Effect of Salt on the Activity and Stability of Aspartate Aminotransferase from the Halophilic Archaebacterium Haloferax mediterranei

Abstract

The aspartate aminotransferase from Haloferax mediterranei, which is in the cell mainly as apoenzyme, requires high concentrations of salt for both activity and stability. The maximum activity is reached with 3.5 M Kc1 in the assay. The effect of different cations and anions has been studied using several types of salts. Monovalent cations show a significant difference in effectiveness of promoting the activity with the following order: K+ > Rb+ > Na+ > NH4+. Mg++ and polyvalent cations with organic character cause partial activity with maximum effectiveness at 0.1 M, an inhibition at higher concentration is observed. Anions, added as potassium salts, promote enzyme activity with the following order: Cl- > NO3- > I- > SCN-. Like activity, the enzyme stability depends on salt concentrations. Incubation of the enzyme with a low salt concentration leads to inactivation following pseudofirst order kinetics. The inactivated enzyme is partially reactivated by high concentrations of KC1 following second order kinetics. Taking into account the dimeric structure of this enzyme, high concentrations of salt could stabilize the dimer, which is the active form. The salt effects on halophile aspartate aminotransferase are discussed considering hydrophobic and electrostatic interactions. © 1992, Walter de Gruyter. All rights reserved.