Human Serum Low-density Lipoprotein Protein: its Conformation studied by Circular Dichroism

Abstract

THE low-density lipoprotein of d 1.019–1.063 (LDL₂) is one of the principal lipoprotein classes of human serum. It contains about 23 per cent protein by weight and can be separated with a high degree of purity from other serum components (review in ref. 1). Several physical and chemical parameters of this lipoprotein have been determined and its antigenic specificity has been clearly documented¹. The structural properties of LDL₂ are largely unknown. Significant steps toward the elucidation of this problem have been the recent isolation of LDL₂ protein in an essentially lipid-free form and the demonstration of its solubility in various aqueous media^2,3. Such an observation has provided a means of assessing the physico-chemical and functional properties of this protein and its relative contribution to the overall structure of LDL₂. We report here experiments carried out by the technique of circular dichroism (CD) in an attempt to define the conformation of LDL₂ protein in its natural lipid environment and after the removal of lipids. Significant spectral differences were noted between lipid-rich and lipid-free products, and such differences could be markedly accentuated by either modifying the protein chemical or by changing the nature of the medium around the optically active chromophores.