Aqueous Dispersions of Phosphatidylserine. Ionic Properties.

The importance of acidic lipids as tissue ion exchange agents has been recently emphasized by a number of investigators (14). In addition, evidence is accumulating that these lipids may play a role in tra-nsmembrane cation transport (3, 5, 6). Consequently, it was felt desirable to study the ionic structure and ion exchange capabilities of the purified acidic lipids. Phosphatidylserine was selected for study. Ultrasonicated sols provide the means for reproducible, quantitative measurement of the ionic properties of the acidic lipids. The ultrasonication produces visually clear, stable dispersions of lipids in water (7, 8). Such lipids were formerly studied in crude aqueous suspensions (9), or in mixed solvent systems (10). The latter system permitted reasonable extrapolations of pK values, and hence a correct deduction of the ionic form in aqueous suspensions. Our study of phosphatidylserine shows that ultrasonicated dispersions of this acidic lipid contain micelles with an average molecular weight of 4 x 106. Essentially, all of the ionogenic polar groups are readily available for titration, and are therefore probably present on the micellar surface. The micelles are strongly acidic, and their isoelectric point is at pH 1.2. Structural evidence for the existence of the following three forms will be presented. CHZ-OCO-R