Sources of sulfur in the thioglucosides of various higher plants

Abstract

L-Methionine-35S and DL-cysteine-35S were good sources of sulfur for the biosynthesis of sinigrin in Armoracia lapathifolia Gilib. and were incorporated with approximately the same efficiency. Homomethionine-35S was a slightly poorer source of sulfur than methionine, while taurine-35S was a very poor source. When homomethionine, methionine, and cysteine were employed as sources of sulfur, the distribution of radioactivity between the two sulfur atoms in sinigrin was approximately 80% in the isothiocyanate moiety and 10–15% in the sulfate moiety. When taurine was employed, the distribution was reversed. 1-Thio-β-D-glucose (1-thioglucose) also was a poor source of sulfur for sinigrin. The carbon skeleton of 1-thioglucose was only slightly incorporated into this thioglucoside. The experiments indicated that 1-thioglucose was not a direct precursor of the 1-thioglucosyl residue of sinigrin.The administration of doubly labeled methionine-2-14C-35S clearly demonstrated that this amino acid was not incorporated intact into sinigrin. The carbon-2 and sulfur atoms were metabolized by two different routes.Preliminary studies related to sulfur incorporation into the thioglucosides of Nasturtium officinale R.Br., Tropaeolum majus L., and Reseda luteola L. indicated that there were some differences depending on the source of sulfur; DL-cysteine was a better source of sulfur than either methionine or 1-thioglucose. Tropaeolum majus L. appeared to utilize the sulfur of cysteine for the production of thioglucoside as efficiently as Armoracia lapathifolia Gilib.; however, the other two species utilized the sulfur from the amino acids very poorly.

The Gene Controlling theIndole Glucosinolate Modifier1Quantitative Trait Locus Alters Indole Glucosinolate Structures and Aphid Resistance inArabidopsis

“…CYP83B1 N-hydroxylates indole-3-acetaldoxime to the corresponding aci-nitro compound, 1-aci-nitro-2-indolyl-ethane Hansen et al, 2001;Naur et al, 2003). This compound is conjugated with an S-donor, most likely Cys (Wetter and Chisholm, 1968). The respective genes and enzymes are not yet known (Halkier and Gershenzon, 2006).…”

CYP83B1 Is the Oxime-metabolizing Enzyme in the Glucosinolate Pathway in Arabidopsis

“…First, in vivo feeding studies have shown that among other sulfur donors, cysteine is most efficiently incorporated into glucosinolates (23). Second, the enzyme that converts S-alkyl thiohydroximate to thiohydroximate is likely to be a C-S lyase, and the characterized C-S lyases from plants hydrolyze Ssubstituted cysteine derivatives and have an absolute requirement for the presence of the ␣-hydrogen atom and an unsubstituted amino group in the cysteine moiety (10,24).…”

Applied Cell Biology of Sulphur and Selenium in Plants

“…It is, however, worth mentioning that despite an advanced understanding of the glucosinolate pathway at the biochemical level, the source of reduced sulphur in the glucosinolate structure has remained elusive and was thought to be cysteine for decades (Geu-Flores et al 2011;Sønderby et al 2010). This assumption was mainly based on feeding experiments using 35 S-labelled cysteine in which the radiolabelled S from cysteine was found to be more efficiently incorporated into glucosinolates compared to radiolabelled S from methionine or thioglucose (Wetter and Chisholm 1968). Such experiments, however, were inconclusive due to the incorporation of the label into GSH even in the presence of the γ-Glu-Cys synthetase inhibitor buthionine sulfoximine (Bednarek et al 2009;Schlaeppi et al 2008).…”