Purification and properties of a cholesteryl ester hydrolase from rat liver microsomes

This report describes a purification procedure for a cholesteryl ester hydrolase (CEH) from female rat liver microsomes, and some structural, immunological, kinetic, and regulatory properties of the enzyme that distinguish the microsomal CEH from other hepatic cholesteryl ester-splitting enzymes. CEH was purified 12.4-fold from reisolated microsomes using sequential solubilization by sonication, polyethylene glycol precipitation, fractionation with hydroxyapatite, anion exchange chromatography, and chromatography on hydroxyapatite, with an overall yield of 3.2%. CEH activity was purified 141-fold over nonspecific esterase activity and 56-fold over triacylglycerol lipase activity. In sharp contrast with most esterases and lipases, CEH did not bind to concanavalin A-Sepharose and heparin-Sepharose. After polyacrylamide gel electrophoresis, the purified enzyme exhibited two silver-stained bands, but only the protein electroeluted from the low mobility band had CEH activity. Affinity-purified polyclonal antibodies raised to electroeluted CEH inhibited 90% of the activity of liver microsomal CEH and reacted with a 106 kDa protein band on Western blot analysis. This 106 kDa CEH contains a unique N-terminal amino acid sequence. The purified enzyme had optimal activity at pH 6 and no taurocholate requirements, and was inhibited by the serine active site inhibitor phenylmethylsulfonyl fluoride and by free sulfhydryl specific reagents. It hydrolyzed cholesteryl oleate much more efficiently than trioleine, and hydrolytic activity with p-nitrophenyl acetate was higher than with p-nitrophenyl butyrate. These results indicate that rat liver microsomes contain a bile salt-independent catalytic protein that is relatively specific for cholesteryl ester hydrolysis.