The surface charge of apolipoproteins, phospholipid liposomes, and human very low density lipoproteins

The surface charge of isolated apolipoprotein A-I, apo-A-II, apo-C-II, and apo-C-III2 as well as of liposomes from synthetic highly purified phospholipids and isolated very low density lipoprotein (VLDL) particles from different donors (n = 35) was determined by polyelectrolyte titration. The particle size of apolipoproteins was evaluated from their molecular weight and specific volume, while that of VLDL particles was determined by photon correlation spectroscopy. The surface charge density of apolipoproteins and of VLDL at pH 7 was calculated from the number of surface charges of the particle and the surface area. The experimental net charge versus pH curves for apo-A-I and apo-A-II are very similar to the theoretical data. The differences between experimental and calculated results for apo-C-II and apo-C-III2 are believed to result from "polyelectrolyte" effects of neighboring charged centers within the protein molecule causing a decrease of dissociation of carboxylic residues. Neutral phospholipid liposomes do not exhibit anionic or cationic properties between pH 3 and 9. Liposomes from anionic phospholipids behave similar to polymeric carboxylic acids, i.e. their degree of dissociation increases with pH. The number of surface charges of VLDL particles increases with the particle size, while their surface charge density is about 1.10 +/- 0.36 charges/nm2 of the surface area.