Sulphur metabolism in Garlic: Integrating complexity of Flavour Precursors

Author Name: Sagar Chhabria, Krutika Desai
Author Email: Krutika_desai@hotmail.com

Abstract

The major flavour precursors in the genus Allium include alliin, isoalliin, proliin and methiin, members of the alkyl Cysteine Sulfoxide group (CSOs). The path of synthesis of alkyl Cysteine Sulphoxides, or flavour precursors, in the Alliums are still speculative. There are two proposed routes for alliin biosynthesis, one is from serine and allyl thiol while the other is from glutathione and an allyl source via glutamyl peptides.The addition of allyl thiol to differentiating garlic tissue cultures resulted in the appearance of detectable levels of both S-allyl cysteine and alliin and also demonstrated that S-allyl-cysteine was oxidised stereospecifically to alliin by garlic tissue cultures, indicating the presence of a specific oxidase in the cells. Although these reports provide good evidence that S-allyl cysteine can be converted to alliin by garlic tissue cultures, it does not indicate whether γ–glutamyl-S-allyl cysteine or S-allyl cysteine is the substrate for the oxidation in vivo. Several approaches have been combined to investigate the intermediates and enzymes involved in the biosynthetic pathways, which have allowed to build up, for the first time, a picture of the dynamic uptake, allocation and distribution of flavour precursors during the life cycle. This indicated that CSOs are remobilised to the new cloves in a late stage of development, following uptake of the bulk of sulphur by the plant. Studies involving feeding potential intermediates to garlic callus tissue, that generally lack flavour precursors, indicated that several alk(en)yl thiols could be converted to alk(en)yl cysteine and alk(en)yl cysteine sulphoxide by callus. This may indicate that synthase and oxidase enzyme(s) with broad substrate specificity may exist in Allium. Cysteine synthase (CS) and serine acetyltransferase (SAT) may play a role in flavour precursor biosynthesis. Alliinase is the enzyme that initiates the conversion of alliin to allicin and its derivatives. These results provide further insight into the biosynthetic pathway of garlic flavour compounds.

Keywords

Allium sativum, Alliin, Cysteine Synthase, Sulfur Metabolism, γ-glutamyl Cysteine Sulfoxid

Introduction

Garlic is grown and consumed in all parts of the world. It has long been considered as beneficial for health and it is recognized as having many properties that can prevent and cure diseases. Researchers have reported pharmacological evidence of the use of garlic as anti thrombotic, anti- atherosclerosis, anticancer and antiinflamatory [1]. Garlic sulphur compounds have shown potential in combating cardiovascular disease and some cancers, and there is therefore an interest in improving the content of these compounds in garlic.
Most of the biological properties of garlic are ascribed to organosulphur compounds. These compounds are derived from alliin which is converted to allicin by the enzyme alliinase when a garlic clove is crushed or cut. Allicin is an unstable compound that breaks down into numerous sulphur compounds such as polysulphides.
Although considerable knowledge about the beneficial properties of garlic has been accumulated, but understanding of the biosynthetic pathway is minimal, especially when compared with current knowledge of the assimilation of sulphur from sulphate to cysteine.

Conclusion

Research into flavour precursor biosynthesis in Allium is at a stage where developments in understanding of cysteine and glutathione metabolism over the last few decades can be applied with the aim of increasing CSO synthesis. We have focused on the role of CS, since a role for this enzyme in the synthesis of substituted alanines has been demonstrated. It is clear that CSs from many plants are able to synthesise alk(en)yl cysteine when provided with suitable substrates. However, more detailed kinetic studies of recombinant Allium CSs from each cell compartment are needed to provide a better appreciation of their biosynthetic potential.
The importance of CS in CSO synthesis in vivo in Allium sp. remains to be determined. Over-expression or silencing of endogenous CS genes might provide an answer, although the effect of transformation of other plant species with genes for SAT and/or CS has had varying results. Some reports little effect on phenotype while others have demonstrated plants exhibiting elevated levels of the OAS, cysteine, glutathione or other metabolites. Additional information on sulphur resource allocation during growth and bulbing would assist in improving agricultural systems for garlic production, and could be used, in conjunction with knowledge about the biosynthetic pathways, to improve garlic varieties. Although garlic can only be propagated vegetatively at present, if fertile varieties became available these aspects of sulphur biochemistry might provide markers to assist breeders. If transgenic garlic were to be developed as a commercial crop, these studies could determine whether targeting transgenes to particular organelles, tissues or times during development would be beneficial in influencing CSO synthesis.

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