Lipolytic acyl hydrolase

Fig. 1. Biosynthetic pathway of C6 and Cg-compounds from Ci8"fatty acids. LAH, Lipolytic Acyl Hydrolase ADH, Alcohol Dehydrogenase IF, Isomerization Factor.

The mechanism by which xenobiotic alcohols or esters are converted to fatty acid esters has not been studied. They could be formed by the action of lyase enzymes in the presence of fatty acid glyceryl esters, as in the conversion of farnesol to farnesol fatty acid esters (150). Some lipolytic acyl hydrolase enzymes from plants readily catalyze the transfer of lipid-bound fatty acids to low MW alcohol acceptors (150.151) and enzymes of this class could be responsible for the occasional formation of fatty acid conjugates of xenobiotic alcohols. Mechanisms involving fatty acid acyl CoA, phospholipids, or direct esterification with fatty acids might also be involved (1 ). 

The following experiments illustrate that when studying the involvement of phospholipase in the host-pathogen interaction, the total contribution of enzyme of host origin may be considerably higher than previously realized. Rodionov and Zakharova (32) recently reported very high rates of autolytic hydrolysis of membrane lipids in homogenates of potato leaves (26-37% of the phospholipids were hydrolyzed after 2 h at 0-1 ). Our laboratory recently confirmed this observation and proceeded to study sosie of the properties of the lipolytic acyl hydrolase activity in potato leaves (6). Lipolytic acyl hydrolase activity is apparently inactivated by polyphenol oxidase or its toxic quinone products. 

True lipases from plants will hydrolyze these partial glycerides, but other enzymes that attack monoacylglycerols (but not triacylglycerols) have been described. However, in most cases the full substrate specificities of these enzymes have not been studied. In one case a lipolytic acyl hydrolase from potato tubers was shown to hydrolyze mono- and diacylglycerols in addition to a range of polar lipids. Thus, to avoid introducing a class of hydrolytic... 

Hg.3. Effect of methanol concentration on acyltransferase activity of lipolytic acyl hydrolase. Amounts of free fatty acids (circles) and fatty acid methyl esters formed (triangles) or lyso-phosphatidylcholine deacylated (squares) in 10-min incubations at 2S°C are given as percentage of substrate (lysophosphatidylcholine) added. (Reproduced from Galliard and Dennis, 1974, by permission.)... 

The same observation can be made with the lipolytic acyl hydrolases which also contaminate washed or sucrose-purified mitochondria (Figure 2). Such hydrolases produce rapid hydrolysis of membrane phospholipids and dramatically impair the mitochondrial functionning, and release large amounts of free fatty acids which are then oxidized to fatty acid hydroperoxides in the presence of molecular oxygen. Figure 2 clearly demonstrates the ability of percoll to remove such vacuolar enzymes which have harmful effects on the glycerolipid composition of plant organelles. 

GLYCOPROTEIN NATURE OF LIPOLYTIC ACYL HYDROLASES IN POTATO TUBERS AND LEAVES... 

The leaves and tubers of potatoes contain high levels of lipolytic acyl hydrolase activities (1,2,3). These enzymes are capable of hydrolyzing all endogenous phospholipids and galactoliplds. It was recently reported that all of the lipolytic acyl hydrolase activity in potato tubers is associated with a glycoprotein fraction called "patatin" which comprises about 30% of the soluble protein in tubers (4). This study was undertaken in order to verify whether this finding was valid for other varieties of potatoes (in ref 4 Kennebec was the only variety studied). 

LAH, lipolytic acyl hydrolase. IF, isomerization factor. ADH, alcohol dehydrogenase. 

Matsuda H, Hirayama O. Purification and properties of a lipolytic acyl-hydrolase from Potato leaves. Biochim Biophys Acta 1979 573 155-165. 

Sahsah Y, Pham Thi AT, Roy-Macauley H et al. Purification and characterization of a soluble lipolytic acyl hydrolase from Cowpea leaves. Biochim Biophys Acta 1994 (in press). 

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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