A UNIQUE FEATURE OF LIPID DYNAMICS IN SMALL-INTESTINAL BRUSH-BORDER MEMBRANE

The lipid composition of the brush border membrane (BBM) or apical pla sma membrane of enterocytes is characterized by a remarkably high glyc osphingolipid content (glycosphingolipid: phospholipid:neutral lipid m ole ratio of about 1:1:1). A manifestation of the high glycolipid cont ent of the BBM is the lipid fluidity which is low compared to other ma mmalian plasma membranes and related to it a steep flexibility gradien t: hydrocarbon chain segments close to the lipid-water interface have quasi-crystalline packing while hydrocarbon chain segments close to th e centre of the lipid bilayer behave like a fluid. An important functi on of the BBM is the absorption of dietary lipids. The absorption of c holesterol from bile salt micelles has been shown to be protein-mediat ed. The integral membrane protein responsible for this activity has fe atures similar to non-specific lipid transfer proteins. Another remark able property of the BBM is described here: phospholipids are exchange d between the lipid bilayer of the BBM and the lipid bilayers of small unilamellar egg phosphatidylcholine (PC) vesicles. In the course of t his probably 1:1 exchange, endogenous BBM phospholipids move out of th e BBM and the lipid loss is compensated by the insertion of exogenous PC from the small unilamellar vesicles. This exchange activity is prob ably due to the same protein(s) responsible for lipid absorption in th is membrane or at least related to the absorptive capacity of the BBM. The unique feature of small intestinal BBM is that the on- and off-ra te of certain lipids is remarkably high: the underlying structure of t his activity is still unknown.