The Biochemical and Physiological Genesis of Alliin in Garlic

The health-giving properties of garlic are thought to be primarily derived from the presence and subsequent breakdown of the alk(en)ylcysteine sulphoxide (CSO), alliin and its subsequent breakdown to allicin. Two biosynthetic pathways have been proposed for CSOs, one proceeds from alkylation of glutathione through γ-glutamyl peptides to yield S-alkyl cysteine sulphoxides while the alternative is direct thioalkylation of serine followed by oxidation to the sulphoxide. 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-Sallyl cysteine or S-allyl cysteine is the substrate for oxidation in vivo. Garlic contains several cysteine synthases and at least one has the capacity to synthesise alliin. Studies on alliin distribution during bulb development are consistent with a process within which CSOs are synthesised primarily in leaves and translocated to garlic clove tissues during bulb development. Alliinase is the enzyme that initiates the conversion of alliin to allicin and its derivatives. Although multiple sequences can be identified within a single variety, the expression of the dominant isoform in both clove and leaf tissue does not vary significantly with stage of development, consistent with allinase genes being constitutively expressed.