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Uptake and Assimilation of Sulphate (continued)

The enzyme serine acetyltransferase has been reported in extracts of a number of higher plants (Smith et al (1969)) and is responsible for catalyzing the reaction shown in equation (2). Cysteine synthase catalyses equation (3) and has been found in onions (Granroth (1974)) and other plants.
Cysteine is the principal starting metabolite for the synthesis of other sulphur-containing metabolites; from cysteine are formed protein cysteine, methionine, protein methionine and glutathione via their intermediate compounds as shown in Figure 1. In most plants 90% of sulphur is found in form of methionine and cysteine whereas in garlic and other alliums most of the sulphur is found in the form of non-protein, amino acid derivatives.

It is one class of secondary metabolites, the S-alk(en)yl-L-cysteine sulphoxides (flavour precursors), that give rise to the characteristic aroma and flavour of garlic. In the intact cell the sulphoxides are located in the cytoplasm and the hydrolytic enzyme alliinase in the vacuole (Lancaster et al (1981)). Disruption of the cell results in the release of alliinase and the subsequent hydrolysis of the sulphoxides to the primary flavour compounds, the thiosulphinates.
In the biosynthetic scheme shown in Figure 2, sulphate is reduced and assimilated into cysteine and thence into the glutathione cycle. In pulse-chase experiments with 35SO42- Lancaster et al showed γ-glutamyl cysteine and glutathione to be the first peptides labelled and were thus proposed as the starting compounds for the pathway to all sulphoxides.


γ-Glutamyl Peptides and Alk(en)yl Cysteine Sulphoxide Biosynthesis

The γ-glutamyl-S-alkyl-L-cysteines are a group of non-volatile sulphur compounds that are not acted upon by alliinase (S-alk(en)yl-L-cysteine sulphoxide lyase) and which do not directly give rise to flavour. Whilst their true significance in plant metabolism is unclear, the γ-glutamyl-S-alkyl-L-cysteines are considered to provide a reserve for the alkylcysteine sulphoxides and to function as reserves of nitrogen and sulphur in the plant (Kasai & Larson (1980))

The distinctive flavour characteristics of garlic are conferred on it by the array of organosulphur compounds generated from the reservoir of S-alk(en)yl-L-cysteine sulphoxides found within the cloves. It is the profile and quantities of these sulphoxides that are responsible for the nature and intensity of flavours derived from fresh and processed garlic tissue.

All of the organosulphur compounds of intact garlic cloves contain the amino acid cysteine (except for trace amounts of methionine), and include approximately equal amounts of the S-alkylcysteine sulphoxides and γ-glutamyl-S-alkyl-L-cysteines, the alkyl groups being strictly allyl (2-propenyl), methyl and trans-1-propenyl (Lawson et al (1991)). Although it has been known for many years that garlic contains S-allyl and S-methyl derivatives of cysteine sulphoxide (Stoll et al (1947), Fujiwara et al (1948)) and γ-glutamylcysteine (Virtanen & Mattila (1961), Suzuki et al (1961)) the presence of trans-1-propenyl homologues was not known until 1990 (Lawson & Hughes).
S-trans-1-propenylcysteine compounds were thought to be unique to onions and other non-garlic alliums until the presence of S-trans-1-propenylcysteine sulphoxide (isoalliin) was indicated by the discovery of trans-1-thiosulphinates in garlic homogenates (Lawson et al (1991), Lawson & Hughes (1990), Lawson et al (1991)).
γ-Glutamyl-S-trans-1-propenylcysteine, the most abundant γ-glutamylcysteine and the second most abundant sulphur compound in garlic, was discovered and isolated in garlic by Lawson et al in 1991 subsequently confirmed by Mütsch-Eckner et al in 1992.

 

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