STABILITY STUDIES ON MAIZE LEAF PHOSPHOENOLPYRUVATE CARBOXYLASE - THEEFFECT OF SALTS

The solution stability of phosphoenolpyruvate carboxylase (PEPC) has b een determined in the presence of various salts by temperature-acceler ated enzyme inactivation and also by using high-performance size-exclu sion chromatography. Kosmotropic (water structuring) anions in the Hof meister series (HPO42-, citrate(3-), SO42-, F-, OAc-) and glutamate st abilized the enzyme most effectively, while Cl- (a borderline Hofmeist er anion) and Br- (a chaotropic anion) were destabilizing. The effects of the cations on PEPC stability ranged from relatively inert (Na+, K +) to destabilizing ((CH3N+, NH4+, Li+). The observed stabilization of PEPC by specific salts has been interpreted in terms of the positive surface tension increment and the water-structuring effects conferred on the solution by the specific stabilizing reagents. Both these effec ts enhance hydrophobic interactions of proteins and increase the energ y required to enlarge the surface area of the solvent cavity in which the protein resides. The destabilization of PEPC by some salts at a co ncentration of 0.5 M was associated with the dissociation of the tetra meric enzyme into its dimeric and monomeric forms, a process most prob ably occurring as a result of ion-peptide dipole binding, which promot es protein-solvent interaction and a subsequent reduction in the free energy of cavity formation. The stabilization of enzyme activity by ko smotropic salts depended on the salt concentration with maximum stabil ization of PEPC in solution at 52 degrees C observed with 0.6-0.8 M so dium glutamate, 2 M KF, and 2.2 M KOAc. Higher concentrations of these salts resulted in decreased activity. This reduction in activity of P EPC in the presence of high concentrations of kosmotropic salts appear s to be associated with irreversible conformational changes of the tet rameric enzyme.