Allium sulphur chemistry and traditional use of wild onions along the silk road

The genus Allium L. (onions) contains more than 800 species, which were divided in several subgenera; Many of them have been used as phyto-pharmaceutics, seasonings, and vegetables. Most prominent are common onion (A. cepa L.) and garlic (A. sativum L.). The health benefits of Allium vegetables are mainly related to sulphur containing compounds as well as saponins.

The species-rich genus Allium has a main centre of distribution reaching from Southwest Asia to the high mountains of Central Asia, were several wild species are used by the local population, as one can be concluded from casual remarks in some floras. Cysteine sulphoxides of these plants are believed to be mainly responsible for these health benefits. These compounds are converted to thiosulphinates like allicin by the enzyme alliinase, when plant material is disrupted.

In this investigation, Allium samples mostly belonging to the subgenera Allium, Melanocrommyum and Reticulatobulbosa were chemically analysed. Amino acids as well methiin, alliin, isoalliin, homoisoalliin, propiin, marasmin, a pyrrole cysteine sulphoxide and cysteine pyridinyl N-O were determined by HPLC-MS/MS (Figure). As the only cysteine sulphoxide, marasmin contains two sulphur atoms. Comparable to all cyteine sulphoxides, the pyridinyl N-O is prone to allinase digestion. Most investigated species belonged to the subgenus Melanocrommyum. Usually methiin occurs in significant concentrations. Allium species of the subgenera Allium, Rhizirideum and Cepa did show higher concentrations of alliin, isoalliin and propiin. Substances of highest interest were cysteine derivatives with alkenyl or heteroaromatic residues. In total, several of the investigated species showed relative large amounts of cysteine sulphoxides (higher than 0.25%). Many of these species do show a traditional use.