Evidence for the separation of albumin- and apo A-I-dependent mechanisms of cholesterol efflux from cultured fibroblasts into human plasma.

The role of albumin has been studied in the plasma-mediated efflux of cholesterol from cultured fibroblasts. Immunoaffinity chromatography of plasma on immobilized anti-albumin antibody decreased by 25-50% total efflux catalyzed by plasma. The remainder of the efflux-promoting capacity of plasma was deleted by immunoaffinity chromatography on antibody to apolipoprotein A-I, the major apoprotein of high density lipoprotein. Both components of efflux were saturable with half-saturation at 0.5-1.0% (v/v) plasma. However, the net transport of sterol from cells to medium catalyzed by lecithin:cholesterol acyltransferase, was not reduced by the deletion of the albumin-catalyzed component of efflux. This finding was confirmed with congenitally analbuminemic plasma. These results indicate that efflux to albumin and to high density lipoprotein in plasma represent independent mechanisms; only the latter is coupled to net transport.