Charge heterogeneity of human low density lipoprotein (LDL)

Plasma samples were obtained from patients with known history of atherosclerosis as well as from a group of Bantus of South Africa and a group of medical students without any history of atherosclerotic vascular disease. The isoelectric point (IEP) of LDL of the different persons was determined by electrophoresis in agarose gel at different pH values. The IEP of the LDL of the students or of the Bantus, were fairly constant at pH 5.28; for the patients the IEP varied widely. The IEP of LDL from hypercholesteremic persons (Fredrickson type II) showed consistently a significantly higher IEP than for the controls. Electrophoretic mobility of the different samples of LDL was determined at pH 6.8 at different ionic strengths using NaCl or CaCl2. The effect of ionic strength (using NaCl) on the mobility was different for LDL isolated from different persons. Low concentrations of CaCl2 reverse the charge of lipoprotein from a negative to a positive one at pH 6.8. The drastic effect of Ca2+ on the net charge of LDL at pH 6.8, possibly due to the exposed phospholipid groups of LDL, shows that this ion may be important in the interaction between the lipoprotein and the mucopolysaccharides; such an interaction has been suggested by other workers as occurring in the arterial wall