Methionine sulfone

Methionine sulfoxide [454-41-1, 62697-73-8] M 165.2, m >240 (dec). Likely impurities are dl-methionine sulfone and dZ-methionine. Crystd from water by adding EtOH in excess. 

S-Methyl-L-methionine chloride (Vitamin U) [1115-84-0] M 199.5, [a]p +33 (0.2M pK[ 1.9, pKj 7.9. Likely impurities are methionine, methionine sulfoxide and methionine sulfone. from water by adding a large excess of EtOH. Stored in a cool, dry place, protected from light. 

FIGURE 1. The chemical oxidation of the methionine to methionine sulfoxide and methionine sulfone. 

HOCl-mediated protein oxidation accelerates under pathophysiological conditions. Thus, proteins from extracellular matrix obtained from advanced human atherosclerotic lesions contained the enhanced levels of oxidized amino acids (DOPA and dityrosine) compared to healthy arterial tissue [44], It was also found that superoxide enhanced the prooxidant effect of hypochlorite in protein oxidation supposedly by the decomposition of chloramines and chlor-amides forming nitrogen-centered free radicals and increasing protein fragmentation [45], In addition to chlorination, hypochlorite is able to oxidize proteins. The most readily oxidized amino acid residue of protein is methionine. Methionine is reversibly oxidized by many oxidants including hypochlorite to methionine sulfide and irreversibly to methionine sulfone [46] ... 

Catalases have proven to be a treasure trove of unusual modifications. The first noted modification was the oxidation of Met53 of PMC to a methionine sulfone (77). Met53 is situated in the distal side active site adjacent to the essential His54 in a location where oxidation by a molecule of peroxide would not be unexpected. Among the catalases whose structures have been solved, PMC is unique in having the sulfone because valine is the more common replacement in other catalases. The sulfone does not seem to have a role in the catalytic mechanism and is clearly generated as a posttranslational modification. A small number of catalases from other sources, principally bacteria, have Met in the same location as PMC, and it is a reasonable prediction that the same oxidation occurs in those enzymes as well, although this has not been demonstrated. 

These techniques have been used successfully in the micro-Zdman degradation of the enzyme mouse sarcoma dihydrofolate reductase to obtain the amino acid sequence of the first 25 amino acids 455). Similarly, RPC has been used in coqjunction with the automated Edman technique for sequencing 32 residues of myoglobin 456). Methionine and its oxidation products, methionine sulfoxide and methionine sulfone, in methionine fortified foods have been analyzed as their dansyl derivatives 457). Lysine has been determined as its dansyl derivative in a study in which the stability of lysine in fortified wheat flour was evaluated (458). 

Oxidize methionine and cysteine to methionine sulfone and cysteic acid using performic acid prior to acid hydrolysis. 

Methionine and half-cystine were determined in duplicate as methionine sulfone and cysteic acid on the performic acid-oxidized protein [8. Moore, JBC 238, 235 (1963)). 

The analysis of methionine and cysteine is problematic. The sulfur containing side chains of these amino acids are prone to oxidation. The standard hydrochloric acid hydrolysis will cause the partial conversion of these amino acids into cystine, cysteine, cysteine sulfinic acid, cysteic acid, methionine, methionine sulfoxide, and methionine sulfone. The classic strategy (79) for dealing with this problem is simply to drive the oxidative process to completion (i.e., convert all the cyst(e)ine to cysteic acid) and then to analyze chromatographically for cysteic acid and/or methionine sulfone. This is traditionally accomplished by a prehydrolysis treatment of the sample with performic acid. While this method has sufficed over the years, the typical recovery (85 -90%) and precision (4% intra- and 15% interlaboratory) have been poor (80). 

More recently, there have been numerous collaborative studies (81-84) that have attempted to improve the accuracy and precision of this method. A typical example by MacDonald et al. (81) reported a collaborative study by seven laboratories. Samples were oxidized with performic acid for 16 hours over ice bath. After oxidation, HBr was used to destroy excess performic acid. Samples were then roto-evaporated to dryness, dissolved in 6N HC1, nitrogen purged, and then hydrolysed for 18 hours at 100°C. Interlaboratory precision for cysteic acid determination in six food ingredients ranged from 7 to 10%. For methionine sulfone, interlaboratory precisions ranged from 1 to 13% for the same six food ingredients. The mean recovery of cysteine was 95% and of... 

Finally, it is very common for methionine to be determined as part of the standard hydrochloric acid hydrolysis. Indeed, methionine is not nearly as labile to oxidation as cysteine is. While this is appropriate for many samples, there are studies (27,91) that indicate that seriously flawed recoveries (10-40% low) may result from methionine determination by standard acid hydrolysis if the samples contain high levels of carbohydrate. For these sorts of samples, it is recommended that determination of methionine as methionine sulfone (by performic acid oxidation) be pursued. 

Methionine 7 -lyase Methionine Homocysteine S-methylcysteine Methionine sulfoxide Methionine sulfone S-methylmethionine (weak) Methanethiol HtS Methanethiol DMS... 

Two oxidation products can be formed—methionine sulfoxide (NH2— CH-(COOH)-CH2-CH2-SO-CH3), which can be determined after alkaline hydrolysis (54) but not after acid hydrolysis, which regenerates methionine, and methionine sulfone (NH2-CH-(COOH)-CH2-CH2-S02-CH3), which can be determined after acid hydrolysis since it is acid stable. 

Nutritional Effects of Oxidized Sulfur Amino Acids. In 1937, Bennett (59, 60, 61, 62) already showed that the different oxidation products of methionine and cystine did not have the same biological effects to promote the growth of rats fed on diets deficient in sulfur-containing amino acids. Methionine sulfone and cysteic acid did not promote growth while the lower oxidation products had a positive effect and could replace methionine and cystine to a certain extent (see Table I). 

It was confirmed later that free cysteic acid and free methionine sulfone were not biologically available (63, 64,138) and that free methionine sulfoxide was partly available. Miller and Samuel (64) observed that the food efficiency of a mixture of free amino acids was lower when the methionine source was replaced by methionine sulfoxide. The food efficiency was restored when 50% of the methionine sulfoxide was replaced by free methionine. Gjoen and Njaa (66) confirmed that free methionine sulfoxide was nearly as available as methionine when the amino acid mixture contained cystine. This suggests that methionine sulfoxide is reduced before it is used for protein synthesis. In order to elucidate this point, we have compared the metabolic transit of free methionine sulfoxide with that of free methionine. 

Fig. 10. The first step of methionine oxidation to methionine sulfoxide is reversible owing to methionine sulfoxide furtehr oxidation to methionine sulfone is irreversible.

Methionine sulfoxide formation may occur without noticeable changes in physical or immunochemical properties of the protein. Thus reduction of sulfoxide to thioether often completely restores the lost protein function. Many cells, including human polymoprhonuclear neutrophilic leukocytes, contain enzyme methionine sulfoxide reductase, which is able to convert methionine sulfoxide to the reduced methione form in a variety of proteins (B25, F8). Methionine reacting with a strong oxidant effects methionine sulfone production, which in vivo is not reduced back to methionine. 

There has been no report on the spectrum of methionine sulfone, which is found in hydrolyzates of performate-oxidized proteins. It will probably be not very different from cysteic acid at wavelengths > 2000 A, since simple sulfones are generally transparent to 1800 A (Koch, 1950). Methionine sulfoxide is a stable amino acid found in human urine, in a variety of plant tissues, and as an intermediate in the oxidation of methionine to the sulfone. Its absorption spectrum has not been recorded. The sulfoxide chromophore is usually a broad band, located around 2100 A and of about the same intensity as alkyl sulfides (Koch, 1950). An interesting... 

CH3SH Methionine, methionine sulfoxide, methionine sulfone, S-methyl cysteine... 

If appropriate precautions have been taken in the preparation of a protein, and if oxygen is completely removed before hydrolysis, methionine will usually be recovered from acid hydrolysates in yields greater than 95 %. However, in some proteins (particularly those that are chemically modified) and in many peptides the methionine may be at least partially oxidized to the sulfoxide or sulfone forms, and even though these may be analyzed with amino acid analyzers (see below), the total yield of methionine (and oxidized products) is usually somewhat low. A good check on total methionine content in a peptide or protein is obtained by analyzing for methionine sulfone after performic acid oxidation, since methionine and its sulfoxides are quantitatively converted to the sulfone by this procedure. 

After oxidation of unmodified proteins with performic acid ( 3.8.1) the methionine will all be present as methionine sulfone, and the cysteine and cystine will be present as cysteic acid. The method for oxidizing proteins with performic acid on a preparative scale ( 3.8.1) has been modified for analytical studies (Moore 1963), to allow rapid destruction of excess performic acid. However, tryptophan is still destroyed by the performic acid in this modified procedure, and quantitation of tyrosine may be complicated by the formation of halogenated derivatives ( 2.2.3). 

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