Lipid Composition of Plasma Membranes and Tonoplasts Isolated from Etiolated Seedlings of Mung Bean ( Vigna radiata L.)

Abstract

The lipid composition of plasma membranes and tonoplasts from etiolated mung bean hypocotyls was examined in detail. Phospholipids, sterols, and ceramide monohexoside(s) were the major lipid classes in both membranes. The content of phospholipids on a protein basis was higher in the tonoplast, but the content of total sterols was similar in both membranes. Accordingly, the sterol to phospholipid molar ratio in the plasma membrane was higher than that of the tonoplast. Phosphati-dylethanolamine and phosphatidylcholine comprised the major phospho-lipids in both membranes. Phosphatidylinositol, phosphatidylserine, and phosphatidylglycerol were identified as minor phospholipid components. The content of phosphatidylinositol and phosphatidylglycerol was rela-tively high in the tonoplast, comprising 11 and 5% of the total phospho-lipids, respectively. Although special care was taken against the degra-dative action of phospholipase D and phosphatidic acid phosphatase during the isolation of these membranes, by adding EDTA, EGTA, KF, choline, and ethanolamine to the homogenizing medium, significant amounts of phosphatidic acid, about 15% of the total phospholipids, were detected in the plasma membrane. On the other hand, the content of phosphatidic acid in tonoplasts and other membrane fractions was very low. This fact may indicate that high levels of phosphatidic acid occur naturally in plasma membranes. Phosphatidylglycerol in both membranes and phosphatidylinositol in the tonoplast contained high levels of palmitic acid, which comprised more than 50% of the total fatty acids. Significant differences were observed in the sterol compositions of plasma mem-branes and tonoplasts. More than 90% of the sterols in the plasma membrane were unesterified, while the tonoplast was enriched in glyco-sylated sterols, especially acylated sterylglycosides. Ceramide mono-hexoside was found to be specifically located in these membranes, in particular, in the tonoplast, in which it comprised nearly 17% of the total lipids. Lipids are recognized as major chemical components of bio-membranes and play an important role in cellular activities through control of permeability, fluidity, and membrane-bound