Lipid composition of the vacuolar membrane of Acer pseudoplatanus cultured cells.

Tonoplast was prepared by osmotic lysis of a pure vacuolar fraction isolated from protoplasts derived from Acer pseudoplatanus cultured cells. After their extraction, neutral and polar lipids were separated by a thin layer chromatography. Phospholipids, glycolipids and neutral lipids represented 44.5%, 39.1% and 16.4% of total lipids, respectively. Sterols (glycosylated plus non-glycosylated forms) constituted 30.8% of total lipids; 75% of sterols were glycosylated. The most prominent lipids were phosphatidylethanolamine (20.8%), phosphatidylcholine (13.5%), ceramide monohexoside (12.8%), steryl glycoside (12.2%) and acylated steryl glycoside (10.9%). Glucose was the only sugar released by acid hydrolysis of these three later compounds. The major neutral lipids were free sterols, triacylglycerols and steryl esters. The phospholipid composition was characterised by a high content of phosphatidylethanolamine (50% more than phosphatidylcholine). The fatty acid composition of phospholipids revealed two major components: palmitic and linoleic acids; they accounted together for 60 to 80% of fatty acids. When tonoplast was incubated at pH 5.5 with 5 mM Ca2+, the free fatty acid content (12% of neutral lipids) increased by 10% and lysophospholipids were detected. This indicated the presence of a calcium-dependent phospholipase A2 associated with the tonoplast.