Interaction of Plasma “Arginine-Rich” Apolipoprotein with Dimyristoylphosphatidylcholine

Very low density lipoproteins isolated from the plasma of cholesterol-fed rabbits contain abnormally high amounts of cholesterol, phospholipid, and an apoprotein referred to as the "arginine-rich" protein (ARP). It is generally assumed that the major interaction between apolipoproteins and lipids is between the protein and the phospholipids. Therefore, we have studied in the present report the lipid-binding properties of ARP to dimyristoylphosphatidylcholine (DMPC) vesicles in order to determine the importance of this interaction for ARP. The interaction was studied by ultracentrifugal flotation, circular dichroism, and microcalorimetry. The binding studies were performed using low protein-to-lipid ratios so as to minimize protein-protein interaction and vesicle disintegration. The ARP-DMPC complexes were isolated by salt density ultracentrifugation in KBr and had an average DMPC to protein molar ratio of 625 to 1. The complexes were stable for several days. The addition of DMPC to ARP induced an increase in the α helicity of the protein; the maximal change (from 45% to 65%) in α-helical content required 90 min with a t_1/2 of approximately 15 min. The enthalpy of association of ARP with DMPC was highly exothermic with a value ΔH = -614 kcal/mol of protein. The rate of heat release in this measurement was time dependent, requiring in excess of 20 min; however, the enthalpic changes were totally finished when the helical increase was only about one-half complete. Based on the kinetics of interaction, we suggest that the high enthalpy of binding may be associated with the increase in helicity of the protein; these two processes, though, are not sufficiently concomitant to account unequivocally for the heat release in terms of either protein-lipid interaction or protein structural changes.