The role of lecithin: cholesterol acyltransferase in high density lipoprotein3/high density lipoprotein2 interconversion.

Serum was incubated in vitro with and without inhibition of lecithin:cholesterol acyltransferase (LCAT, EC 2.3.1.43). High density lipoprotein2 (HDL2) and high density lipoprotein3 (HDL3) were separated by zonal ultracentrifugation and analysed for lipid and apoprotein contents. The incubation of fresh sera resulted in a time-dependent decrease in HDL3 and an increase in HDL2. At the end of 24 h incubation HDL3 disappeared completely and the HDL2 peak had reached its maximum. The newly formed HDL2 was relatively enriched in total protein (apoprotein A-I, C-apoproteins) and cholesteryl esters, and depleted in phosphatidylcholine. Its migration in polyacrylamide gel electrophoresis was identical with HDL2 contained in fresh serum or HDL2 isolated from serum by zonal ultracentrifugation. The generated HDL2 particles exhibited the same electron microscopical characteristics as reference HDL2 samples prior to incubation. Addition of Ellman's reagent to the incubation mixture or heat inactivation of the samples prior to incubation resulted in a complete inhibition of HDL3/HDL2 interconversion, whereas addition of 1 mol/l NaCl had no detectable influence. There was also a substantial increase in HDL2 when VLDL-deficient serum was incubated at 37 degrees C. Similarly, in fresh serum from a patient affected with familial lipoprotein lipase deficiency, HDL3 was completely converted to HDL2. Our experiments demonstrate that LCAT promotes HDL3/HDL2 interconversion in native serum irrespective of the presence or absence of triglyceride-rich lipoproteins and lipoprotein lipase.