SALT CONCENTRATION AND IONIC STRENGTH 



837 



at high polyghitamate concentration are perhaps due to different molecules 

 binding to the cationic groups so that bridges are not formed and the sub- 

 strate can reach the active site. Several proteins (serum albumin, /?-lacto- 

 globulin, hemoglobin, and ovalbumin) at low concentrations can inhibit 

 chymotrypsin (Hofstee, 1960 b). In the absence of salt, 0.01% serum albumin 

 inhibits around 80%, giving a K^ of approximately 10~' M. The addition 



Table 15-6 

 Effects of Ionic Strength on the Inhibition of Trypsin by Poly-L-glutamate " 



" The concentration of poly-L-glutamate is given in terms of glutamate residues. 

 From Kornguth and Stahmann (1960). 



of KCl at 100 milf completely prevents this inhibition indicating a fun- 

 damental electrostatic mechanism for the binding. Furthermore, the inhi- 

 bitions progress slowly and may be completely reversed by KCl if they have 

 not been allowed to proceed too far. In other words, there is an initial 

 salt-sensitive inhibition, followed by an irreversible inhibition. Similar 

 protection is given by NaCl and more effective protection by MgCL, SrClg, 

 CaClg, and MgS04. The inhibitions by carboxymethylcellulose and nucleic 

 acids are also reversed by increasing salt concentrations. Pancreatic car- 

 boxypeptidase is inhibited by several dipeptides (Yanari and Mitz, 1957). 

 The iC; for D-leucyl-L-tyrosine is 4 niM at an ionic strength of 0.2 and falls 

 to 2 mM at an ionic strength of 0.5. Since this is a competitive inhibitor and 

 K„, is also decreased by increasing ionic strength, the inhibition does not 

 rise as much as would be expected from the greater affinity of the enzyme 

 for the inhibitor. 



One example of an interesting specific ion effect on enzyme inhibition 

 may be cited. Serum cholinesterase is activated by Ca++ and this effect is 

 probably explained by a facilitation on the binding of the enzyme and the 

 substrate. Several phenothiazines inhibit the enzyme and counteract the 

 activating effect of Ca++ (Hofstee, 1960 a). It is likely that the phenothia- 

 zines and Ca++ bind to the same enzyme site. In the absence of Ca++, 

 phenothiazines are completely noncompetitive, but in the presence of Ca++, 



