Effect of monovalent cations on the stability and activity of Kluyveromyces lactis beta-galactosidase

The effect of sodium. potassium and ammonium on the stability and activity of a whole-cell Kluyveromyces lactis lactase in buffer solutions was studied. In the absence of these cations the enzyme was rather unstable Stabilization increased with the following sequence: Na+ > NH4+ > K+. Maximum protection was attained with a cation concentration of 100 - 200 mmol/L. Na+ did not affect enzyme stability either in presence of K+ or NH4+, but NH4+ enhanced enzyme stability when K+ was not at the saturating level. Activity was not observed when these cations were absent. The effect of cations on enzyme activity depended on the substrate used in the assay. The rate of lactose hydrolysis increased as follows: Na+ > NH4+ > K+ while maximum rates with ONPG were attained either with K+ or Na+. The reaction rate did not depend on the cation concentration within the range 40 - 200 mmol/L. In the presence of K+, lactose hydrolysis was inhibited by Na+ but not by NH4+, while ONPG hydrolysis was inhibited by NH4+. With both substrates, inhibition of the enzyme activity depended on the concentration ratio between the cations. (C) 1996 Academic Press Limited