 The release is due to the breaking open of the onion cells and their releasing enzymes called alliinases, which then break down amino acid sulfoxides, generating sulfenic acids.A specific sulfenic acid, 1-propenesulfenic acid, formed when onions are cut, is rapidly rearranged by a second enzyme, called the lachrymatory factor synthase or LFS, giving "syn"-propanethial "S"-oxide.The gas diffuses through the air and, on contact with the eye, it stimulates sensory neurons creating a stinging, painful sensation.