Hydrodynamic properties of phospholipid vesicles and of sucrase isomaltase-phospholipid vesicles.

The hydrodynamic properties of phosphatidylcholine vesicles prepared by the cholate removal method (Brunner, Biochim Biophys. Acta 455, 322-331) were determined by analytical ultracentrifugation and compared with those of sonicated vesicles. The former were homoge- neous as characterized by a narrow particle size distribution, but showed some fluctuation in the average particle size from one preparation to another. In con-trast, sonicated vesicles showed a wide particle size distribution with their average particle diameter depending on the conditions of the ultrasonic irradiation. The molecular weight of the enzyme complex sucrase- isomaltase, which is a major intrinsic protein of small intestinal brush border membrane, was determined in the analytical ultracentrifuge in the presence of two different detergents: sodium cholate and octyl tetraox- yethylene. The values thus obtained were 2.7 X 10‘ and 2.8 X lo‘, respectively. Upon removal of the detergent, the protein aggregated to regular oligomeric structures which were speroidal and had a diameter of 28 +. 4 nm (ie. about twice the diameter of the sucrase-isomaltase complex). A model membrane system was reconstituted from phosphatidylcholine and sucrase-isomaltase by the cholate removal