Nature of the interaction between low-density lipoproteins and polyanions and metal ions, as exemplified by heparin and Ca2+.

I examined the effect of lipid concentration on the interaction of the very-low density and low-density lipoproteins in serum with heparin and calcium. A precipitate forms when partly delipidated serum is subjected to the polyanion-metal ion system. Although this precipitate is less turbid than the complexed lipoprotein-polyanion-metal ion in undelipidated serum, the two precipitates contain identical amounts of apolipoprotein. Totally delipidated serum produces only a slight precipitate with heparin and calcium, and this precipitate contains only a fraction of the appolipoproteins. Cholesterol and triglycerides are the major determinants of turbidity when serum is mixed with heparin and calcium, but have no effect on the precipitation of the protein moiety. Phospholipids contribute a small proportion of the turbidity in the lipoprotein-polyanion-metal ion interaction. Precipitation of the low-density lipoproteins by heparin and calcium depends on the protein moiety and on the chemical composition of the lipid moiety. Phospholipid is required, but cholesterol and triglyceride are not, for total precipitation of the complex.