PIG LIVER ESTERASE . PHYSICAL PROPERTIES

The molecular weight of pig liver esterase, purified by the method of Adler and Kistiakowsky, was found to be 168.000 by equilibrium sedimentation, approach to equilibrium, and sedimentation-diffusion, based on a partial specific volume of 0.74 that was calculated from amino acid analysis; this molecular weight is in agreement with that of other esterase preparations. Near pH 4.5 the enzyme dissociates reversibly in a few minutes to active half-molecules of mol wt 85.000 according to equilibrium sedimentation, and ca. mol wt 75.000 according to sucrose gradient sedimentation and measurement of the diffusion coefficient by immunodiffusion. Dissociation to half-molecules, of mol wt 85,000-90,000 by gel chromatography on Sephadex G-100, occurs at pH 7-8 over several hours in extremely dilute solutions or in the presence of salts. The same equilibrium constant of approximately 4 × 10-7 m was found in the presence of 0.5 m sodium chloride or 0.5 m lithium bromide, suggesting that interactions with peptide groups are not responsible for dissociation. Concentrated solutions of enzyme give a single band upon electrophoresis in polyacrylamide gel at pH 8.3, but dilute solutions undergo slow dissociation upon standing to subunits with an increased mobility. Below pH 4 the enzyme undergoes irreversible denaturation to inactive half-molecules of altered shape. In 6 m guanidine hydrochloride-0.1 m mercaptoethanol, the uncorrected molecular weight is 53,500. This is interpreted as evidence for dissociation of the enzyme to unfolded quartermolecules of mol wt 42,000, which exhibit a preferential interaction with guanidine hydrochloride. Different preparations of pig liver esterase exhibit closely related, but significantly different, physical, immunological, and kinetic properties. © 1969, American Chemical Society. All rights reserved.