â„–16496314[Quote]
Mechanical disruption of onion cellular membranes allows the vacuolar enzyme alliinase to react with cytosolic S-1-propenyl-L-cysteine sulfoxide, generating 1-propenesulfenic acid. This intermediate is rapidly converted by lachrymatory factor synthase into syn-propanethial-S-oxide, a highly volatile compound that vaporizes into the surrounding air. Upon contacting the aqueous film of the cornea, this chemical irritant stimulates the nociceptors of the trigeminal nerve's ophthalmic branch, inducing a burning sensation and triggering a reflex arc that stimulates the lacrimal glands to secrete tears to dilute and flush the foreign agent.