Characterization of Rat Plasma Esterase ES-1A Concerning Its Molecular and Catalytic Properties

To characterize esterase ES-1A from rat plasma with regard to its molecular and catalytic properties, the enzyme was purified. The degree of purification, as measured by a densitometric gel scanning assay for ES-1 activity, was about 142-fold with a recovery of 9%. The ES-1A preparation was free of other plasma esterases but not of other plasma proteins. The enzyme shows microheterogeneity after staining for esterase activity in gradient polyacrylamide gel electrophoresis and isoelectric focusing. The native ES-1A protein is a carboxylesterase (EC 3.1.1.1) with a molecular mass of about 59 kDa as determined by gel filtration and gradient polyacrylamide gel electrophoresis. ES-1A exhibits a pI of 4.73 and optimum pH of 8.6 for p-nitrophenylbutyrate hydrolysis. With various p-nitrophenyl esters as substrate for ES-1A, it was found that the Michaelis constant decreased with increasing number of C atoms of the unbranched fatty acid moiety, whereas the maximum reaction velocity peaked with p-nitrophenylvalerate. The high degree of similarity of the properties of rat ES-1A with those reported earlier for mouse ES-2B, rabbit EST-2F, and human ESB2 suggests that these four esterases have a common evolutionary origin