Identification of aspirinase with one of the carboxylesterases requiring a thiol group.

Aspirin-hydrolysing activity in guinea-pig liver is located mainly in the microsomal fraction. This activity was found by electrophoresis to be due to a single carboxylesterase band, out of 12 bands revealed with alpha-naphthyl acetate as substrate. The activity is inhibited completely and irreversibly by the carboxylesterase inhibitor bis-(-4-nitrophenyl) hydrogen phosphate, and also by thiol-blocking reagents.