SALT CONCENTRATION AND IONIC STRENGTH 



827 



modifications in the kinetic constants because these results have bearing 

 on inhibition studies. It should be pointed out, however, that only in rare 

 instances are the salt effects solely due to variations in the ionic strength 

 and the examples discussed here are mostly of a mixed nature with respect 

 to mechanism. 



The activity of acetylcholinesterase was show^n by Mendel and Rudney 

 (1945) to be decreased by increasing concentrations of KCl or NaCl at 

 relatively low concentrations of the substrate (Fig. 15-18). At higher sub- 



150 



leOmM 



80mM 



Fig. 15-18. Effects of different concentration of KCl on 

 the substrate inhibition of mouse brain acetylcholines- 

 terase. The figures on the curves represent the KCl con- 

 centrations in addition to the 25 mM NaHCOg. (From 

 Mendel and Rudney, 1945.) 



strate concentrations the effects are complicated by the substrate inhibition 

 and this will be discussed in a later section. A thorough analysis of human 

 erythrocyte acetylcholinesterase was later made by Myers (1952 c), who 

 determined the various pertinent constants as the NaCl concentration was 

 increased to 500 mM (Table 15-5). With increase in NaCl there is a marked 

 rise in the first substrate constant indicating a decreased afiinity of the 

 enzyme for acetylcholine, which would be expected on the basis of the 

 binding of a positively charged acetylcholine to an anionic group on the 

 enzyme. It is this effect that reduces the rate. However, the rate does 

 not decrease as much as would be expected because there is another effect 

 to increase F„; and presumably k^. This might indicate that the charge on 



