Activation of acyl-CoA cholesterol acyltransferase: Redistribution in microsomal fragments of cholesterol and its facilitated movement by methyl-beta-cyclodextrin

Acyl-CoA cholesterol acyltransferase (ACAT) (EC 2.3.1.26) in the yolk sac m embrane of chicken eggs plays an important role in the transport of lipids, which serve as both structural components and as an energy source during e mbryogenesis. ACAT from the yolk sac membrane of chicken eggs 16 d after fe rtilization has higher activity and better stability than its mammalian liv er counterpart. During our study of the avian enzyme, ACAT was round to be activated up to twofold during storage at 4 degrees C. The activation was i nvestigated, and data suggest that redistribution of cholesterol within mic rosomal vesicles leads to the increase. Methyl-beta-cyclodextrin (M beta CD ) increases activation an additional twofold, possibly by facilitating the movement of cholesterol within microsomal fragments and allowing redistribu tion of cholesterol in lipid bilayers to a greater extent. Treatment of mic rosomes with M beta CD removes cholesterol from the membranes. Controlled a mounts of cholesterol can be restored to the membranes by mixing them with cholesterol-phosphatidylcholine liposomes in the presence of M beta CD. Und er these conditions, the plot of ACAT vs. cholesterol mole fraction in the liposomes is sigmoidal. The finding that M beta CD can enhance cholesterol transfer between liposomes and microsomes and reduce the limitation of slow movement of nonpolar molecules in aqueous media should make cyclodextrins more useful in in vitro studies of apolar molecule transport between membra ne vesicles.