Enzymic Oxygenative-cleavage Reaction of Linolenic Acid in Leaves — Chloroplastic Lipoxygenase and Fatty Acid Hydroperoxide Lyase in Tea Leaves

Abstract

Since (2E)-hexenal (leaf aldehyde) and (3Z)-hexenol (leaf alcohol) were first found in some bushes and tea leaves(1–3), many early works have been reported that these C6-volatile compounds are originating from plant tissues and largely responsible for the characteristic odor of various fruits and green leaves of vegetables and trees. Liholeic acid and linolenic acid having (1Z, 4Z)-pentadiene moiety were reported to be precursors of hexanal and hexenals, respectively, in 1966.(4) The reactions involved in the major biosynthetic pathway for C6-aldehydes consist of four sequential steps; acyl hydrolysis of lipids, hydroperoxidation of linoleic acid and linolenic acid, cleavage of the fatty acid hydroperoxides and isomerization of (3Z)-hexenal to (2E)-hexenal (Fig. 1).(5–9)) The three enzymes and one non-enzymic factor involved in the pathway are lipolytic acyl hydrolase(LAH), lipoxygenase, fatty acid hydroperoxide lyase (hydroperoxide lyase) and an isomerization factor (Fig. 1). Volatile C9-aldehydes are formed by a similar pathway.(7, 10, 11) Among these enzymes lipoxygenase and hydroperoxide lyase are the most important. The substrate specificities and the product specificities of these enzymes determine the composition of volatile aldehydes formed from linoleic acid and linolenic acid. The present report describes occurrence of, and substrate and product specificities of, lipoxygenase and hydroperoxide lyase in leaves, particularly in tea leaves.