Changes in the physical behavior of low density lipoprotein in the presence of glycosaminoglycans and high density lipoprotein.

Temperature dependent techniques - differential scanning calorimetry (DSC), polarizing microscopy - were used to study the properties of human serum low density lipoprotein (LDL) and its glycosaminoglycan (GAG) complexes, and to investigate the influence of the addition of high density lipoprotein (HDL) to the complex system. In the LDL molecule a reversible endothermic transition took place with its peak at 33 degrees C. Cholesteryl esters within the LDL core existed as an isotropic solution above this temperature (i.e. around body temperature), and in the form of smectic liquid crystals below it. When LDL was converted in vitro into GAG-LDL complexes by the addition of chondroitin-6-sulfate, dermatan sulfate, heparin or heparan sulfate, the DSC curves showed an evaluation of the transition temperature: the peak values in these samples were found at 40 degrees C and under the polarizing microscope a birefringence developed, typical of smectic liquid crystals. In chondroitin-4-sulfate-LDL complexes no alteration of the physical structure of the LDL molecule could be demonstrated. HDL decreased the transition temperature of GAG-LDL complexes and the disappearance of birefringence indicated that in the presence of HDL the lipids within the GAG-LDL complexes existed in a liquid phase at a temperature in the vicinity of body temperature.