Improved direct determination of serum cholesterol in low-density lipoproteins with use of polycations

The previously described procedure of direct determination of LDL-cholesterol after selective extraction of VLDL and HDL with polycations and a polycation-exchange resin has been improved by using dodecoylated poly(ethyleneimine) instead of poly(ethyleneimine). The acyl derivative has a higher affinity for lipoproteins. Disc-electrophoretic patterns of lipoproteins separated by this procedure correspond to patterns of apolipoproteins in VLDL and HDL, a finding that corroborates the conclusion that the mechanism of the selective binding of certain lipoproteins to lipophilic water-soluble polycations is based on hydrophobic interaction between the polymer and lipids located at the surface of lipoprotein particles. The procedure offers a simple possibility for direct LDL-cholesterol and LDL-triglyceride determination and for the isolation of VLDL and HDL. The mechanism of the reaction has a biochemical correlate in the action of phospholipase A2 on phospholipids.