Characterization of aspirin hydrolase of guinea-pig liver cytoplasm

Previous subcellular fractionation studies of guinea-pig liver had shown aspirin hydrolysing activity to be located mainly in the microsomal fraction, and was due to a single carboxylesterase (EC 3.1.1.1) (White, K.N. and Hope, D.B. (1981) Biochem. J. 197, 771-773). However, activity had been found in all cell fractions, and in this study they were analysed simultaneously by slab gel electrophoresis for aspirin hydrolysing activity. Two enzymes were identified, one of which was associated only with the particulate cell fractions and was the microsomal carboxylesterase described previously. The other activity was located exclusively in the cytoplasmic fraction, and could be inhibited by bis(4-nitrophenyl)phosphate, so identifying it as a carboxylesterase. It has an unusually low molecular weight of 35 000, compared with values of 60 000, or multiples thereof, normally found for liver carboxylesterases. It contributes about 14% of the total aspirin hydrolysing activity of liver homogenates, and could be distinguished from its particulate counterpart by differences in molecular weight and in sensitivity to inhibition by bis(4-nitrophenyl)phosphate (see White, K.N. and Hope, D.B. (1984) Biochim. Biophys. Acta 785, 138-147).