838 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



they tend to displace the Ca++ from the enzyme and the inhibition is semi- 

 competitive. An anionic group adjacent to the active site is suggested: 

 this interferes in some manner with the interaction of the enzyme and the 

 substrate, so that combination of the group with Ca++ favors the formation 

 of the ES complex. The phenothiazines prevent the binding and effect of 

 Ca++. This is actually a case of an inhibitor that is competitive with an 

 activator that happens to be a common physiological ion. Instances in which 

 ions complex with the inhibitor and thus reduce the inhibition are fairly 

 common; the binding of Ca++ and Mg++ to malonate is a typical example. 

 Substrate inhibition can also be modified by changes in the ionic strength. 

 This was observed by Mendel and Rudney (1945) in their work on mouse 

 brain acetylcholinesterase (Fig. 15-18). The substrate concentration for 

 the maximal rate, (Sq), is shifted upward by KCl; 160 mM KCl increases 

 (So) from 0.25 mM to 3 mM. Myers (1952 c) found the same behavior with 

 erythrocyte cholinesterase and showed that the binding of the second 

 substrate molecule is weakened at higher ionic strengths (Table 15-5). 

 The effect of KCl on the rate thus depends on the substrate concentra- 

 tion, a depression of the rate occurring at low substrate concentrations, 

 and an activation at high substrate concentrations. This situation has been 

 discussed in Chapter 4 with respect to the mechanism of substrate inhibition. 

 The svibstrate inhibition of yeast enolase by 2-phosphoglycerate in the ab- 

 sence of added salts has been shown to be due to an ionic strength effect, 

 the inhibition disappearing when the medium contains 500 mM KCl. 

 This is a good illustration of the danger in interpreting inhibitions in media 

 of low salt content where the addition of the inhibitor can alter the ionic 

 strength and the activity coefficients strongly. 



Effects of Salts on Metabolic Systems 



It is at present impossible to interpret ionic strength effects in com- 

 plex metabolic systems and the likelihood of specific ion effects is increased 

 with the number of enzymes involved. However, it is well known that sev- 

 eral types of metabolism are quite sensitive to changes in the salt con- 

 centration and one would expect that such changes would alter the respon- 

 ses to certain inhibitors, although no experimental results are available. 

 The marked effects often seen with the common i^hysiological ions are 

 well illustrated in the excellent work of Utter (1950) on the anaerobic gly- 

 colysis in brain homogenates. Na+ is inhibitory and K+ first stimulates and 

 then depresses at higher concentrations. The Na+ inhibition is already al- 

 most 50% at 35 mM. The mechanisms responsible for the inhibition are 

 several in number. (1) A stimulation of ATP Isreakdown leading to a fall 

 in the ATP level and a depression of phosphorylations. (2) An increase in 

 the level of AMP which is inhibitory to glycolysis. (3) A decreased rate of 

 phosphorylation of AMP and ADP by inhibition of phosphopyruvate trans- 



