Modification of the essential amino acids of human pancreatic carboxylic-ester hydrolase.

Chemical modification of human pancreatic carboxylic-ester hydrolase (EC 3.1.1.1) were performed using organophosphorus compounds, ethoxyformic anhydride and Woodward's K reagent. It has been shown that: (1) the inhibition of the enzyme activity by organophosphorus compounds is due to the phosphorylation of only one alcohol residue, probably a serine residue which may represent the acylable group of the enzyme: (2) 35-36 free carboxyl groups are modified by Woodward's K reagent but only one is responsible for the loss of enzyme activity. This carboxyl group has a pKa of 5.2.: (3) carbethoxylation of histidine residues leads to the inhibition of the enzyme activity. All nine histidine residues are reactive but only one is essential for activity. Taking into account the probable formation of an acyl-enzyme intermediate (Lombardo, D. and Guy, O. (1981) Biochim. Biophys. Acta 657, 425-437) during substrate hydrolysis and the present data we discuss a possible mechanism for carboxylic-ester hydrolase catalysis, a mechanism similar to that described for chymotrypsin.