Sulphur Chemistry of Drumstick Onions (Allium Subgenus melanocrommyum)

Mountainous areas of Central Asia show a high variety of plants belonging to the genus Allium (1). Out of this, the subgenus melanocrommyum is most prominent, but only a few species were noticed as ornamentals in the western world. In contrast, several species are highly estimated by local populations as food or medicinal plant. However, the concentration of sulphur compounds as shown in the Figure is usually rather low. Typically, bulbs contain the cysteine sulphoxide methiin (1) in noteworthy amounts. Alliin (2) and propiin (4) could be detected in trace amounts, whereas some species also produce isoalliin (3). Normally, plant material exhibits no or only a weak onion or garlic like smell. However, there are exceptions: Aliium stipitatum Regel and A. suworowii Regel emit a strong (and unpleasant) smell but have lower cysteine sulphoxide contents than odourless taxa like A. jesdianum Boiss. et Buhse and A. hollandicum R.M. Fritsch. Besides cysteine sulphoxides, the activity of the enzyme alliinase must be also considered for the formation of odour compounds. In addition to cysteine sulphoxides listed above, a number of species like A. giganteum Regel, A. rosenorum R.M. Fritsch, A. jesdianum, and A. komarowii Lipsky produce a red-staining substance with the chemical structure of a sulphurpyrrol (5). This compound showed some bioactivity, but the function inside the plant is unknown yet.

Acknowledgements: Research was supported by the German VolkswagenStiftung as part of the “PharmAll“-project.

Reference: 1. Fritsch, R.M., Keusgen, M. (2006), Phytochem. (in press).