Effect of Ionic and Non-ionic Co-Solutes on the Activity of β-Galactosidase

The hydrolysis of the lactose analog o-Nitrophenyl-beta-D-Galactopyranoside into o-Nitrophenol and beta-D-Galactose by the action of beta-Galatosidase is a well known enzymatic reaction. We have studied this reaction with varying concentrations of substrate in the presence of several ionic and non-ionic co-solutes considered chaotropes and kosmotropes, and examined their effects on the enzyme activity. In the presence of increasing concentrations of up to 1 M NaCl, beta-Galactosidase showed an activity that was barely affected by the salt concentration and was basically similar to that under stripped conditions. In contrast, in the presence of NaI, while at concentrations of 0.1 M NaI and below the effect on the enzyme activity was comparable to that of 0.1 M NaCl, in the presence of 1 M NaI the enzyme showed a marked deactivation. This fact suggested that the difference in effects was caused by the presence of iodide ions. On the other hand, the activity of this enzyme was also investigated in the presence of several polyols differing in the number of hydroxyl groups. The kosmotropes used were glycerol (3), erythritol (4), xylitol (5), and sorbitol (6), in which the number in parentheses indicate the number of hydroxyl groups present in each molecule. At concentrations of 7 M, except for sorbitol (6), all polyols showed various degrees of deactivation of the enzyme. It is interesting to note that this effect showed no correlation with the number of hydroxyl groups. The increasing order in deactivation was glycerol (3), xylitol (5), erythritol (4). Sorbitol (6) showed no impact on the activity of beta-Galactosidase at any concentration. These results will be discussed in terms of the effect of co-solutes on the structure of water as well as on the structure-function correlation of the enzyme.