J. Biochem, 1988, Vol. 104, No. 1 141-148
© 1988 Japanese Biochemical Society
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Modulation of Cholesterol Microenvironment with Apolipoproteins Induced by the Presence of Cholesteryl Ester in Lipid Microemulsion1
Department of Etiology and Pathophysiology, National Cardiovascular Center Research Institute Suita, Osaka 565
2 Present address: Technical Research and Development Division, Terumo Co., Shibuya-ku, Tokyo 151
3 To whom correspondence should be addressed
In order to investigate the effect of cholesteryl ester (CE) accumulation in plasma lipoprotein on its metabolism, change of the cholesterol (CHOL) microenvironment was studied by using a lipid microemulsion model system (J. Biol. Chem. 258, 10073–10082, 1983 and 260, 16375–16382, 1985) in the presence of CE and apolipoproteins. Solubility of CHOL in the triolein (TG) core of the emulsion was limited (0.4 weight percent), so that most of the CHOL in the emulsion was found to be associated with the phosphatidyicholine (PC) surface membrane. CE was associated almost exclusively with the TG core without any significant effect on the partitioning of cholesterol between the core and the surface. However, membrane-associated CHOL seems to be present in the TG core adjacent to the surface membrane in the microemulsion without CE, and it is likely to be shifted into the membrane by the presence of CE in the core according to the compositional analysis. Binding parameters of apolipoproteins (apo) A-I, A-II, C-III and E were not significantly different among the emulsions with and without CHOL and/or CE at CHOL/PC ratios up to 0.17 (w/w). Susceptibility of CHOL to cholesterol oxidase was observed as an enzymatic probe for CHOL microenvironment. In the absence of apolipoproteins, CHOL reacted similarly to the enzyme regardless of its shift by CE. When apolipoproteins bound to the emulsion containing only CHOL, the rate of CHOL oxidation was decreased by 40% with apoE but not with the others. In the presence of CE, it was decreased by 80% with apoA-I and E, and by 65 and 35% with apoC- and apoA-II, respectively. These results indicated that apolipoproteins modulate CHOL environment more potently when lipoprotein con tains more CE and that this modulation is the strongest with apoE.
1 This work was supported in part by Grants-in-Aid for Scientific Research (Nos. 59580111 and 60580161) from the Ministry of Education, Science and Culture of Japan.
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