The factor that make the eyes water when onions are chopped up was thought to be a by-product of the enzyme alliinase, from the biochemical pathway that produces the compounds responsible for the onion's characteristic flavour. But in October 17 Nature, S. Imai and colleagues from Somatech Center, House Foods Corporation, Takanodai, Yotsukaido, Chiba, Japan, show that the lachrymatory factor in onions is synthesized by a previously undiscovered enzyme, lachrymatory-factor synthase (Nature, 419:685, October 17, 2002).

Imai et al observed that the fraction with lachrymatory-factor forming activity could be completely separated from the alliinase activity by passing the crude onion alliinase preparation through a hydroxyapatite column. They identified three distinct proteins, and a single gene that they assume encodes them — lachrymatory-factor synthase.

When they expressed the lachrymatory-factor synthase gene in Escherichia coli, the resulting recombinant protein produced lachrymatory factor (propanthial S-oxide) from only 1-propenyl-L-cysteine sulphoxide...

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