Cholesterol esterase: a cholesterol transfer protein

Rat pancreatic cholesterol esterase was examined for its ability to effect sterol transfer between small unilamellar vesicle (SUV) preparations. Sterol exchange was determined using SUV composed of palmitoyloleoylphosphatidylcholine/sterol (65:35) with or without 10 mol % phosphatidylserine or phosphatidic acid. This recently developed assay does not require separation of donor and acceptor vesicles (Butko et al., 1992). Cholesterol esterase stimulated cholesterol exchange when SUV contained phosphatidylserine and even more so in the presence of phosphatidic acid. Cholesterol esterase increased the initial rate of sterol transfer between phosphatidic acid-containing SUV by approximately 80%. The enzyme increased sterol exchange by significantly decreasing the half-times of sterol transfer and by significantly increasing the initial rates of sterol exchange. In the absence of negatively charged phospholipids, cholesterol esterase was ineffective at increasing sterol transfer. Monolayer studies showed that negatively charged phospholipids seem to play a key role in cholesterol esterase adsorption to lipid interfaces. Finally, a mutant cholesterol esterase lacking a histidine (435) residue essential for esterasic catalysis was found to be equally capable of increasing sterol transfer and binding to charged monolayers. In summary, cholesterol esterase enhances sterol transfer in SUV containing negatively charged phospholipids, independent of esterasic activity.