The association of D-glucose with unilamellar phospholipid vesicles.
The equilibrium uptake of hydrophilic solutes, D-glucose and L-carnitine, by large unilamellar phospholipid vesicles composed of egg lecithin (PC), phosphatidic acid (PA), and various concentrations of cholesterol (Chol) has been measured. Calculation of the encapsulated volume of PC-PA and PC-PA-Chol vesicles, based on electron-microscopy data, agreed with the values directly measured by fluorescence techniques. Likewise, vesicle surface areas determined directly and from electron microscopy were in good agreement. Equilibrium uptake experiments by these well-characterized vesicles showed that glucose was taken up in excess of that amount predicted on the basis of the encapsulated aqueous volume. In contrast, the equilibrium uptake of carnitine can be predicted solely on the basis of the vesicle encapsulated volume. Each excess glucose molecule was found to be associated with from 7 to 5200 phospholipid molecules for 100 and 0.1 mM glucose, respectively. Uptake of glucose by PC-PA-Chol vesicles is independent of the cholesterol concentration and is similar to that observed in PC-PA vesicles. The cholesterol concentration independence and oil/buffer partitioning studies with octane and octanol, coupled with previous studies, strongly suggest that excess glucose is located in the vicinity of the phospholipid head group. A probable mechanism would have phospholipid, water and glucose all involved in the interaction rather than a competition between water and glucose for the phospholipid surface, as has been suggested in the literature.
Bummer
P M
PM
School of Pharmacy, University of Wisconsin-Madison 53706.
Zografi
G
G
D001692:Biological Transport; D002331:Carnitine; D002784:Cholesterol; D005947:Glucose; D008081:Liposomes; D008433:Mathematics; D008954:Models, Biological; D010712:Phosphatidic Acids; D010713:Phosphatidylcholines; D014316:Tritium
D008081:Liposomes; D010712:Phosphatidic Acids; D010713:Phosphatidylcholines; D014316:Tritium; D002784:Cholesterol; D005947:Glucose; D002331:Carnitine