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 t o influence cell permeability, although the mechanisms of this process are unclear. Here, the glycoprotein effects on the permeability of os motically stressed phospholipid liposomes are studied as a model of me mbrane permeability. Liposomes containing glycoprotein were found to b e osmotically sensitive to water and chloride salts of some monovalent (Na+, K+) and bivalent (Mg2+, Ca2+) ions. The permeations of these su bstances were determined by light-scattering measurements of the volum e changes in liposomes after mixing with hyperosmotic solutions of chl oride salts. The time courses of scattered light were recorded by mean s of stopped-flow spectrophotometry. Two processes were studied: the f ast water outflow from liposomes and slower ion permeations through th e lipid membrane. The second order permeation rate constants were dete rmined at different glycoprotein concentrations for both processes. Va lues from 66 to 250 x 10(3) for water outflow and 2-500 M-1 sec(-1) fo r the different ion permeations were obtained in order to characterize the permeations of solutes across the Lipid membrane. The apparent ac tivation energies also were calculated between 18 and 33 degrees C. Th e mercurial sulphydryl reagent pCMBS inhibited the ion permeations in the slow phase. When pCMBS was present in this phase, higher activatio n energies were obtained, indicating more difficult permeations. An in terpretation of these results is that membrane permeability is mediate d by aqueous pores. Membrane selectivity to monovalent metal ions also was demonstrated, but no correlation was observed between the ion rad ius of the corresponding metal cation and permeation rate constants. T he discovery of non-specific pores in liposomes containing glycoprotei n shows that they can serve as vehicles for the water and ions in the processes of passive transport through lipid membranes. (C) 1997 Elsev ier Science Ltd.