Electrophoretic mobility-ionic strength studies of proteins. IV. The effect of lipids on the electrophoretic patterns of human serum albumin at acid pH.

In previous publications from this laboratory, we have investigated the anomalous behavior of human serum albumin (l), various animal albumins (2) and human -y-globulin (3) in the Tiselius electrophoresis apparatus at acid pH. Because of increasing interest in the behavior of proteins in the acid region below their isoelectric point, this phenomenon has been receiving increasing attention from several groups of investigators as discussed in a recent review article by Brown and Timasheff (4). The main characteristics of the electrophoretic patterns of proteins in the isoelectric point region are the complex boundary patterns obtained and the lack of symmetry between the ascending and descending boundaries (5-7). Longsworth and Jacobsen (8) suggest that the nonenantiographic patterns obtained for bovine serum albumin under such conditions indicate continuously readjusted equilibria across the moving boundaries. Charlwood (9) studied the variations in the electrophoretic pattern of human serum albumin (2%) in 10 different organic buffer anion systems at pH 4.0 and 0.02 ionic strength. He concluded that the complex ascending patterns obtained must involve specific interactions between buffer anions and protein although they may be partly dependent on buffer capacity. Aoki and Foster (10-12) investigated the electrophoretic behavior of bovine serum albumin in the pH region between 3.0 and 4.5 with 0.2% protein solution at 0.02 ionic strength of chloride, thiocyanate, and acetate. Two main boundaries were observed whose percentage composition varied with pH although the patterns obtained were fairly enantiographic at the low protein concentration employed. These authors explained the anomalous electrophoretic behavior of bovine serum albumin at pH 4.0 as due to an isomerization reaction. Cann and Phelps (13-15) have also performed studies of the electrophoretic behavior of bovine serum albumin and ovalbumin in low and high ionic strength acetate buffers at pH 4.0 and at protein concentrations of approximately 1.3%. These investigators explain the complex nonenantiographic patterns at these higher protein concentrations as being due to false boundaries, i.e. those not due to single components, which result partly from the conductance and pH gradients in the electrophoresis cell but are also associated with the complex forming of the protein with the undissociated buffer acid (16). Recently, Woods (17) investigated the electrophoretic behavior of a number of proteins