Alliin

Alliin (S-allyl-L-cysteine-S-oxide) is a derivative of the sulfur-containing amino acid cysteine, that acts as a free radical scavengers and is compound of onions and garlic.

In onions, alliin is converted by the enzyme alliinase to the tears provoking (lachrymatory) compound Propanthial S-oxide. In garlic however, alliin is converted to allicin (also catalyzed by alliinase). This different reaction of aliin is the result of the different constitution of onions and garlic.