Osmotic water permeability through liposomes in the presence of alpha 1-acid glycoprotein

Purification and characterization of a novel human 15 kd cholesterol crystallization inhibitor protein in bile

Crystallization-inhibiting proteins can explain longer nucleation times associated with bile from gallstone-free subjects as compared with bile from patients with cholesterol gallstones. We partially characterized and examined the crystallization inhibitory potency of a newly purified 15 kd human biliary protein. Gallbladder bile was passed through an anti-apolipoprotein A-I (apo A-I) immunoaffinity column to extract lipid-associated proteins. The bound fraction was separated by 30 kd ultrafiltration. Sodium dodecyl sulfate-polyacrylamide gel electrophesis (SDS-PAGE) was performed under nonreducing and reducing conditions. Cholesterol crystallization activity was tested in a photometric cholesterol crystal growth assay. Isoelectric focusing was performed by using a standard gel. The purified 15 kd protein was subjected to N-terminal amino acid sequencing. Although the whole apo A-I-bound fraction contained a variety of proteins and lipids, its 30 kd filtrate yielded a nearly pure 15 kd protein with only minor contamination from apo A-1. Amino acid sequencing showed that the protein was unique. Enzymatic deglycosylation revealed no evidence for glycosylation. At a protein concentration of 10 micrograms/ml, crystallization time was delayed as compared with control and apo A-I, and final crystal mass was reduced to 75% of control. Its isoelectric point was 6.1 without isoforms. Under nonreducing conditions, the protein formed a 30 kd dimer and a 60 kd tetramer. We conclude that this protein is a novel potent biliary crystallization inhibitor protein.

Identification of human biliary alpha 1-acid glycoprotein as a cholesterol crystallization promoter

BACKGROUND/AIMS

We have recently outlined the biochemical features of a human 42-kilodalton biliary glycoprotein that shows concentration-dependent cholesterol crystallization-promoting activity. The goal in this work was to establish its identity and to examine some aspects of its biochemical properties relative to its activity.

METHODS

Internal amino acid sequencing following tryptic digestion was performed. Based upon this result, immunoreactivity against the 42-kilodalton glycoprotein was examined using a relevant antibody. With the same antibody, the 42-kilodalton glycoprotein was isolated from bile and assayed for activity. Sequential enzymatic deglycosylation of successive terminal glycans of the purified glycoprotein was performed, and the effects on both reductions in molecular radius (M(r)) and on comparative promoter activities were examined.

RESULTS

Both amino acid sequence and immunochemical data identify the 42-kilodalton glycoprotein as a biliary form of alpha 1-acid glycoprotein. When purified by immunoaffinity chromatography, potent promoting activity shown was proportionately reduced by successive removal of terminal glycans that also reduced the M(r)s.

CONCLUSIONS

The 42-kilodalton cholesterol crystallization-promoting glycoprotein is now identified as a biliary form of alpha 1-acid glycoprotein. Further, some aspects of the important role of glycans in this extensively glycosylated protein have been explored.

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.

Osmotic water permeability in glycoprotein containing liposomes

The kinetics of osmotic water permeability in proteoliposomes containing alpha 1-acid glycoprotein was investigated by means of stopped-flow spectrophotometry. A biphasic time-course of scattered light with time was registered. The rate constants calculated from fits to an exponential function in the first phase were proportional to the final medium osmolarity. The apparent second order rate constants Kapp (Osm-1 sec-1) were determined at different glycoprotein concentrations in the original mixture for preparation of proteoliposomes. The value of Kapp at lipid:glycoprotein weight ratio = 1 was plotted in Arrhenius coordinates. The calculated activation energy for water permeation through the lipid bilayer suggests that eventual channel mechanism may be involved due to the presence of glycoprotein molecule in the liposomes.