the quantity of sulfate formed. This agreement is an argument in favor of the hypothesis that methionine sulfur cannot be oxidized to sulfate unless its methyl group is previously. accepted by another compound. Furthermore, this oxidation can only take place if an acceptor for the sulfhydryl group exists in the system. 

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. - ... 

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