Dissociation of lipid-free apolipoprotein A-I from high density lipoproteins.

Abstract

Conditions under which apolipoprotein (apo) A-I dissociates from human high density lipoproteins (HDL) during incubation in vitro have been investigated. Dissociation of apoA-I was demonstrated by non-denaturing gradient gel electrophoresis followed by immunoblotting for apoA-I and by size-exclusion chromatography. It was quantitated after ultracentrifugation as the loss of apoA-I from the fraction of d 1.25 g/ml isolated by ultracentrifugation and of the lower molecular weight fractions recovered after size-exclusion chromatography revealed that the dissociated apoA-I was not associated with significant quantities of either cholesterol, phospholipids, or other apolipoproteins. When the dissociated apoA-I was subjected to agarose gel electrophoresis, it migrated to a prebeta position comparable to that of purified, lipid-free apoA-I. This contrasted with the original HDL that exhibited alpha migration. Thus, CETP-mediated transfers of cholesteryl esters from HDL to VLDL and LDL are accompanied not only by a reduction in HDL size but also by the progressive dissociation from HDL of a pool of prebeta-migrating, essentially lipid-free apoA-I.