MEANS OF EXPRESSING RESULTS 253 



in this chapter will in almost all cases be apparent inhibitor constants be- 

 cause the true K/s have generally not been calculated in the reports, 

 although in many instances they must be very close to the true K/s because 

 of the nature of the analog or the conditions of the experiments. I have 

 taken the liberty in certain cases where the data are adequate of calculating 

 the K/s from the inhibitions reported. In every instance where plotting 

 procedures have been used to determine the type of inhibition, it would 

 have been possible to determine the appropriate constants, but these con- 

 stants have been seldom reported. 



An especially unsatisfactory means of expressing the results is to give the 

 inhibition for a certain ratio of inhibitor to substrate concentrations, 

 (I)/(S), which has often been called the inhibition index. If the actual con- 

 centrations are not given, it is not possible to visualize the inhibition quan- 

 titatively, because (I)/(S) is not constant for a certain degree of inhibition 

 even when the inhibition is competitive. This has been clearly pointed out 

 in Chapter 1-3 (page 106) but it is important to re-emphasize it here. The 

 ratio, (I)/(S), is constant and meaningful only at high substrate concen- 

 trations which saturate the enzyme. This may be seen from the following 

 expression, obtained by rearranging the equation for competitive inhibition: 





1 1 



~k7 ^'Wi 



1 



(2-7) 



It has sometimes been assumed tliat for 50% inhibition, (I)/(S) = KJKg, 

 but this is not necessarily true, which can be easily seen by rewriting Eq. 

 2-7 for i = 0.5 and (S) = nK,: 



(I) 



l:[v + ^l 



, -. (2-8) 



(S) Ks ' ' 



(I)/(S) = KJK^ only when the substrate concentration is high relative to 

 Kf. (i.e., when n is much greater than unity). This can also be seen in another 

 way: If K, = 1, K^ = 0.3, and (I)/(S) is kept constant at 0.3, the inhibition 

 will vary with the absolute magnitudes of the concentrations as shown in 

 the following tabulation: 



