Biosynthesis of S-Alk(en)yl-l-Cysteine Sulfoxides in Allium: Retro Perspective

The biosynthesis of S-alk(en)yl-l-cysteine sulfoxides is one of the most characteristic feature of plants that belong to the genus Allium. Upon tissue damage, these compounds are hydrolyzed by the enzyme alliinase to generate their respective sulfenic acids, which are spontaneously converted to a series of volatile sulfur-containing compounds with a range of health-beneficial activities. Therefore, the molecular understanding of the mechanism for the biosynthesis of S-alk(en)yl-l-cysteine sulfoxides is important for both basic and applied pharmaceutical researches. Information from chemical analysis and radiolabeling experiments, conducted in the latter half of last century, has suggested that S-alk(en)yl-l-cysteine sulfoxides are biosynthesized from glutathione via γ-glutamyl-S-alk(en)yl-l-cysteines; however, the molecular components that contribute to this biosynthetic pathway and their exact reaction order have long been unclear. Very recently, some genes encoding enzymes involved in the biosynthesis of S-alk(en)yl-l-cysteine sulfoxides have been identified through transcriptome-based approaches, and the characterization of these genes and the encoded enzymes has provided insights into this biosynthetic “black box”. Here we briefly summarize the current knowledge on the molecular basis of the generation of bioactive sulfur-containing compounds and the biosynthesis of S-alk(en)yl-l-cysteine sulfoxides in Allium plants.