Nature of the interaction of dextran sulfate with high and low density lipoproteins in the presence of Ca2+.

The interactions of dextran sulfate with high density lipoproteins (HDL) and with low density lipoproteins (LDL) were studied to differentiate their mechanisms and to provide some information of the charge characteristics of these lipoproteins. The insoluble complex formation with high density lipoprotein 3 (HDL3) progressively increased with increases of the Ca2+ concentration; complete conversion occurred with 60 mM CaC12. Between the two major subclasses, HDLz was more effectively converted to insoluble complex at lower CaClz concentrations than HDL3. It appeared that primarily the phospholipid components of HDL were involved in the formation of the insoluble complex and that the protein components reduced the complex forming ability of HDL. However, both apolipoproteins A-I (apo-A-I) and A-II (apo-A-II) possessed a definite reactivity with dextran sulfate in the presence of divalent metal ions, and a reduction in the ionic strength progressively enhanced their complex formation. In addition, modification of the apo-A-I carboxyl groups with glycine methyl ester, after activation with carbodiimide, was found to greatly increase its complex forming ability. Although LDL strongly interacted with dextran sulfate, primarily involving the positive charges of the protein moiety, increases in the Ca2+ concentration seemed to cause a progressive increase in the participation of the zwitterionic polar heads of the LDL phospholipids in complex formation. The study of the interaction of the preincubated apo-A-I and lysolecithin mixtures with dextran sulfate in the presence of Ca2+ revealed that these two components produced a molecular complex which interacted with dextran sulfate as an organized unit.