The enzymic cleavage of linoleic acid to C9 carbonyl fragments in extracts of cucumber (Cucumis sativus) fruit and the possible role of lipoxygenase.

1. Homogenates and acetone powders of cucumber fruits catalyse the enzymic conversion of linoleic acid to aldehyde and oxoacid fragments in high yield, up to 60% with acetone powder extracts. 2. The major products are trans2-nonenal--a major component of the characteristic odour of cucumber--and 9-oxononanoic acid. 3. The cleavage reaction is a heat-labile, aerobic process, optimal at pH 6 (approx.). 4. Substrate specificity studies indicate that a lipoxygenase-type of reaction is involved in the cleavage process. 5. The acetone powder extracts have lipoxygenase activity and the proportion of linoleic acid hydroperoxide to carbonyl fragments depends upon incubation conditions. 6. Linoleic acid hydroperoxide isomers are also converted to carbonyl fragments by acetone powder extracts; the 9-hydroperoxide is cleaved at the 9-10 position whereas 12-13 cleavage is predominant with the 13-hydroperoxide isomer.