We believe that hydrolysis of low density lipoprotein (LDL) cholesteryl ester may be one of the mechanisms that contributes to the accumulation of unesterified cholesterol-containing vesicular particles in the vessel wall during atherogenesis (Z01 HL 02826-06 EA). The purpose of this project is to study the change in structure of the LDL particle when its oily cholesteryl ester core has been enzymatically hydrolyzed. Human LDL is a spherical lipid particle with size ranging between 20 to 25 mn. It has a surface layer formed with protein, phospholipid and unesterified cholesterol and a core that contains predominantly cholesteryl ester. The cholesteryl ester core of LDL was not hydrolyzed when incubated with cholesterol esterase alone at 37 degree C for up to 6 hrs. When LDL was treated with trypsin for up to 2 hrs to degrade its surface protein, the lipid composition and structure of the particle was also not changed. However, after treatment with trypsin, LDL cholesteryl ester became susceptible to hydrolysis by cholesterol esterase. Hydrolysis of cholesteryl ester resulted in a change in LDL structure. With 10 min of cholesterol esterase treatment, the hydrolyzed LDL appeared to be a somewhat flattened spherical structure which was slightly larger than unhydrolyzed LDL. After 2 hrs of cholesterol esterase treatment, the hydrolyzed LDL transformed into large flattened vesicles with irregular shape and size (60-300 nm), even though the lipid composition of the particles did not further change with the additional incubation time. These large vesicles contained unesterified cholesterol at their surfaces as detected by reaction with filipin. The results show that LDL can be transformed to a larger vesicular particle when its cholesteryl ester has been partially hydrolyzed. The hydrolyzed LDL particles resemble the unesterified cholesterol- rich lipid particles we have isolated form atherosclerotic vessels.
