600 



12. RATES OF INHIBITION 



such curves to equations of the types discussed provides constants tliat can 

 be used to characterize inhibitors. However, there is some doubt that nuicli 

 vahiable information about the mechanisms involved can l^e obtained by 

 these means. Let us see first what types of curves would be expected from 

 the simple inhibition of isolated enzymes. An irreversible inhibition (reac- 

 tion 12-15) rate can be expressed as: 



1000 



TIME (MINUTES)- 



FiG. 12-42. Log-log plot of the concentration-time curve for the killing of paraniecia 



by arsenite. [(I) — (I)„J"< = C where n = 1.38. (I)„, ^ 1 mM and C = 1.21 moles/ 



liter/min. The points are from the data of Xeuschlosz (1919). 



so that for anv degree of inhibition: 



(IK 



1 



1 



(12-92) 



and thus {l)t = constant. For 50% inhibition, {l)t = 0.693/A'i. If n mole- 

 cules of inhibitor reacted with each molecule of enzyme to produce the in- 

 hibition, E -\- nl -> EI,,, the relation (I)"^ = C should be obeyed. However, 

 a reversible inhibition (reaction 12-8) where the inhibition is (riven bv: 



i^ [I _ (.-/Cit[{i)+Ki] 



(12-93) 



