The effect of albumin on the state of aggregation and phase transformations in phosphatidylcholine-sodium cholate mixtures

Dilution of phospholipid-detergent mixed micellar systems results in an increase of the fraction of monomeric detergent and, consequently, in a decrease of the effective ratio R_e between non-monomeric detergent and phospholipid. The value of R_e has been previously shown to be the main determinant of the state of aggregation in such mixtures: at R_e values below a critical value R_e^SAT, the mixture is vesicular; at R_e values higher than R_e^SOL, the mixture is micellar, whereas within the range of R_e^SAT - R_e^SOL, vesicles and micelles co-exist. Albumin binds bile salts. Therefore, in PC-cholate mixtures, R_e is reduced by the presence of albumin in the system. Within the range of PC concentrations of 2-23 mM, cholate concentrations of 2-15 mM and BSA concentrations of 0-100 mg/ml, binding of cholate to BSA results in reduction of the effective cholate concentration to the extent of 0.11 mM cholate per 1 mg/ml BSA, namely up to 7 cholate molecules bind to each BSA molecule. Yet, the values of R_e^SAT and R_e^SOL are essentially independent of BSA. In addition, at any given R_e value, the size of vesicles made by dilution of mixed micelles is a complex function of albumin and PC concentrations. Possible mechanistic details which may cause this effect are discussed. These effects of albumin on the state of aggregation of PC-cholate mixtures must be taken into account in studies of such mixtures in the presence of albumin when other effects of albumin (e.g., on phospholipolysis) are investigated. Practical conclusions are reached with respect to the procedures that can be used to prepare vesicles of identical composition and size in the presence of different concentrations of albumin.