Phosphatidylcholine exchange between brush border membrane vesicles and sonicated liposomes

Abstract

The brush border membrane is that part of the plasma membrane of the enterocytes which is most highly specialized in both digestive and transport functions. For a better understanding of its physiolog- ical functions a knowledge of the membrane architec- ture is essential. Phospholipid exchange activity is present in rat small intestine [l] and the isolation of phospholipid exchange proteins from rat small intestinal mucosa has been reported [2]. Phospholipid exchange has been assumed to play a role in the distribution of newly synthesized phospholipids between subcellular membranes. It may also be important for the process of fat absorption in the intestine [I]. Here we present evidence for the first time that in the presence of phosphatidylcholine exchange protein significant amounts of phosphatidylcholine can be incorporated into brush border membranes from rabbit small intestine when incubated with sonicated phosphatidylcholine liposomes. The extent of lipid transfer observed can be accounted for if it is assumed that practically all of the phosphati- dylcholine in brush border membrane is available for exchange as a single pool. Measurements of D-glucose uptake show that the brush border membrane vesicles remained intact and sealed under the conditions of intermembrane phospholipid exchange. Possible inter- pretations of our results are either that all phosphati- dylcholine is located on the outer surface of the mem- brane and readily accessible to the exchange protein or, that, if some phospholipid is present on the inner