720 6. O-IODOSOBENZOATE 



If only the affinities of the substrates are altered by the inhibitor, we may 

 write for the inhibited rate: 



(S) + aK, 



where a is a factor indicating the magnitude of the effect of the inhibitor 

 on the substrate binding (a > 1). The inhibition is then given by: 



a - 1 



(6-1) 



(S') + a 



where (S') is the specific concentration of the substrate, (^)IK,. Thus, even 

 though a is the same for each substrate, the inhibition will vary with (S'). 

 Superficially it might appear that an irreversible inhibitor reducing sub- 

 strate binding would produce inhibitions independent of the substrate con- 

 centration, but such is not the case. Some of the confusion arises from as- 

 sociating this type of inhibition with noncompetitive inhibition, which it 

 is not in any sense. The inhibitions by o-iodosobenzoate, and probably most 

 SH reagents, are usually competitive, as shown by the protection afforded 

 by the substrate when it is present during the incubation with the inhibitor, 

 and K, is altered rather than k^, the rate constant for the breakdown of the 

 ES complex into products. It may be noted that even though Kj„ is not Kg, 

 but the more complex {k_i + li2)jk-^, a similar expression for the inhibition 

 will be found, and the substrate concentration will play a role in the degree 

 of inhibition produced. Furthermore, if the inhibition actually is noncom- 

 petitive and k^ is altered rather than K„ it can easily be shown that the 

 inhibition is given by: 



'"^ - (6-2) 



^k. 



A:_i[(S') + l] 



where /? is the factor by which k^ is changed by the inhibitor (/?<!). Here 

 the variation of the inhibition with the specific concentration of the sub- 

 strate is different than in the previous case, in that the inhibition rises as 

 (S') increases, as long as (S') does not greatly differ from unity. Indeed, at 

 high substrate concentrations, t = 1 — /5, the usually expected purely non- 

 competitive inhibition and, likewise, if iii,,, = K^., ^ = 1 — /?. 



It is consequently not necessary to assume some complex mechanism in- 

 volving steric factors when the inhibition is found to vary with the sub- 

 strate used, unless the specific concentrations of all the substrates are kept 

 equal. It was stated by Singer (1948) in his study of lipase that the sub- 

 strate concentration was chosen so as to "just saturate the enzyme and 

 thereby give optimal activity." However, calculation of the values of (S') 



