Further Investigations on the Subunit Structure of Microsomal Carboxylesterases from Pig and Ox Livers

Using sedimentation equilibrium studies, the molecular weight of pig liver esterase was found to be 163000. Sedimentation velocity experiments yielded a sedimentation coefficient s020,w of 7.9 S; the diffusion coefficient was found to be 40.7 μm2· s–1 by ultracentrifugation and 40.3 μm2· s–1 by analytical gel filtration. From these data a molecular weight of 181 000 was calculated. By analytical disc electrophoresis a molecular weight of 183000 was estimated. Disc electrophoresis of ox liver esterase revealed three enzymatically active bands. The molecular weight of two of them was 54500 and that of the third 187000. Extrapolation of the sedimentation coefficients measured at very low enzyme concentrations to infinite dilution yielded a minimum value of S020,w= 4.0 S. Based on gel filtration results a diffusion coefficient of 66.0 μm2· s–1 was calculated. From these data a molecular weight of 57300 was obtained. Sedimentation equilibrium experiments of the ox enzyme showed a non‐linear plot of In absorbance versus r2 indicating heterogeneity with respect to molecular weight. The smallest species present had a molecular weight of 59000. By sedimentation equilibrium experiments in the presence of 6 M guanidine‐HCl and 0.1 M 2‐mercaptoethanol, the subunit weight of pig liver esterase was found to be 58000. For the maleylated enzyme, values of 52500 (sedimentation equilibrium) and 56500 (from S20,w and D20,w) were obtained. By covalent cross linking of the subunits with dimethyl suberimidate and subsequent polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate, three bands of molecular weights 60000, 120000 and 180000 were found. All these results give strong evidence for a three subunit structure of pig liver carboxylesterase.