SALT CONCENTRATION AND IONIC STRENGTH 



831 



indeed been found to increase rapidly as the ionic strength is raised to 

 around 0.2-0.3 and there is little further change beyond this (Dickman and 

 Ring, 1958). This effect may be through changes in both the substrate and 

 the enzyme. At least, it was found that increase of the ionic strength al- 

 tered the substrate UNA in that the absorptivity at 300 m//. was markedly 

 decreased. This may be due to an increased dissociation of certain acidic 

 groups and this would perhaps increase the interaction with the enzyme. 

 The effects of NaCl on spleen deoxyribonuclease are very complex and 

 vary with the pH (Fig. 15-20) (Shack, 1959). In all cases there are optimal 

 ionic strengths but a decrease in the pH leads to shifts in these optima to 

 higher concentrations. These interrelationships undoubtedly are due to the 

 effects of the ionic strength on dissociable acidic and basic groups on both 

 enzyme and substrate. However, the shapes of the curves and particularly 

 the slopes point to effects other than mediated through the ionic strength. 

 The polynucleotide phosphorylase of Micrococcus lysodeikticus loses most 

 of its activity upon dialysis and the addition of KCl restores the activity 

 (Beers, 1957). The rate increases very rapidly up to 0.1 ionic strength, 

 reaches a maximum around 0.2 and then falls off at higher salt concentration 

 (Fig. 15-21). The effect is mainly on K,,, rather than A'., and it was concluded 



Fig. 15-21. Effects of KCl concentration on the activity of 

 polynucleotide phosphorylase. The rate is given in terms of in- 

 organic phosphate released ( X 10"* M). (From Beers, 1957.) 



