Higher plant lipoxygenase

Basic functions of lipoxygenases and their products in higher plants... 

Lipoxygenases (originally termed lipoxidases) were discovered in higher plants before the first reports on their presence in mammalian cells [1-4]. Nevertheless, the first clear biological function of a lipoxygenase was established for a mammalian enzyme (the arachidonate 5-lipoxygenase) when the biosynthesis of leukotrienes was elucidated [5]. More recently, research into plant lipoxygenases has intensified. 

In an indirect manner, the lipoxygenase reaction in higher plants appears to be implicated in the biosynthesis of abscisic acid, a further senescence hormone [23-25], Abscisic acid is formed from the carotenoid violaxanthin that is converted into xanthoxin, which is a plant growth inhibitor and at the same time a precursor of abscisic acid. The formation of xanthoxin requires the co-oxidative activity of a lipoxygenase-linoleic (or a-linolenic) acid system. 

Table 2 Induction of lipoxygenases in higher plants by pathogen infection ...

The main biological function of these proteins is electron transfer. An important non-haem metalloenzyme with bound iron, which occurs in prokaryotic organisms, higher plants and animals, is lipoxygenase, an enzyme catalysing the peroxidation of unsaturated fatty acids. 

Lipoxygenases (E.C. 1.13.11.12) catalyze the peroxidation of compounds that possess a cis cis-1,4-pentadiene structure. Their principal substrates in higher plants are the di- and tri-unsaturated fatty acids, linoleic (CIS 2) and linolenic (Cl 8 3) acids. Lipoxygenases (LOXs) are also known to catalyze the cooxidation of chlorophylls and carotenoids (Axelrod et al, 1987 Vick and Zimmermann, 1987). 

Mack, A.J., T.K. Peterman and J.N. Siedow. 1987. Lipoxygenase isozymes in higher plants biochemical properties and physiological role. Isozymes Current Topics Biol. Med Res. 13 127-154. 

Lipoxygenases have been known in higher plants for over 60 years (475,476) and more recently were identified in animals (477, 478) and algae (388-392, 479, 480). Much of the interest in the hydroxy fatty acids produced via the lipoxygenase pathway has been generated because of their role in the inflammatory responses of mammals (29-32) and fish (427, 428, 481, 482). A role for these products has also recently been postulated as part of plant defense responses against microbial invasion (483-488). This is just one class of dioxygenase, enzymes which catalyze the introduction of an intact O2 into a variety of substrates (489). 

Kuramshin RA. The Lipoxygenase Pathway of Polyenoic Fatty Acid Metabolism in Higher Plants (Doctoral Thesis). Kazan, 1991 23 pp. 

In higher plants, the lipolytic enzymes and their physiological functions are not well characterized [1]. iMost reports demonstrated that phospholipid catabolism in plants is achieved by the concerted actions of membrane-bound enzymes including phospholipase D, phosphatidate phosphatase, lipolytic acyl hydrolases and lipoxygenases [1,2]. With the exception of the phospholipase D, the literature on plant phospholipases is still very limited. We previously reported that tonoplast from Acer pseudoplatanus cells contains small amounts of phosphatidic acid and lysophospholipids, which were produced together with free fatty acids, particularly after addition of Ca " [31. These data suggested the possible involvement of phospholipase D and phospholipase A in the metabolism of vacuolar membrane lipids. The phospholipase activities were studied by following the hydrolysis of added sn-2-[14c]linoleyl-PC to tonoplast vesicles. Tonoplast was obtained by osmotic lysis of pure preparations of vacuoles isolated from protoplasts derived from Acer pseudoplatanus cells [4]. This present work demonstrated clearly the presence of phospholipase D and phospholipase Ai activities associated with the tonoplast of Acer, The phospholipase Ai showed an optimal activity at pH about 6-6.5, did not necessarily require divalent cations, but was stimulated by Mg- and particularly by Ca. This work presents the first evidence for the presence of phospholipases A in plant cells. 

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