Carboxylesterase was purified from rat kidney in an electrophoretically homogeneous form by acetone precipitation, followed by successive chromatographies on DEAE-cellulose and hydroxyapatite and then isoelectric focusing. The purified enzyme catalyzed the hydro-lyses of monoacylglycerols and short-chain triacylglycerols, such as tributyrin, but not the hydrolysis of long-chain triacylglycerol. Its optimum pH with methyl butyrate as a substrate was 8.0. The relation of its activity to the methyl butyrate concentration differed from those for pancreatic lipase and liver esterase, and also from those for lipolytic enzymes from various other tissues. The relations of methyl butyrate-hydrolyzing activity with methyl butyrate concentration were compared among various carboxylester hydro-lyzing enzymes. Based on the results, these enzymes were classified into four classes.
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