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.
