Interconversion between apolipoprotein A-I-containing lipoproteins of pre-beta and alpha electrophoretic mobilities

Apolipoprotein (apo) A-I-containing lipoproteins can be separated into two subfractions, pre-beta HDL and alpha HDL (high density lipoproteins), based on differences in their electrophoretic mobility. In this report we present results indicating that these two subfractions are metabolically linked. When plasma was incubated for 2 h at 37 degrees C, apoA-I mass with pre-beta electrophoretic mobility disappeared. This shift in apoA-I mass to alpha electrophoretic mobility was blocked by the addition of either 1.4 mM DTNB or 10 mM menthol to the plasma prior to incubation, suggesting that lecithin:cholesterol acyltransferase (LCAT) activity was involved. There was no change in the electrophoretic mobility of either pre-beta HDL or alpha HDL when they were incubated with cholesterol-loaded fibroblasts. However, after exposure to the fibroblasts, the cholesterol content of the pre-beta HDL did increase approximately sixfold, suggesting that pre-beta HDL can associate with appreciable amounts of cellular cholesterol. Pre-beta HDL-like particles appear to be generated by the incubation of alpha HDL with cholesteryl ester transfer protein (CETP) and either very low density lipoproteins (VLDL) or low density lipoproteins (LDL). This generation of pre-beta HDL-like particles was documented both by immunoelectrophoresis and by molecular sieve chromatography. Based on these findings, we propose a cyclical model in which 1) apoA-I mass moves from pre-beta HDL to alpha HDL in connection with the action of LCAT and the generation of cholesteryl esters within the HDL, and 2) apoA-I moves from alpha HDL to pre-beta HDL in connection with the action of CETP and the movement of cholesteryl esters out of the HDL. Additionally, we propose that the relative plasma concentrations of pre-beta HDL and alpha HDL reflect the movement of cholesteryl esters through the HDL. Conditions that result in the accumulation of HDL cholesteryl esters will be associated with low concentrations of pre-beta HDL, whereas conditions that result in the depletion of HDL cholesteryl esters will be associated with elevated concentrations of pre-beta HDL. This postulate is consistent with published findings in patients with hypertriglyceridemia and LCAT deficiency.