EFFECT OF CHOLESTEROL ON APOLIPOPROTEIN-A-I BINDING TO LIPID BILAYERSAND EMULSIONS

The effects of cholesterol (Chol) on the interaction of apolipoprotein A-I (apoA-I) with phospholipid bilayer vesicles and lipid emulsions w ere investigated. ApoA-I bound to phosphatidylcholine (PC) vesicles wi th higher affinity and lower capacity compared to triglyceride-PC emul sions. An increase in surface Chol in triglyceride-PC emulsions decrea sed the binding capacity without changing the binding affinity. In con trast, addition of Chol to PC vesicles caused a marked increase in cap acity and decrease in affinity for apoA-I binding. ApoA-I caused a lar ge release of entrapped aqueous dye, calcein, from PC vesicles, wherea s this apoA-I-induced leakage was relatively small in the vesicles con taining Chol. The incorporation of phosphatidylethanolamine into the v esicles also exerted effects similar to those of Chol on apoA-I bindin g and calcein leakage. The shifts of fluorescence emission maximum of dansyl lysine, probing the surface region of membranes, indicated that Chol as well as phosphatidylethanolamine increased the headgroup spac e of the vesicles. The binding maximum of apoA-I was closely correlate d with the emission maximum of dansyl lysine, not with the fluorescenc e anisotropy of 1-[4-(trimethylamino)phenyl]phenylhexatriene, suggesti ng that the binding capacity of apoA-I to the,bilayer surface was modu lated by the headgroup space than the acyl chain fluidity. These resul ts show that Chol affects the bilayer surface so as to allow more apoA -I to bind to bilayers and may suggest the possibility of the interact ion of apoA-I with Chol-enriched membrane domains.