Amino cysteine sulfoxides

Nishimura and coworkers57-59 studied the y-radiolysis of aqueous solutions of sulfoxide amino acids. Sulfoxide amino acids are the precursors of the flavors of onions (S-propyl-L-cysteine sulfoxide, S-methyl-L-cysteine sulfoxide and S-(l-propenyl)-L-cysteine sulfoxide) and garlic (S-allyl-L-cysteine sulfoxide). In studies on sprout inhibition of onion by /-irradiation it was found that the characteristic flavor of onions became milder. In the y-radiolysis of an aqueous solution of S-propyl-L-cysteine sulfoxide (PCSO)57,58 they identified as the main products alanine, cysteic acid, dipropyl disulfide and dipropyl sulfide. In the radiolysis of S-allyl-L-cysteine sulfoxide (ACSO) they found that the main products are S-allyl-L-cysteine, cysteic acid, cystine, allyl alcohol, propyl allyl sulfide and diallyl sulfide. The mechanisms of formation of the products were partly elucidated by the study of the radiolysis in the presence of N20 and Br- as eaq - and OH radicals scavengers, respectively. 

Scheme 7.3 Enzymatic production of sulfur-containing flavour compounds in Allium species from amino acid flavour precursors, a S-Alk(en)yl cysteine sulfoxides and b (+)-S-l-propenyl cysteine sulfoxide (isoalliin) P-5 -Ppyridoxal-5 -phosphate...

Several sulfiir-containing amino acids, other than cysteine, cystine, and methionine, are found in higher plants. Allium (Liliaceae) species often contain sulfur compounds that are lachrymatory in nature (Fig. 13.17). Among these species are chives, garlic, onion, and leeks. The compounds involved are 5-substituted cysteine sulfoxides and their hydrolysis... 

Thomas, D.J. and Parkin, K.L. 1994. Quantification of alk(en)yl-L-cysteine sulfoxides and related amino acids in Alliums by high-performance liquid chromatography. JAgric Food Chem 42(8) 1632-1638. 

Some vegetables also contain alk(en)ylthio substituted L-cysteine sulfoxides in small amounts. For example, ( (-.Sjj-S-methyl-thiocysteine sulfoxide (R=S-CH3, 2-30), (Rc,S5)-S-propylthio-cysteine sulfoxide (2-30,R=S-CH2-CH2-CH3) and Rq,S, 1E)-S-(prop-1 -en-1 -ylthiocysteine sulfoxide (2-30, R=S-CH=CH-CH3) occur in onions at concentrations of 0.19, 0.01 and 0.56 g/kg fresh weight, respectively. These amino acids and the corresponding peptides (see Section 2.3.3.1.2) are formed by reaction of thiosulfinates (see Section 8.2.9.1.4) with cysteine residues. 

Among the most important volatile sulfur compounds are hydrogen sulfide (sulfan), various thiols, sulfides, isothiocyanates and heterocyclic sulfur compounds. Precursors of volatile sulfur compounds are usually non-volatile, sensory indifferent sulfur compounds, especially sulfur-containing amino acids cysteine, cystine, methionine and their derivatives, such as S-alk(en)yl cysteine sulfoxides, glucosinolates, thiamine and other compounds. An important sulfur compound is also sulfur dioxide, which is used as a preservative and an inhibitor of enzymatic browning reactions or the Maillard reaction. It also occurs in a small amount as a metabolite in fermentation processes. Sulfur compounds may be accompanied by their selenium analogues at very low concentrations. 

Some sulfur amino acids or their selenium analogues may also exhibit toxic effects in livestock. An example of a sulfur-containing amino acid that is toxic to cattle, sheep, goats and other ruminants is a derivative of L-cysteine, S-methyl-L-cysteine sulfoxide, which is known as methiin (2-23). Methiin commonly occurs in forage brassicas, such as forage rape Brassica napus), leaf turnips or forage... 

Contains 0.1-0.36% (usually ca. 0.2%) volatile oil, alliin (5-allyl-L-cysteine sulfoxide), 5-methyl-L-cysteine sulfoxide, enzymes (e.g., alliinase, peroxidase, and myrosinase), ajoenes ( ,Z-ajoene, ,Z-methylajoene, and dimethylajoene), protein (16.8%, dry weight basis), minerals, vitamins (thiamine, riboflavin, niacin, etc.), lipids, amino acids, and others (jiangsu karrer list and horhammer marsh martindale). ... 

Amino acid variants of IL-2 have been used to investigate the relationship between retention and protein structure in gradient RPLC.22 The protein contains three cysteine residues in its primary sequence at positions 58, 105, and 125. The two located at positions 58 and 105 are linked in a disulfide bridge in the native molecule. A series of variants in which the three cysteinyl residues were replaced with serines were compared. Substitution with serine at positions 58 or 105 forces the molecule to form an unnatural disulfide between positions 125 and 58 or 105. A methionine residue located at position 104 can also be oxidized to the sulfoxide... 

The oxidation of cysteine, as well as other amino acids, was studied by Mudd et a/. Individual amino acids in aqueous solution were exposed to ozone the reported order of susceptibility was cysteine, methionine, tryptophan, tyrosine, histidine, cystine, and phenylalanine. Other amino acids were not affected. This order is similar to that for the relative susceptibility of amino acrids to radiation and to lipid peroxides. Evaluation of the ozonization products revealed that cysteine was converted to cysteic acid, as well as cystine methionine to methionine sulfoxide tryptophan to a variety of pioducrts, including kynurenine and N-formylkynurenine tyrosine also to a variety of products, includiitg dihydroxyphenylalanine histidine to ammonia, proline, and other compounds and cystine in part to cysteic acid. In some cases, the rate and end products depended on the pH of the solution. 

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). 

Male Sprague Dawley rats, seven days post weanling (Bantin and Kingman, Inc., Fremont, CA) were fed pelleted rodent chow (5001, Ralston Purina Co., St. Louis, MO) or modified AIN-76 diets (BioServ, Inc., Frenchtown, NJ) with or without added sulfur amino acids. Chow, diets, and water were supplied to the rats ad libitum. The AIN-76 diet (16) was modified to contain 12% casein (normally 20% casein), and the weight difference made up with cornstarch. The normal methionine supplement was not included. The cornstarch and sucrose portions were held separately from the casein/vitamin/mineral (CVM) mixture for the purpose of mixing amino acids with the cornstarch and sucrose (CsS). Methionine (MET), methionine sulfoxide (MSO) and cysteine sulfinic acid (CSA) were obtained from Sigma Chemical Co., St. Louis, MO. Cysteine monoxide (CMO) was prepared by the method of Savige et al., (17). 

Some amino acids may be oxidized by reacting with free radicals formed by lipid oxidation. Methionine can react with a lipid peroxide to yield methionine sulfoxide. Cysteine can be decomposed by a lipid free radical according to the following scheme ... 

S - Alky Icy steines, while not cleaved by the onion enzymes, inhibit the cleavage of the sulfoxides (27, 42) in a partially competitive manner —1/v does not increase hnearly with inhibitor concentration but approaches a maximum (42). The eflFect has not been explained. Other amino acids, including L-cysteine itself, did not inhibit the onion enzyme although L-cysteine did inhibit the garlic enzyme (40). 

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