Characteristics, biosynthesis, decomposition, metabolism and functions of the garlic odour precursor, S-allyl-L-cysteine sulfoxide

S-Allyl-L-cysteine sulfoxide (ACSO) is an odour precursor in garlic bulbs. One plausible pathway for the biosynthesis of ACSO involves S−2-carboxypropyl glutathione produced from glutathione and methacrylic acid via valine or from γ-glutamyl cysteine. The elimination of glycine and glutamic acid from S−2-carboxypropyl glutathione produces S-2-carboxypropyl cysteine, which is converted to S-allyl cysteine by decarboxylation and oxidation. S-Allyl cysteine is also biosynthesized via the elimination of glutamic acid from γ-glutamyl S-allyl cysteine by γ-glutamyl transpeptidase. The sulfur oxidation of S-allyl cysteine by flavin-containing monooxygenase forms ACSO. When cells are damaged by slicing or grating, ACSO in the cytoplasm or cytoplasmic vesicle is immediately converted to allylsulfenic acid, pyruvic acid, and ammonia by alliinase (C-S lyase), which is located in the vacuoles of vascular bundle sheath cells. Two molecules of allylsulfenic acid form diallyl thiosulfinate (allicin), which exhibits potent antimicrobial activity. Allicin eventually yields garlic odour compounds, such as diallyl disulfide (DADS) and diallyl trisulfide (DATS). Although these sulfides are known to exert various physiological functions, their strong odour limits their use in foods. On the other hand, ACSO is water-soluble and odourless and enhances sweet, salty, and umami tastes, characteristics of which are desirable for food additives. Upon consumption, ACSO is primarily absorbed from the small intestine in the intact form, but is also partly decomposed to allylsulfenic acid, pyruvic acid and ammonia. Allylsulfenic acid is then further converted to DADS and diallyl monosulfide (DAS). ACSO has numerous in vivo functions, such as the prevention of diabetes, myocardial ischaemia, hepatic injury, platelet aggregation and blood ethanol elevation. Although some of these effects may be attributed to its metabolites, ACSO itself contributes to many of these physiological functions.