Organization of Lipids and Proteins in Lipophorins

The elucidation of the structure of any lipoprotein, i.e., the organization of the protein and lipid components, is a challenging problem, because it relies on several techniques that give only partial and approximate information. Structural models should be consistent with experimental data that characterize the physiological role and the physicochemical properties of the lipoprotein and its components. Considerable effort has been expended to fit experimental data to structural models of mammalian lipoproteins (Zilversmit, 1965 Sata et al., 1972 Schneider et al., 1973 Havel, 1975 Verdery and Nichols, 1975 Shen et al., 1977  

Edelstein et ai, 1979 Oeswein and Chun, 1981). The concept of a lipoprotein core of nonpolar lipids surrounded by an outer shell of phospholipids and proteins is a common theme in all these studies. The existence of the lipid core is supported by the properties of lipids, SAXRS studies, and is apparent for large lipoproteins with high lipid contents. On the basis of SAXRS studies this concept has also been applied to lipoproteins with low lipid contents, such as human HDL (Scanu, 1972 Laggner, 1982). In addition to the lipid core, experimental support for a surface localization of phospholipids and cholesterol has been obtained from NMR studies (Henderson et al., 1975 Yeagle et ai, 1978 Lund-Katz and Phillips, 1986). 

Among the problems involved in constructing a general model for vertebrate lipoprotein structure, some of the more important are the heterogeneity in size and apolipoprotein composition and the small number of examples of lipoproteins containing similar apolipoprotein composition, which can be used to validate the model. Thus, in most cases, data as elementary as the stoichiometry of the apolipoproteins cannot be included in the models (Shen et ai, 1977) or, if stoichiometry data are included, only a partial fit of the data to the model is observed (Edelstein etal., 1979). 

One of the most intriguing features of lipophorin composition is the large variation in lipid content and composition that can be accommodated without modifications in the apolipoprotein composition of the particles (Table I). This feature makes lipophorin a good system in which to analyze the structure of lipoproteins and the physicochemical factors that govern their structure and properties. In addition to the previously discussed data on the size and shape of lipophorins, several studies on other aspects of lipophorin structure have been performed and need to be discussed before describing models for lipophorin structure. 

Experimental evidence for a core localization of hydrocarbon was obtained from NMR, calorimetry, and SAXRS studies (Katagiri et al., 1985, 1987). Thus, from the similarity in thermotropic behavior of pure hydrocarbons isolated from locust lipophorin, and the same hydrocarbons in the native lipoprotein, it was concluded that hydrocarbons are partially segregated from the other lipophorin components forming a hydrocarbon-rich cluster. The presence of an internal region of low electronic density was observed by SAXRS in lipophorins containing hydrocarbons, suggesting that hydrocarbons form part of an inner lipid core, which is not exposed to the aqueous environment. 

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