Substrate specificity of purified rabbit liver esterase ES‐1

1. Substrate hydrolysis by two purified rabbit liver esterase-1 allozymes (ES-1A and ES-1B) was compared under conditions differing in substrate, pH and temperature. ES-1A and ES-1B activities had a similar pH and temperature dependency and similar thermal stability profile. 2. There were marked differences in specific activity of ES-1A and ES-1B. ES-1A hydrolysed procaine more rapidly than ES-1B, but was less active towards aspirin. The acetate and propionate esters of p-nitrophenyl were hydrolysed slower by ES-1A than by ES-1B. 3. The effect of substrate concentration on ES-1A activity did not comply with the Michaelis-Menten kinetics, which may be due to so-called substrate activation. 4. At identical substrate concentration, pH and temperature, selected artificial esters were better substrates for ES-1A than selected physiological substrates. Beta-Naphthyloctanoate was found to be a suitable substrate for ES-1A. 1,3-Dioctanoylglycerol was hydrolysed at a rate of only 2% of that of beta-naphthyloctanoate. 5. With methyl, p-nitrophenyl, beta-naphthyl and 4-methylumbelliferyl esters as substrates, ES-1A activity is influenced by length and structure of the acyl moiety. Likewise, ES-1A activity is influenced by the nature of the alkyl moiety of acetate esters. With acetate and methyl esters, branched chains when compared with unbranched chains reduced the esterase activity of ES-1A. Elongation of the acyl moiety up to four or five C atoms gradually raised the velocity of methyl, p-nitrophenyl, and 4-methylumbelliferyl ester hydrolysis by ES-1A. A similar pattern was found for the length of the alkyl moiety of acetate esters. 6. The high degree of similarity between the observed substrate specificity of rabbit ES-1A and that reported earlier for rat ES-10, suggests that these two esterases have a common evolutionary origin.