X-Ray Small Angle Scattering on Human Plasma Lipoproteins 1 PETER LAGGNER, 2 KARL Mf3LLER, AND OTTO KRATKY Institut fu'r R~ntgenfeinstrukturforschung der Osterreichischen A kademie der Wissenschaften und des Forschungszentrums Graz, Steyrergrsse 17, A-g010 Graz, Austria AND GERHARD KOSTNER AND ANTON I-IOLASEK Institut far Medizinische Biochemie der Universitdt Graz Received May 21, 1975; accepted October 14, 1975 Two immunochemically defined lipoprotein flactions from human plasma (LpB from Low Density Lipoprotein and LpC from High Density Lipoprotein HDL2) were studied by X-ray small angle scattering. The molecular weights were found to be 2.4 X 106 for LpB, and 4.6 )< 10 ~ for LpC. In both cases the scattering curves indicated spherical symmetry of the particles. The radial electron density distributions were obtained by Fourier transformation. LpB showed an overall radius of approx 120 ~, and three electron density maxima around 30, 65, and 105 .~. LpC showed a low electron density core and a high electron density shell extending from approx 55 to 70 ~ radii. I. INTRODUCTION In recent years several reports have been published on the structural analysis of human plasma lipoproteins by X-ray small angle scat- tering (1-5). The majority of these studies was concerned with the subclasses of high density lipoproteins, HDL8 and HDL2 ~ and the results indicated a common structural principle for these lipid-protein complexes, that is, a spherical micellar arrangement of the constit- uent lipids and a surface location of the phospholipid polar head groups and the protein 1 Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975. To whom all correspondence should be addressed. 3 Abbreviations used: HDL~, high density lipopro- tein of buoyant density range d 1.125 1.21 g cm-3; HDL~, high density lipoproteins of buoyant density range d 1.063-1.125 g cm-3; LDL, low density lipo- proteins, buoyant density range d 1.006-1.063 g cm3; LpB, LDL of which the apoprotein consists of more than 96% of Apo B ; LpC, HDL2 of which the apopro- tein consists of more than 80% of Apo C. constituents (1-4). The first x-ray small angle scattering results on low density lipoprotein LDL were interpreted essentially in terms of a spherical lipid bilayer with most of the protein at the particle surface (5). The possibility of some protein in the core of the particle was also suggested. The present article is a preliminary account of our recent studies on the structure of LpB from LDL and of the LpC fraction of HDL23. To obtain reliable structural information from x-ray small angle scattering of macromolecules in dilute solutions, it is of great importance to ensure the monodispersity of the system as far as possible. Thus, we have chosen to investi- gate lipoprotein species that, in addition to being fractionated according to the buoyant density class system, are homogeneous with re- spect to their immunochemical behavior (6). This approach seems of particular importance in the case of LDL because of its inherent heterogeneity both with respect to buoyant density and molecular weight (7, 8). 102 Journal of Colloid and Interface Science. VoI.55, No. 1, April 1976 Copyright ~) 1976by Academic Press. Inc. All rights of reproductionin any form reserved