SALT CONCENTRATION AND IONIC STRENGTH 835 



phosphate, sulfate, and fluoride — inhibit with both substrates and a com- 

 petitive mechanism is likely. Some anions thus interfere with the binding 

 of the substrate and some with the breakdown of the ES complex. A potent 

 activation of fly muscle dehydrogenases by K+ at 1 mM has been noted 

 (Chefurka, 1957). Both glucose-6-phosphate and 6-phosphogluconic dehy- 

 drogenases are activated severalfold by concentrations of K+ which pre- 

 clude any major ionic strength effect. A different sort of effect is seen in the 

 more complex yeast hexokinase system where increases in Na+ concentra- 

 tion at constant ionic strength stimulate the rate (Melchior and Melchior, 

 1958). Here the active enzyme complex is E-Mg-ATP-glucose and a Mg- 

 ATP complex may be formed independently; Mg++ and ATP can also com- 

 plex separately with the enzyme. Many ionic interactions are thus involved. 

 The effect of Na+ is believed to be due to interactions of the ions with ATP 

 and the E-ATP complex, and this effect is in addition to ionic strength ef- 

 fects which are interpretable in terms of changes in the activity coefiicients. 



In studies on salt effects, it is often informative to test several salts 

 of different types. This is illustrated by the work of Repaske (1954) on 

 the succinic dehydrogenase of Azotobacter. It was found that 100 mM 

 NaCl and KCl inhibited the enzyme about 33%. From these data alone 

 one might conclude that either it was the CI" ion that was inhibitory or 

 that the increase in the ionic strength reduced the affinity of the enzyme 

 for succinate. However, K2SO4 was also used and at 50 mM did not affect 

 the enzyme; thus the ionic strength effect could be eliminated. It is, by 

 the way, rather remarkable that no ionic strength effect was observed in 

 a system where a negatively charged substrate interacts with positively 

 charged enzyme groups. The most likely explanation is that the hydrogen 

 transfer to 2,6-dichlorophenol-indophenol was accelerated by the salts 

 and that this compensated for the reduction in the binding of the succinate 

 to the dehydrogenase. 



It may be concluded that the common i:»hysiological ions can interact 

 quite specifically with the enzyme, substrate, or cofactors, and that either 

 binding may be altered or the rate of breakdown of the active enzyme com- 

 plexes may be modified. Activation or inhibition may result and these ef- 

 fects should always be considered in inhibition studies, especially when these 

 ions are added with the inhibitor (as the addition of the sodium or potas- 

 sium salts of malonate). 



Effects of Changes in the Ionic Strength on Enzyme Inhibitions 



Very few accurate studies on inhibition with respect to salt concentra- 

 tion have been reported, but in these few the results are usually interesting 

 and interpretable. Cholinesterase presents an ideal situation for such work 

 because the structure and charge distribution of the active center are to 

 some extent understood. One of the most thorough investigations of ionic 



