Net transfer of phosphatidylcholine from plasma low density lipoproteins to sphingomyelin-apolipoprotein A-II complexes by bovine liver and human plasma phospholipid exchange proteins

Bovine liver phosphatidylcholine exchange protein facilitated the net transfer of dipalmitoyl phosphatidyl[N-methyl-¹⁴C]cholme from radiolabeled low density lipoproteins (LDL) to complexes of sphingomyelin and apolipoprotein A-II. Transfer of the radiolabeled phosphatidylcholine was dependent on temperature and incubation time, as well as the ratio of LDL to sphingomyelinapolipoprotein A-II. When unlabeled LDL was incubated with sphingomyelinapolipoprotein A-II complex containing [N-methyl-¹⁴C]sphingomyelin, negligible amounts of the radiolabel were transferred to the LDL, implying that the transfer of phosphatidylcholine to the sphingomyelin-apolipoprotein A-II complex was not coupled with a return of sphingomyelin to the LDL. Bovine liver phosphatidylcholine exchange protein also facilitated the net transfer of dipalmitoyl phosphatidyl[N-methyl-¹⁴C]choline from sonicated vesicles of 1-palmitoyl-2-palmitoleoyl phosphatidylcholine to sphingomyelin-apolipoprotein A-II complex. Dansyl phosphatidylethanolamine incorporated in the donor vesicles was not transferred to the sphingomyelin-apolipoprotein A-II complex, indicating that phospholipid transfer was not due to a disruption of the vesicles and subsequent fusion of vesicle remnants with the sphingomyelin-apolipoprotein A-II complex. Bovine liver phosphatidylcholine exchange protein did not transfer dipalmitoyl phosphatidyl[N-methyl-¹⁴C]choline or [³H]cholesteryl esters from high density lipoproteins (HDL) to sonicated vesicles of sphingomyelin. Fluorescence polarization studies showed that relative to the sphingomyelin-apolipoprotein A-II complexes, the sphingomyelin vesicles have a rigid structure which may account for this lack of transfer of the phosphatidyl[¹⁴C]choline. Phospholipid exchange protein(s) isolated from human plasma also facilitated the net transfer of dipalmitoyl phosphatidyl[¹⁴C]choline from LDL to sphingomyelin-apolipoprotein A-II complexes.