Complete exchange of phospholipids between microsomes and plasma lipoproteins mediated by liver phospholipid-exchange proteins

Abstract

It is known that cholesterol and phospholipids are in dynamic equilibrium and spontaneously exchange both in vitro and in vivo between individual classes of plasma lipoproteins [l-4] . Spontaneous transfer of phospholipids has also been shown between lipoproteins and erythrocytes [5],mitochondria [6] ,liver microsomes [7] , liver slices [8] and cells in culture [9]. In these examples of phospholipid exchange it is presumed that transfer occurs upon formation of a collision complex. Evidence for a protein-facilitated transfer of lipoprotein phospholipids in serum is controversial [7,10]. On the other hand, eukaryotic cells contain proteins designated as phospholipid exchange proteins (PLEP) that do facilitate the transfer of phospholipids from one membrane to another [ 1 I-131 . A purified PLEP from beef liver shows a highly specific affinity for phosphatidylcholine (PC-PLEP [ 141). Two proteins have been isolated from bovine cerebral cortex which stimulate both the transfer of phosphatidylinositol (PI) and PC (PI-PLEP [IS] ). Recently, exchange proteins have been isolated from rat liver [ 161 and hepatoma [ 171 which lack specificity in the transfer of phospholipid. The purpose of the present study was to determine whether and if so, to what extent, purified PCand PI-exchange proteins catalyze the transfer of phospholipids between rat liver microsomes and the individual classes of lipoproteins. It will be shown that the protein-mediated transfer of labeled PC and PI from microsomes to very low density lipoproteins (VLDL), LDL and HDL was 5-lo-fold higher than