MODIFICATION OF THE CHOLESTEROL EFFLUX PROPERTIES OF HUMAN SERUM BY ENRICHMENT WITH PHOSPHOLIPID

To investigate the importance of phospholipid in promoting cholesterol efflux from cells, phospholipid multilamellar vesicles were incubated with normal human serum and the efflux ability of these lipid-modifie d sera was tested. When incubated under appropriate conditions, both d imyristoyl-phosphatidylcholine (DMPC) and bovine brain sphingomyelin ( BBSM) were shown to combine with components of human serum to form new protein:lipid complexes and to markedly enhance the ability of serum to promote efflux: of cholesterol from Fu5AH cells. In particular, the high density lipoprotein (HDL) particles were altered in their compos ition and electrophoretic properties and the or-migrating species, whi ch were reactive with antibodies to apoA-I, were converted to larger, pre-beta-migrating particles, similar in electrophoretic properties to pre beta(2)-HDL. DMPC, but not BBSM, also generated particles with mo bility similar to pre beta(1)-HDL; these species were demonstrably dif ferent from the discoidal complexes formed by reaction of DMPC with pu rified apoA-I. However, no change in cholesterol efflux potential was observed when serum was mixed with phospholipids that failed to intera ct or when cells were incubated with phospholipid multilamellar vesicl es alone. To further identify the components of serum that become alte red in their efflux potential after reaction with phospholipid, isolat ed lipoprotein fractions were incubated with DMPC or BBSM and it was f ound that only interaction with HDL caused enhancement of cholesterol efflux. In summary, cholesterol removal from the Fu5AH cells by serum can be promoted by adding phospholipid under conditions where new HDL- like complexes can be formed between the phospholipid and serum compon ents, most notably apolipoprotein A-I.