Lipoprotein model systems

T,he stoichiometric characterization of detergent-protein complexes has been the object of many studies over the past 30 years (6). Recent studies have placed more emphasis upon developing a molecular-kinetic description of the complex (2, 8). The importance of such descriptions lies in the fact that detergent-protein complexes can be considered as lipoprotein model systems. Indeed, virtually all conceptions of the microscopic nature of lipid-protein interactions are based on the properties of detergent-protein complexes (3). 

Most stilbenoids possess antioxidant activities because they possess polyphenol functions in the molecules. Some of their beneficial effects, hepatoprotective action, cardiovascular protection, for instance, are in close relation to their antioxidant activities. Several models have been employed in the assay such as lipid peroxidation system, human low-density lipoprotein model, xanthine oxidase system and l,l-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging model, which is the most commonly used protocol. 

Net transfer of lipid occurs from the plasma to the erythrocyte membrane, presumably because of a shift in the equihbrium as the plasma lipoproteins become saturated with the excess cholesterol and phosphatidylcholine. This leads to membrane abnormalities and cholesterol-phospholipid ratios of up to 2 1. Changes in cellular physiology of the type referred to in section IV have also been reported [94,96,161]. These must reflect an alteration in lipid-protein interactions within the membranes. The molecular arrangement of the excessive amounts of cholesterol present in the cell membranes in diseased liver cells is not known. In model systems cholesterol is not present in molar amounts greater than 1 1. In liver disease a major change is in cellular morphology with the formation of abnormally shaped erythrocytes, as discussed earlier. 

Overturf, M. L., and D. S. Loose-Mitchell. 1992. In vivo model systems The choice of experimental animal model for the analysis of lipoproteins and atherosclerosis. Current Opinions in Lipidology 3 179-185. 

We selected Pseudomonas pictorum (ATCC 23328) as another model system because it can degrade cholesterol. The standard encapsulation method does not result in a high-porosity membrane that would allow lipoprotein-cholesterol to cross. Therefore, we devised a modified method to prepare high-porosity agar microspheres. There was no evidence of leakage of the enclosed bacteria. Open pore agar beads were incubated in serum, and the bacterial action did not significantly... 

Because of its occurence in diseased tissue, the mode of association of cholesterol esters with biomembranes is of interest. Possible modes of association could be droplets within the hydrophobic core of the membrane bilayer, binding to membrane protein or as part of membrane attached serum lipoproteins. A potentially useful model system for investigating this association is the membrane of the microorganism Mycoplasma capricolum. The Mycoplasma due to their simplicity have served as model membrane systems in many studies. As mentioned previously, cholesterol esters show complex behavior that is a function of thermal history, impurities and physical packing constraints. Using DSC on native membranes and extracted membrane material, it was possible to demonstrate that the majority of cholesterol esters associated with the membranes of M. capricolum exist as relatively large and pure liquid droplets (17). 

Negre-Salvayre A, Alomar Y, Troly M, Salvayre R. Ultraviolet-treated lipoproteins as a model system for the study of the biological effects of lipid peroxides on cultured cells. HI. The protective effect of antioxidants (probucol, catechin, vitamin E) against the cytotoxicity of oxidized LDL occurs in two different ways. Biochim Biophys Acta 1991 1096 291 300. 

It is now well established that very low density and low density lipoproteins are synthesized in the liver. Data supporting this conclusion have been obtained from studies on plasma triglyceride metabolism, the fatty liver and its origin, and protein biosynthesis in the liver. Many of the studies used animals which have a somewhat different lipoprotein pattern than the human. Metabolic sequences differ in detail between species and unique model systems are required (Farquhar et al. 1965). Nevertheless fundamental aspects of lipoprotein metabolism such as liver synthesis have been confirmed in several animals. 

Rapeseed phenolics isolated by Vuorela et al. (2004) were tested for radical scavenging and for liposome and low-density lipoprotein (LDL) model systems. The inhibition of hexanal and conjugated diene hydroperoxides formation was reported (>90% and >80%, respectively). All isolates also exhibited inhibition of LDL particles oxidation by >90%. The antioxidant activity of methanol and acetone extracts of canola hulls in a P-carotene-linoleate model system was comparable to that displayed by butylated hydroxyanisole (Naczk et al., 2005). These extracts showed more than 95% scavenging effects (at 40 p/assay on DPPH radical). Vuorela et al. (2005a,b) indicated that rapeseed phenolics were excellent antioxidants towards oxidation of phosphatidylcholine membrane (liposomes) and rapeseed oil (crude) phenolics were effective radical scavengers (DPPH test). The authors suggested that these phenolic isolates from rapeseed are safe and bioactive for possible food applications including functional foods intended for health benefit. 

Because of their relatively large hydrophobic surface area, apolipoproteins, in the absence of lipids, readily self-associate in aqueous solution (Stone and Reynolds, 1975 Vitello and Scanu, 1976). The rate of desorption of apolipoproteins from lipoprotein surfaces has not been studied systematically. Extensive studies of the reverse process, which is the assembly of lipid apoprotein complexes, have been conducted in considerable detail. The dynamic of lipid-protein interactions have been studied primarily with in vitro model systems. Analysis of the association of apolipoproteins with various phospholipid aggregates have provided important clues about the nature of the kinetically important steps in the transfer of apolipoproteins between lipoproteins (Pownall et al., 1977 1978a Massey et al., 1981a Mantulin et al., 1981). 

Heparin Sulfate Proteoglycans Hepatic Lipase Hepatitis Hepatitis C Heptahelical Domain Heptahelical Receptors HERG-channels Heterologous Desensitization Heterologous Expression System Heterotrimeric G-Proteins Hidden Markov Model High-density Lipoprotein (HDL)... 

Lipidated peptides embodying the characteristic linkage region found in the parent lipoproteins and bearing additional functional groups, which could be traced in biological systems or which allowed for their use in biophysical experiments, were used successfully in model studies. However, such model studies only provide a limited amount of information. In order to approximate the situation in a biological system more precisely, experiments with differently lipidated proteins are required. 

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. 

The ability of flavonoids to enhance the resistance to oxidation and to terminate free-radical chain reactions in lipophilic systems can be monitored using low-density lipoproteins (LDL) as a model (Rice-Evans et al., 1996). The LDL oxidation is initiated either by copper or by a peroxyl radical [2,2 azobis(2-amidinopropane hydrochloride) (AAPH)] (Abuja et al., 1998). Hexanal liberated from the decomposition of oxidized n-6 polysaturated fatty acids in LDL may be determined by static headspace gas chromatography (Frankel and Meyer, 1998). Also, bleaching of P-carotene (Velioglu et al., 1998 Fukumoto and Mazza, 2000) and the tracing by HPLC (Fukumoto and Mazza, 2000) of malonaldehyde formed in lipid emulsion systems in the presence of iron (Tsuda et al., 1994) have been used to measure antioxidants in lipophilic systems. 

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