| Abstract: | The carboxylesterase (carboxylic-ester hydrolase, EC 3.1.1.1) and monoacylglycerol lipase (glycerol-monoester acylhydrolase, EC 3.1.1.23) activities, measured against ethyl butyrate and emulsified monooleoylglycerol respectively, were determined for chicken liver microsomes and highly purified chicken liver carboxylesterase. The activity ratio (ethyl butyrate activity/monooleoylglycerol activity) was approx. 5 for microsomes and approx. 400 for carboxylesterase. Homogenization of microsomes in 0.1 M Tris-HCl buffer (pH 7.92) released all of the ethyl butyrate activity and about half of the monooleoylglycerol activity into a soluble form. Both activities eluted from a Sephadex G-200 column with the same elution volume as that of pure carboxylesterase. This fraction (fraction B) had an activity ratio of approx. 15, an average pI of 5.01 (cf. 4.75 for carboxylesterase), and ran on polyacrylamide gel electrophoresis at pH 8.6 as a number of closely spaced esterase bands with mobilities considerably less than those of the esterase bands present in the carboxylesterase. Fraction B activities against both substrates were completely inhibited by diethyl p-nitrophenyl phosphate and completely precipitated by antibody to carboxylesterase. The remaining half of the monoacylglycerol lipase activity of microsomes was solubilized by treatment with 1.5% (w/v) Triton X-100. This solubilized monoacylglycerol lipase was completely inhibited by diethyl p-nitrophenyl phosphate, showing it to be a serine-dependent enzyme like the carboxylesterases. However, it had no detectable activity against ethyl butyrate, indicating that it is not closely related to the carboxylesterases. |
