Topological distribution of bovine serum albumin in multilamellar and unilamellar liposomes.

The topological distribution of bovine serum albumin (BSA) in multilamellar vesicles (MLV) and unilamellar vesicles (ULV) composed of 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DML)/cholesterol (molar ratio, 3:1) was studied by ESR using hydrophobic spin-labelled lecithins and hydrophilic Tempocholine. A spin-labelled BSA was also prepared, characterized and used as a probe. Results with hydrophobic spin-labelled lecithin probes showed no significant phospholipid-albumin interaction, indicating the virtual absence of albumin from the phospholipid bilayer of MLV and ULV. Reduction with L-ascorbic acid showed that MLV contained about 50% and ULV 90% of spin-labelled albumin on the surface. The distribution of Tempocholine in MLV and ULV was similar to that of entrapped BSA. These findings were confirmed by results using liposomes treated with nickel which broadened the ESR spectra of probes on the surface of vesicles. This study and our previous work suggest that the immunoadjuvant effect of liposomes can be mediated by surface antigens and can be maximized by preferential surface distribution as in ULV-associated BSA.