Freeze-etching electron microscopy of serum lipoproteins

Characterization of native and drug-loaded human low density lipoproteins

Low-density lipoproteins (LDLs), the physiological vehicles for lipids, are potentially useful drug delivery devices for (hydrophobic) drugs. The physicochemical characteristics of LDL loaded with the adriamycin derivative AD 32 or the N-mustard derivative WB 4291 were compared to that of native and reconstituted LDL at different temperatures. X-ray solution scattering indicates that loading with AD 32 has no detectable effect on the particle structure at room temperature, in contrast to WB 4291. According to 19F NMR data, AD 32 molecules are located in two distinct chemical environments with restricted motional freedom of the CF3 groups in samples stored as lyophilisates. 1H NMR signals from AD 32 were not observed, while those from WB 4291 could be distinguished from those of LDL constituents. WB 4291 molecules are in an environment with a higher motional freedom than AD 32 molecules. 1H NMR data suggest a higher fluidity of the core components for the WB-loaded LDLs compared to the other LDL preparations. While the motional freedom of the phospholipid head groups seems to be temperature independent, there is an increase in the mobility of the lipid components in the core region of the LDL particles with temperature.

Isolation and characterization of three monoclonal antibodies to human serum low density lipoprotein apoprotein B

Human serum low density lipoprotein (LDL) is a large (Mr = 2-3 X 10(6), complex particle composed of lipid, protein and carbohydrate. We obtained about 40 mouse spleen-myeloma hybrid cell lines which produce antibodies against LDL. Three of them, SC2, SC3 and SC10, have been cloned and subcloned and their antibody products characterized. They recognize three non-overlapping epitopes in native LDL. Two of them, SC3 and SC10, also are capable of recognizing very low density lipoprotein, (VLDL), whereas SC2 reacts only weakly with VLDL. All three antigenic determinants remain intact, and accessible to antibodies on the LDL protein apo B, prepared by delipidation in a 'non-denaturing' detergent, sodium deoxycholate. However, apo B prepared by organic solvent, ether-ethanol, or sodium dodecyl sulfate (SDS) delipidation, while reacting strongly with SC10, is only poorly recognized by SC2 or SC3. Proteolysis of LDL with trypsin, chymotrypsin, Staphylococcus aureus protease, papain or thermolysin gives, in each case, several non-identical protein fragments which are separable by SDS-polyacrylamide gel electrophoresis. Upon immunoblotting, some of these fragments are now recognized by either SC3 or SC10 but not SC2, some are recognized by both SC3 and SC10, and others are immunologically unreactive. The protein bands that are separated by SDS gel electrophoresis are composed of several non-identical fragments and contain the antigenic sites to differing degrees. Some of the immunologically reactive fragments do not appear to contain carbohydrate. Reduction and carboxymethylation do not destroy the immunoreactivity of LDL toward any of the antibodies; however, modification of lysine residues by citraconic anhydride markedly diminishes the reactivity of LDL toward SC3. It is likely that the two antibodies SC3 and SC10 are directed against different linear amino acid sequences or very stable domains, whereas the third, SC2, is directed against a more fragile conformational domain of apo B.

Electron microscopic and solution X-ray scattering observations on the structure of hepatitis B surface antigen

The structure of the small, spherical hepatitis B surface antigen was studied by negative staining, freeze-fracture and freeze-etching electron microscopy and solution X-ray scattering techniques. The protein appears to be organized at the surface into a small number of morphological subunits which display two- and threefold axes of symmetry. The mean particle size was 18.3 nm by negative staining and 19.6 nm by freeze-fracture electron microscopy. The diameter of the individual subunits was about 7.5 nm with an intersubunit distance of about 10.0 nm. The lipid is distributed more homogeneously. Some heterogeneity of the particle structure is apparent which may be due to a slightly variable lipid-protein composition or incomplete or defective particle formation.

Freeze-fracture electron microscopic and low temperature x-ray scattering studies of the effect of cryofixation upon serum low density lipoprotein structure

We report here a correlated X-ray diffraction and freeze-fracture electron microscope study of the effects of several cryofixation procedures upon human serum low density lipoprotein (LDL2) structure. Only when the LDL2 solutions contained 75%, by weight, glycerol were the room temperature and post cryofixation low temperature LDL2 X-ray scattering curves indistinguishable from one another. Other cryofixation procedures, slow or rapid, with or without glycerol, resulted in differences between the room temperature and low temperature LDL2 X-ray scattering curves, in part due to the effect of quenching upon the solvent. Freeze-etching electron microscopy of the slowly cryofixed LDL2 showed marked aggregation of the particles and an unusual morphological appearance. In contrast, after rapid cryofixation or cryofixation in the presence of glycerol, freeze-etch electron microscopy revealed well-isolated particles which had a knobby morphology. The results demonstrate that under certain conditions (in the presence of 75% glycerol) cryofixation results in minimal, if any, structural alteration of, at least, the LDL2 lipid moiety. Further, this study underlines the more general conclusion that any high resolution structural study employing a cryofixation step must be interpreted with caution and the effect of cryofixation upon the sample structure need be evaluated by independent means.

Freeze-fracture electron microscopic analysis of solutions of biological molecules

Freeze-fracture electron microscopy was used to study the morphology of proteins in solution. The size of the particles appearing on the fractured surface, replicated with tungstentantalum, were measured in a direction perpendicular to the shadowing angle. The distributions of the measured particle sizes could be correlated with the known shape and dimensions for each protein. It is concluded that freeze-fracture electron microscopy is a useful technique to study the morphology of biological molecules in solution, particularly hydrophobic proteins which may be difficult to study by other microscopic techniques.

Density and composition models for lipoproteins

Two theoretical models for lipoprotein particle structure are proposed, based on density and composition and utilizing the Stokes' coefficient as evaluated from the Maude-Whitemore expression, using analytical ultracentrifugation. These density and composition models are compared with the Corey-Pauling-Koltun space-filling model proposed by Verdery and Nichols, and that proposed by Shen et al., which has a sharply defined boundary between the hydrophobic core and the amphipathic layer surrounding it. Results of our calculation of the percentage of total apolar lipid accommodated in the outer hydrophobic core -28, 64 and 94% for LDL, HDL2 and HDL3, respectively - suggest that there may be differences among these three lipoproteins classes in both the packing of cholesteryl ester and triglyceride as well as the conformation of apolipoproteins at the particle's surface.

Freeze-fracture study of water-soluble, standard proteins and of detergent-solubilized forms of sarcoplasmic reticulum Ca2+-ATPase

Conventional freeze-fracturing electron microscopy was used to study water-soluble proteins and different forms of Ca2+-ATPase-detergent complexes. Freeze-fracture images of solutions containing proteins larger than myoglobin showed the presence of distinct, randomly dispersed particles on smooth fracture surfaces. The distribution of sizes of these particles was closely to Gaussian, with a mean size which was correlated to the Stokes diameter. Monomeric Ca2+-ATPase from sarcoplasmic reticulum, solubilized by deoxycholate or a non-ionic detergent, showed a bimodal distribution of particle sizes. Even more complex distributions were found for dimeric and trimeric preparations of Ca2+-ATPase. The results can be interpreted on the assumption that the Ca2+-ATPase molecule is elongated, with an overall length of about 110 A and a width in its largest part of about 75 A. It is concluded on the basis of the presented results that freeze-fracture electron microscopy can be successfully used for morphological studies of protein molecules in solution.