S-methyl-L-cysteine sulfoxide

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. 

Onions Allium cepa) were shown to contain similar compounds, S-methyl and S-propyl-L-cysteine sulfoxide (20). The principal flavor precursor in onion is fmn -S( + )-l-propenyl-L-cysteine sulfoxide 21, 22, 23). It is responsible for the lachrymatory properties and bitter taste of freshly cut onion (22). All these compounds were cleaved by an S-alkyl-L-cysteine sulfoxide lyase from onion 24, 25) which yielded pyruvate and ammonia in addition to a sulfur compound. The enzyme has also been demonstrated in Bacillus subtilis 26) and in a number of the Cruciferae where the only substrate known is S-methyl-L-cysteine sulfoxide (27). The product presumably gives rise to dimethyl disulfide which is the odor of cooked cabbage. 

Ostermayer, F. and Tarbell, D., 1959. Products of acidic hydrolysis of S-methyl-L-cysteine sulfoxide the isolation of methylmethane-thiosulfonate and mechanism of hydrolysis. J. Am. Chem. Soc., 82 3752—3755. 

Figure 2. Formation of methanethiol-related volatile sulfur compounds from S-methyl-L-cysteine sulfoxide via the action of cysteine sulfoxide lyase (C-S lyase) in cruciferous vegetables.

Methyl Methanethiosulfinate. Methanethiol-related compounds are secondary products of the primary C-S lyase action on S-methyl-L-cysteine sulfoxide (19). The primary product, presumably methanesulfenic acid, is very unstable and facilely convert to methyl methanethiosulfinate which has been found in macerated Brussels sprouts (20). Methyl methanethiosulfinate has also been reported to be present in a model system composed of S-methyl-L-cysteine sulfoxide and partially purified cabbage C-S lyase (20). 

Carson, J. F., and F. F. Wong Isolation of (+) S-methyl-L-cysteine sulfoxide and of (-I-) S-n-propyl-L-cysteine sulfoxide from onions as their N-2,4-dinitrophenyl derivatives. J. Org. Chem. 26, 4997 (1961). 

This scarcity in thiosulfinates results in the low production of sulfides. Sulfides majorly detected in onion are dipropyl disulfide, dipropyl trisulfide, methyl propyl disulfide, methyl 1-propenyl disulfide, 1-prope-nyl propyl disulfide, and methyl propyl trisulfide (Boelens et al. 1971 Kallio and Salorinne 1990). Interestingly, more amount of sulfides with propyl group is produced in spite of the lower amount of S-propyl-L-cysteine sulfoxide than that of S-methyl-L-cysteine sulfoxide (Randle etal. 1995). 

Sugii, M., Nagasawa, S., and Suzuki, T. 1963. Biosynthesis of S-methyl-L-cysteine and S-methyl-L-cysteine sulfoxide from methionine in garlic. Chem Pharm Bull 77 135-136. 

The Brassica plants also obtain a flavor contribution from alkyl cysteine sulfoxide precursors. The Brassica group includes broccoli, brussel sprouts, cabbage, cauhflower, and rutabagas. S-Methyl-L-cysteine sulfoxide is the primary cysteine sulfoxide derivative found in this family. An investigation of fresh cabbage flavor found that 6% of the volatiles was dimethyl disulfide, 6% dimethyl trisulfide, 3% dimethyl tetrasulfide, and 1.5% methyl ethyl trisulfide [6]. 

C4H9NO3, L(S)-Threonine, 13, 488 40B, 434 C HjNOa, L-Allothreonine, 41B, 525 CftHgNOa, L-Threonine - L-allothreonine, 41B, 526 C4H9NO3S, (+)-S-Methyl-L-cysteine sulfoxide, 27, 765 CftH9NOsS, DL Homocysteic acid, 43B, 589... 

Ci,H8N20i,Pt, a-c is Diglyc inatoplat inum( 11), 41B, 1027 Ci,H8N20i,Pt, trans Bis(glycinato)platinum(II), 34B, 538 Ci,HgCl2N03PdS H2O, Dichlorobis( (S-methyl-L-cysteine)sulfoxide)pal-ladiumdl) monohydrate, 46B, 1033... 

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... 

For DMS and DMTS, S-mothyl-L-cysteine sulfoxide is a precursor action of a C-S lyase enzyme yields methanesulfenic acid, CH3-S-OH, and hence methyl methanethiosulfinate, CH3-SO-S-CH3. Disproportionation reactions yield polysulfides such as DMS.56... 

Urinary metabolites are S-methylthioacetic acid sulfoxide, V-acetyl-.S -methyl-L-cysteine and /-(methylthioacetyl)glycine, which are metabolites of Y-methyl-i-cysteine and 5-methylglutathione. These last two compounds were found after incubation of methyl chloride with rodent liver, kidney and brain homogenates. The methyl group of methyl chloride is metabolized via -methyl-L-cysteine to formate which is found in urine and blood of rats, whereas formaldehyde is found in rat liver microsomes and blood of mice and rabbits (lARC, 1986). 

A similar enzyme has been purified 26-fold from the bacterium Pseudomonas cruciviae (53). It shows a constant ratio (about three) between activity on S-methyl-L-cysteine and on the sulfoxide through purification. It cleaves a wide range of S-alkyl and S (2-carboxyalkyl) cysteines but not L-cysteine itself (or other amino acids). 

This paper (S4) compares the two silylating reagents bis (trimethyl-silyl) acetamide (BSA), first described by Klebe et ad. (K6), and bis-(trimethylsilyl) trifluoroacetamide (BSTFA), for the preparation of volatile trimethylsilyl (TMS) derivatives of 12 sulfur-containing amino acids. BSTFA was recommended as the reagent of choice for taurine, cysteic acid, homocystine, djenkolic acid, ethionine, methionine sulfone, L-2-thiolhistidine, cysteine, and cystine. For S-methyl-L-cysteine, methionine sulfoxide, and methionine, BSA was used as silylating reagent. 

Similar to the S-allyl-L-cysteine sulfoxide in garlic, the C-S lyase can also catalyze the flavor formation from the 5 -methyl-L-cysteine sulfoxide. Therefore, when garlic is cut or crushed, methyl sulfenic acid and allyl sulfenic acid can be produced from 5 -methyl-L-cysteine sulfoxide and S-allyl-L-cysteine sulfoxide, respectively (Figure 18.5). As sulfenic acids are quite reactive, methyl sulfenic acid and allyl sulfenic acid will quickly form the metiiyl 2-propenethiosulfinate and allyl methaneth-iosulfinate. With the similar reaction to that from diallyl thiosulfinate, these thiosulfinates produce allyl methyl disulfide and allyl methyl trisulfide. 

Precursors of importance for the aroma of onions, other than compound I, are S-methyl and S-propyl-L-cysteine sulfoxide. Precursor I is biosynthesized from valine and cysteine (cf. reaction sequence 17.12). 

The key precursor for garlic aroma is S-allyl-L-cysteine sulfoxide (alliin) which, as in onions, occurs in garlic bulbs together with S-methyl-and S-propyl-compounds. The allyl and propyl-compounds are assumed to be synthesized from serine and corresponding thiols ... 

All these enzymes have similar specificity and require the L-cysteine sulfoxide portion of the molecule. S-alkyl cysteines are not substrates 18, 24, 27, 39) neither are sulfoxides of N-substituted L-cysteine, / -di-methyl-L-cysteine, y8-thiopropionic acid 18), D-cysteine 19), and dl-methionine 18, 24, 39), nor do the enzymes act on L-cysteine itself 24, 27, 39), cysteic acid, cysteinesulfinic acid 24, 27), or cycloalliin 24, 39). There may be some action on S-ethylcysteine sulfone (42). 

When onion is cut or cmshed, ( )-5 -l-propenyl-L-cysteine sulfoxide (isoalliin) is converted into ( )-l-propene-l-sulfenic acid. Different from garlic, (Z)-thiopropanal S-oxide, a lachrymatory factor, is formed from the sulfenic acid by lachrymatory factor synthase (Figure 18.6). The remaining ( )-l-propene-l-sulfenic acid and methyl sulfenic acid produced from 5 -methyl-L-eysteine sulfoxide ean form methyl 1-pro-pene-thiosulfinate and 1-propenyl methane thiosulfinate that are further converted to sulfides such as 1-propenyl methyl disulfide and 1-prope-nyl methyl trisulfide. 

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. 

Known active constituents of garlic (there are at least 35 other constituents whose actions are unknown) ajoene, allicin, aliin, allixin, allyl mercaptan, allyl methyl thiosulfmate, allyl methyl trisulfide, allyl propyl disulfide, diallyl disulfide, diallyl hepta sulfide, diallyl hexa sulfide, diallyl penta sulfide, diallyl sulfide, diallyl tetra sulfide, diallyl tri sulfide, dimethyl disulfide, dimethyl trisulfide, dirpopyl disulfide, methyl ajoene, methyl allyl thiosulfmate, propyline sulfide, 2-vinyl-4H-l, 3-tithiin, 3-vinyl-4H-1, 2dithiin, S-allyl cysteine sulfoxide, S-allyl mercapto, cysteine. 

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