498 10. EFFECTS OF MORE THAN ONE INHIBITOR 



What information may be obtained from the plotting of isobols? If the 

 isobol is Hnear, complete binding interference between the two inhibitors 

 is indicated, and the degree of deviation from linearity would be a measure 

 of the effect of each inhibitor on the binding of the other. It is indeed pos- 

 sible to calculate a, if the inhibition types for the individual inhibitors are 

 known, by the following equation: 



(Il)'(l2)' 



(10-20) 



X - y{\,)' - (1,)' 



where for 50% inhibition x = (S)' + 1 and ?/ = 1 when both inhibitors are 

 competitive, x — y = 1 when both inhibitors are noncompetitive, and 

 X — y — (S)' -^ 1 when one inhibitor is competitive and the other is non- 

 competitive. This method could be of value in determining the relative po- 

 sition of binding of inhibitors on an enzyme surface but depends upon an 

 accurate evaluation of iiT,,,, K^ , and K^ . Actually, little information may 

 be obtained directly from these curves but they are one of the most concise 

 and intelligible ways of representing the relations between two inhibitors 

 in both isolated enzyme systems and cellular preparations. Their relation 

 to antagonism and potentiation will be discussed later (page 508). 



TWO INHIBITORS ACTING ON A MONOLINEAR CHAIN 



More interesting patterns of inhibition are produced when two enzymes 

 in a multienzyme system are attacked simultaneously and this situation 

 is commonly encountered when two inhibitors are used in mitochondrial, 

 cellular, or whole animal preparations. In this section the simple monolin- 

 ear enzyme chain will be discussed in some detail as an example of this 

 type of inhibition and in the following section some of the other multien- 

 zyme systems will be treated qualitatively. One of the earliest demonstra- 

 tions of multiple monolinear inhil)ition is that described by Beerstecher 

 and Shive (1947), who showed that tyrosine depressed the synthesis and 

 /?-hydroxyphenylalanine depressed the utilization of phenylalanine in E. 

 coli. A marked potentiation of the inhibition was observed when the two 

 inhibitors were used simultaneously; thus the same degree of growth in- 

 hibition was produced by 0.016 mil/ tyrosine alone, by 1.6 niM /5-hydroxy- 

 phenylalanine alone, and by 0.005 niM tyrosine and 0.05 mM /5-hydroxy- 

 phenylalanine together. They postulated that the potentiation was due 

 to the fact that the /5-hydroxyx)henylalanine inhibition was competitive 

 and the reduction of the phenylalanine synthesis by tyrosine made this 

 competitive inhibition much more effective. Potter (1951) applied the term 

 "sequential inhibition" to this phenomenon and illustrated it by the si- 

 multaneous actions of malonate and fluoroacetate on the tricarboxylic acid 

 cycle. The possible utilization of sequential inhibition in tumor chemother- 



