Carboxylic ester hydrolase activity in hairless and athymic nude mouse skin

The carboxylic ester hydrolase activity was compared in athymic nude mouse skin and hairless mouse skin with respect to hydrolytic ability, heat inactivation, pH optima, and substrate specificity. Five aliphatic 5'-esters of 5-iodo-2'-deoxyuridine (IDU) were incubated with skin homogenate preparations, and the effect of linear chain length and branching of the ester substituent on hydrolysis rate was evaluated. The ester hydrolase activity was three times higher in athymic mouse skin relative to hairless variety. In both mice skin preparations maximum hydrolysis rates were obtained with the valeryl ester followed by butryl, isobutyryl, propionyl and pivaloyl derivatives. Kinetic studies, however, revealed that higher ester hydrolase activity (Vmax) in athymic mouse skin is also associated with higher Km values, while the carboxylic ester hydrolases from these two different strains of mice have similar biochemical properties with respect to heat inactivation and pH optima. Athymic mouse skin resembled hairless mice skin in terms of cholinesterase content. A significant fraction (70-80%) of ester hydrolyzing activities in both strains of mice skin resulted from cholinesterases (true and/or pseudo). The remaining activity was attributed to different ester cleaving enzymes in the two strains of mice. Carbonic anhydrases and arylesterases contributed to the ester hydrolyzing activity of the athymic and normal hairless mice skins, respectively. Product inhibition by the regenerated hydrolytic product, free IDU, was also noticed which resulted in incomplete conversion of rapidly hydrolyzable 5'-esters such as the valeryl and butyryl derivatives.