Osmotic water permeability in glycoprotein containing liposomes

Kinetics and role of alpha 1-acid glycoprotein-dependent osmotic transport of water and ions in palmitoyl-L-oleoyl phosphatidylcholine liposomes

alpha 1-Acid glycoprotein isolated from human blood plasma is known to influence cell permeability, although the mechanisms of this process are unclear. Here, the glycoprotein effects on the permeability of osmotically stressed phospholipid liposomes are studied as a model of membrane permeability. Liposomes containing glycoprotein were found to be osmotically sensitive to water and chloride salts of some monovalent (Na+, K+) and bivalent (Mg2+, Ca2+) ions. The permeations of these substances were determined by light-scattering measurements of the volume changes in liposomes after mixing with hyperosmotic solutions of chloride salts. The time courses of scattered light were recorded by means of stopped-flow spectrophotometry. Two processes were studied: the fast water outflow from liposomes and slower ion permeations through the lipid membrane. The second order permeation rate constants were determined at different glycoprotein concentrations for both processes. Values from 66 to 250 x 10(3) for water outflow and 2-500 M-1 sec-1 for the different ion permeations were obtained in order to characterize the permeations of solutes across the lipid membrane. The apparent activation energies also were calculated between 18 and 33 degrees C. The mercurial sulphydryl reagent pCMBS inhibited the ion permeations in the slow phase. When pCMBS was present in this phase, higher activation energies were obtained, indicating more difficult permeations. An interpretation of these results is that membrane permeability is mediated by aqueous pores. Membrane selectivity to monovalent metal ions also was demonstrated, but no correlation was observed between the ion radius of the corresponding metal cation and permeation rate constants. The discovery of non-specific pores in liposomes containing glycoprotein shows that they can serve as vehicles for the water and ions in the processes of passive transport through lipid membranes.

Temperature-dependent osmotic permeability in glycoprotein containing liposomes

The osmotic water outflow of large multilamellar liposomes containing alpha 1-acid glycoprotein was measured at a temperature near the lipid's phase transition temperature. The liposomes were formed from a mixture of DPPC, cholesterol and glycoprotein in molar ratios 100:20:1, by continuous sucrose density gradient centrifugation. These liposomes captured 35% of the radiolabeled glycoprotein. The temperature-dependent experiments showed that near phase transition temperature the initial rate of water outflow increased drastically in comparison with glycoprotein free liposomes incubated in buffer containing glycoprotein. We suggested that eventual a channel mechanism may be involved due to spontaneous incorporation of glycoprotein into the bilayer.