Mechanism of heparin and serum lipoprotein interaction: effects of calcium, phosphorylcholine, and a serum fraction.

The mechanism of formation of an insoluble complex between heparin and rat serum lipoprotein has been studied. Optical density changes during the reaction, counting of the fatty acid labelled lipoproteins in the precipitates, and complexing of 14C]palmitate-labelled lipoprotein with heparin-CNBr-Sepharose were used to quantitatively determine the formation of insoluble complexes. The maximal heparin--lipoprotein complex formation requires 25--30 mM of Ca2+, but with micromolar amounts of phosphorylcholine, the reaction was saturated at only 10 mM of Ca2+. The effect of phosphorylcholine in promoting the reaction was lost when purified chylomicrons or very low density lipoproteins were used. The effect of phosphorylcholine in promoting the interaction between heparin and pure chylomicrons or very low density lipoproteins was regained when a crude serum protein factor of unwashed chylomicrons was added to the system, suggesting that rat serum contains a protein factor(s) which normally inhibits the heparin--lipoprotein interaction by raising the requirement of Ca2+. Phosphorylcholine counteracted the effect of this protein, thereby favouring the precipitation reaction in the presence of much lower concentration of Ca2+. The results have been discussed with special reference to the possibility of a relationship between mucopolysaccharides, Ca2+, lipoproteins, and arterial phospholipids in the pathogenesis of atherosclerosis.