SULFUR BIOCHEMISTRY OF GARLIC: THE BIOSYNTHESIS OF FLAVOR PRECURSORS

The major flavour precursors in the genus Allium include alliin and isoalliin, members of the alk(en)yl cysteine sulfoxide group (CSOs). There are also large amounts of S-alk(en)yl γ-glutamyl cysteine sulfoxide compounds. It has been suggested that alliin is derived from serine and an allyl thiol source, or from glutathione and an allyl source, while isoalliin is synthesised via glutathione and a series of γ-glutamyl peptides. We have combined several approaches to investigate the intermediates and enzymes involved in the biosynthetic pathways. These include profiling and tracking sulfur components in garlic leaf, clove and root tissue as bulbs develop in the greenhouse. This has allowed us 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 sulfur by the plant. Experiments involving feeding potential intermediates to onion and 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 sulfoxide by callus. This may indicate that synthase and oxidase enzyme(s) with broad substrate specificity may exist in Allium. Using data derived from the scientific literature, we have searched for genes and enzymes that may be involved in the biosynthetic pathway. Cysteine synthase (CS) and serine acetyltransferase (SAT) may play a role in flavour precursor biosynthesis. Sequences with significant homology to two groups of plant CS and to a SAT have been obtained from garlic (cultivar Printanor). The sequence of one further CS has been identified, using partial peptide sequences from a protein that showed allyl cysteine synthase activity. Our results provide further insight into the biosynthetic pathway of garlic flavour compounds.