Abstract
Highly purified plasma membranes (PM) were obtained from barley (Hordeum vulgare L. cv. Kristina) leaves and roots, spinach (Spinacia oleracea L. cv. Viking II) leaves, and cauliflower (Brassica oleracea) inflorescences by partitioning in an aqueous polymer two‐phase system. The sterol and polar lipid composition of the PM, including the fatty acid composition of the glycerolipids, was determined. Dominating lipids were free sterols, glucocerebroside, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), although large variations in content were observed between the PM of the different species and organs. Thus, the spinach leaf PM contained only 7% (mol %) free sterol compared to over 30% free sterol in the other PM analysed, with the barley root PM as the other extreme (57% free sterol). On the other hand, sterol derivatives were more abundant in the spinach leaf PM, containing 13% acylated sterol glycosides. Cerebroside constituted 16% of the lipids in the barley leaf PM but only 3% in cauliflower. The phospholipids PC and PE ranged from 25 and 24%, respectively, in the spinach leaf PM to 8 and 7%, respectively, in the barley root PM. As a result of the large variations in sterol and phospholipid content, the ratio of free sterol to phospholipid varied from 2.2 in the barley root PM to only 0.1 in the spinach leaf PM. Sitosterol, campesterol and stigmasterol were the completely dominating sterols in the barley and cauliflower PM, whereas the unique sterol composition of spinach was dominated by spinasterol. Palmitic (16:0), linoleic (18:2) and linolenic (18:3) acid were the major glycerolipid fatty acids. The fatty acid composition of the barley root PM was the most saturated (44% 16:0, 13% 18:3), whereas that of the cauliflower PM was the most unsaturated (21% 16:0,42% 18:3). Thus, very large variations were observed in both total lipid and fatty acid composition of the PM investigated, which represent both mono— and dicotyledons, as well as both photosynthetic and non‐photosynthetic tissue. The consequences of this large diversity in composition of the lipid bilayer for the function of integral PM proteins are discussed.
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