INHIBITION OF ENZYMES 719 



a disulfide link near the substrate site would not obviously produce a steric 

 effect. Inasmuch as the general problem of dependence of inhibition on the 

 substrate will come up several times with other inhibitors, it will be well to 

 suggest some of the possible mechanisms by which such effects can arise. 



Substrate % Inhibition by o-iodosobenzoate (1 m.M) 



Triacetin 50 



Tripropionin 67 



MonobutjTin 74 



Tributyrin 89 



(1) It is conceivable that the formation of a disulfide structure can dis- 

 tort the enzyme structure at or near the substrate site so that inhibition 

 will result, and this inhibition might not be the same for each substrate, 

 because of either steric factors or changes in the spatial position of the en- 

 zyme groups involved in the hydrolysis. (2) The SH reagent might not pri- 

 marily react with the enzyme but with the substrate, as suggested by Wills 

 (1960) for the inhibition of pacreatic lipase by p-chloromercuribenzoate. 

 Since this enzyme is not inhibited by o-iodosobenzoate, a relation with sub- 

 strate cannot be established. Wills believes that the mercurial is adsorbed 

 onto the glyceride-water interface and, in order to examine this possibility, 

 shook tributyrin with 10 nxM p-chloromercuribenzoate, washed it, and then 

 used this as a substrate; marked inhibition was noted, indicating a rather 

 strong affinity of the glyceride for the mercurial. However, again this ex- 

 planation would not seem to hold for o-iodosobenzoate, since it should in- 

 hibit the pancreatic lipase as well as the wheat germ lipase (with which 

 Singer worked) if the substrates are altered. Also it is not too surprising 

 that a molecule like p-chloromercuribenzoate would adsorb at an oil-water 

 interface, since it possesses polar and nonpolar regions, but o-iodosobenzoate 

 has polar groups at either end. (3) A group, such as — S — I — (p — COO", is 

 actually added to the enzyme near the substrate site and interferes steri- 

 cally as p-chloromercuribenzoate may do. 



Another explanation involves the basic kinetics of such inhibitions. It 

 seems to have been generally assumed that when an irreversible inhibitor 

 reduces the affinities of each substrate of an enzyme equally, the inhibi- 

 tions will all be the same, which can readily be shown not to be true. Let 

 us assume that Kj„ is equal to K^, the true dissociation constant of the ES 

 complex, so that the uninhibited rate is given by: 



F^(S) 

 (S) + K, 



