Onion 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. 

Members of the allium family (onions, garlic, and leeks) contain cysteine sulfoxide derivatives (allyl sulfur compounds), such as allicin and alliin (see Figure 14.9). When the plant cells are damaged, the enzyme alliinase is released from vacuoles and catalyzes the formation of thiosulfinates and thiols, including the lachrymator thiopropanal S-oxide. Their function in the plant is presumably to provide protection against attack by pests. 

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. 

Characteristics of the Enzyme. S-alkyl-L-cysteine sulfoxide lyases prepared from garlic 17, 19) and onion 24, 25, 39) are similar. They differ primarily in their response to pH—the garlic enzyme has a broad pH optimum from 5 to 8 and may be precipitated at pH 4.0 and redissolved without loss of activity (17) while the onion enzyme is sensitive to acid and is most active at pH 8.8 in pyrophosphate buffer 24, 39). The purified garlic enzyme showed a sharper pH optimum at pH 6.5 40). The Brassica enzyme is most active at pH 8.5 in borate buffer (27) and remains soluble and active when other proteins are precipitated at pH 4.0. 

Pigiirc I, The proposed tiiosyniliclic palliway for S-alk(cR)yl cysteine sulfoxides in onions. 

Figure 2. S-alk(en)yl cysteine sulfoxide flavor precursor accumulation from mature onion bulbs in response to increasing sulfur feriilily from applied nutrient solutions. Solid bars are 1-propenyI cysteine sulfoxide. Lined bars are methyl cysteine sulfoxide. Doited bars are propyl cysteine sulfoxide.

Onion cultivars differ in alliinase concentration which may affect flavor development and intensity. Pukekohe Ixmg Keeper, a pungent cultivar, had a 4 fold increase in bulb alliinase content compared to (he mild Granex 33 cultivar (per. com., J.E. Lancaster, Crop and Food, NZ). In addition, onion alliinase decomposes the three cysteine sulfoxide flavor precursors ai different rates (26). The kinetics of 1-Propenyl cysteine sulfoxide decomposition is quicke.st, followed by the methyl and propyl cysteipe sulfoxides. In vivo, 1-propciiyl cysteine sulfoxide decomposition was almost instantaneous and complete, whereas methyl cysteine sulfoxide and propyl cysteine sulfoxide were decomposed to a much lesser extent (27). Between 60 and 80% of the methyl cysteine sulfoxide, and approximately 50% of the propyl cysteine sulfoxide was left intact in onion macerate, even after 2 hours following maceration... 

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... 

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). 

SCHWiMMER, S. S-Allyl-L-cysteine sulfoxide lyase [Allium cepa (Onion)]. Methods Enzymol. 17 B, 475 (1971). 

The flavor precursors, 5 -alk(en)yl-L-cysteine sulfoxides, themselves do not have any specific flavor. However, when garlic or onion cells are damaged by cutting or crushing and the 5 -alk(en)yl-L-cysteine sulfoxides meet the C-S lyase (alliinase), they are converted into various volatile sulfuric compounds. Stoll and Seebeck (1951) first proposed the production of diallyl thiosulfinate (allicin) from the 5 -allyl-L-cysteine sulfoxide (alliin) by the C-S lyase. Through the reaction catalyzed by the C-S lyase, 5 -alk(en)yl-L-cysteine sulfoxides yield alk(en)yl sulfenic acid and aminoacrylic acid, the latter being spontaneously degraded... 

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. 

Even considering the difference in the amount of 5 -alk(en)yl-L-cysteine sulfoxide, the total amount of thiosulfinates produced from onion is significantly lower than that from garlic being approximately 0.01-0.035 mg/g-fresh weight (Table 18.2). The reason for this is because most of the ( )-l-propene-l-sulfenic acid produced from E)-S-1-propenyl-L-cysteine sulfoxide is converted into a lachrymatory factor, and not so much can be used for the production of thiosulfinates. 

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). 

Randle, W.M., Lancaster, J.E., Shaw, M.L., Sutton, K.H., Hay, R.L., and Bussard, M.L. 1995. Quantifying onion flavor compounds responding to sulfur fertility-sulfur increases levels of alk(en)yl cysteine sulfoxides and biosynthetic intermediates. J Amer Soc Hort Sci J20(6) 1075-1081. 

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