Partial purification and characterization of a microsomal carboxylesterase specific for salicylate esters from...

Studies on liver carboxylesterases have predominantly involved the use of uncharged ester and amide substrates to monitor activity. A microsomal carboxylesterase (EC 3.1.1.1) from guinea-pig liver microsomes has been identified which specifically hydrolyses aspirin (White, K.N. and Hope, D.B. (1981) Biochem. J. 197, 771-773), a substrate which is negatively charged at physiological pH, and this work describes its partial purification and characterization. The enzyme is monomeric, it has a molecular weight of approx. 55 000 and is very sensitive to inhibition by the carboxylesterase inhibitor bis(4-nitrophenyl)phosphate. Although it could not be completely separated from contaminating carboxylesterases, substrate specificity was investigated using the negatively charged esters of salicylic acid. The enzyme is not specific for the acetyl ester of salicylic acid, aspirin, but hydrolyses the longer chain esters more rapidly, with the highest Vmax for the n-octanoyl ester. The enzyme was subject to substrate inhibition which increased with increasing chain length of the fatty acid on the ester, and approached 100% inhibition at concentrations of substrate below critical micellar concentrations.