VOLATILE C-6-ALDEHYDE FORMATION VIA HYDROPEROXIDES FROM C-18-UNSATURATED FATTY-ACIDS IN ETIOLATED ALFALFA AND CUCUMBER SEEDLINGS

Etiolated seedlings of alfalfa and cucumber evolved n-hexanal from linoleic acid and cis-3-hexenal and trans-2-hexenal from linolenic acid when they were homogenized. The activities for n-hexanal formation from linoleic acid, lipoxygenase and hydroperoxide lyase were maximum in dry seeds and I - 2 day-old etiolated seedlings of alfalfa, and in 6 - 7 day-old etiolated seedlings of cucumber. n-Hexanal was produced from linoleic acid and 13-hydroperoxylinoleic acid by the crude extracts of etiolated alfalfa and cucumber seedlings. cis-3-Hexenal and trans-2-hexenal were produced from linolenic acid and 13-hydroperoxylinolenic acid by the crude extracts of etiolated alfalfa and cucumber seedlings. But these extracts, particularly cucumber one, showed a high isomerizing activity from cis-3-hexenal to trans-2-hexenal. When the C6-aldehydes were produced from linoleic acid and linolenic acid by the crude extracts, formation of hydroperoxides of these Cwfatty acids was observed. When 9-hydroperoxylinoleic acid was used as a substrate, trans-2-nonenal was produced by the cucumber homogenate but not by the alfalfa homogenate. As the enzymes concerned with C18-aldehyde formation, lipoxygenase was partially purified from alfalfa and cucumber seedlings and hydroperoxide lyase, from cucumber seedlings. Lipoxygenase was found in a soluble fraction, but hydroperoxide lyase was in a membrane bound form. Alfalfa lipoxygenase catalyzed formation of 9- and 13-hydroperoxylinoleic acid (35: 65) from linoleic acid and cucumber one, mainly 13-hydroperoxylinoleic acid formation. Alfalfa hydroperoxide lyase catalyzed n-hexanal formation from 13-hydroperoxylinoleic acid, but cucumber one catalyzed formation of n-hexanal and trans-2-nonenal from 13- and 9-hydroperoxylinoleic acid, respectively. From the above results, the biosynthetic pathway for Cs-aldehyde formation in etiolated alfalfa and cucumber seedlings is established that C6-aldehydes (n-hexanal, cis3-hexenal and trans-2-hexenal) are produced from linoleic acid and linolenic acid via their hydroperoxides by lipoxygenase and hydroperoxide lyase. © 1979 Taylor & Francis Group, LLC.