Combination of delipidized high density lipoprotein with lipids.

A method has been developed for combining lipids in vitro with delipidized high density lipoprotein (apo-HDL) by mixing a concentrated petroleum ether solution of lipids with an aqueous solution of apo-HDL. When a mixture of plasma lipids was used, combination of phospholipids and cholesterol was observed in amounts comparable with those present in native HDL; triglycerides and cholesterol esters combined, but in much smaller amounts. Free fatty acids combined with apo-HDL and with albumin, but other lipids did not combine with albumin or with other proteins tested. Washing the recombined apo-HDL with petroleum ether did not remove lipids. The mobility of HDL on paper electrophoresis was decreased by removal of all lipids, and this change was reversed by recombination. Practically all the lipids present in the aqueous phase traveled with the protein. The ultracentrifugal sedimentation pattern of HDL was changed by delipidization to give an additional peak with a higher sedimentation coefficient, suggesting aggregation; recombination with lipids reversed this change to give a pattern similar to that of native HDL. Combination with phospholipids alone, either as a petroleum ether solution or as a micellar solution, produced the same change in ultracentrifugal pattern of apo-HDL as did combination with plasma lipid mixture. Cholesterol alone combined with apo-HDL in much smaller amounts than when phospholipids were also present, suggesting that combination of phospholipids with apo-HDL is the primary step in the formation of HDL.