Thermal behavior of human plasma high density lipoprotein

Abstract

Human plasma low density lipoprotein displays a reversible thermal transition between 20 and 40° C, due to a phase transition of its core cholesterol ester from a smectic to a more liquid-like state. To determine if the cholesterol ester of high density lipoprotein (HDL) displays similar thermal behavior, the human lipoprotein and its extracted lipid have been examined by differential scanning calorimetry, low angle X-ray scattering and polarizing microscopy. Neither HDL 2 ∗∗ ∗∗ HDL 2 was isolated in the salt density range 1.063−1.125 g/ml and HDL 3 1.125–1.21 g/ml. Apo A-l is apolipoprotein A-1 ( M r 28 300) and apo A-2 is apolipoprotein A-2 ( M r 17 100). ( d 1.063−1.125 g/ml) nor HDL 3 ( d 1.125−1.21 g/ml) show thermal transitions between 0 and 60° C. By contrast cholesterol ester isolated from HDL and mixtures of cholesterol oleate and linoleate show reversible liquid crystalline transitions between 20 and 40° C. X-ray scattering studies of HDL 2 and HDL 3 performed at 10° C show no scattering fringes attributable to a smectic phase of cholesterol ester. When HDL is heated to temperatures above 60° C a broad, double-peaked endotherm is observed. The first component (peak temperature = 71° C) corresponds to a selective release of apoprotein A-1 from the lipoprotein, and the second component (peak temperature = 90° C) to a more generalized disruption of lipoprotein structure with release of cholesterol ester and apoprotein A-2. Following the thermal disruption of HDL, reversible liquid crystalline transitions of cholesterol ester can be seen by differential scanning calorimetry and polarizing microscopy, showing the presence of large domains of cholesterol ester. The absence of cholesterol ester transitions in intact HDL may indicate an interaction of cholesterol ester molecules with the protein-phospholipid surface of HDL that prevents the formation of an organized lipid phase. The high temperature behavior of HDL indicates that apoprotein A-1 is less important than apoprotein A-2 in maintaining the HDL apolar lipids in the form of a stable microemulsion.