510 10. EFFECTS OF MORE THAN ONE INHIBITOR 



and associates (1957; Ariens, 1954) and of Landahl (1958) may be recom- 

 mended. Nowhere could the cooperation between pharmacology and bio- 

 chemistry be more fertile than here. 



SPECIFICITY OF INHIBITION WITH TWO INHIBITORS 



The general problems of inhibitor specificity will be taken up in Chapter 

 17 but it is pertinent at this time to consider the possibility of increasing 

 the selectivity of inhibition by the judicious use of two or more inhibitors. 

 The uninhibited use of multiple inhibitors will generally complicate the 

 situation and reduce the likelihood that a selective block on one enzyme 

 or pathway is being achieved. However, in cases in which the action spec- 

 tra of two inhibitors are reasonably well known, it is occasionally possible 

 to augment the inhibition of a particular system without simultaneously 

 losing specificity. This situation can arise especially when both inhibitors 

 act on a common enzyme but otherwise have different sites of action. The 

 simplest case is where I^ acts on E^ and E., whereas 1^ acts on E^ and E3; 

 a combination of I^ and I2 will have a greater effect on E^ without increase 

 of inhibition on Eg and E3. Of course, the two inhibitors together will in- 

 hibit more enzymes than either one alone but the inhibition on these sec- 

 ondary systems will be relatively smaller compared to the primary ac- 

 tion on E,. Let us assume that K , = K, — 1 niM and K: = K, = 



11 12 21 23 



3 mM, where the numerical subscripts refer to inhibitor and enzyme in 

 order. It is easy to calculate that for noncompetitive inhibitors, a concen- 

 tration of 1 mM would inhibit E^ 50% when acting singly, but together 

 at this concentration the inhibition would be 67%. On the other hand, 

 the inhibitions on either E., or E3 would not be increased, although both 

 would now be depressed 25%. If 50% inhibition of E^ is desired, it will 

 require only 0.5 mM of each inhibitor but now the inhibitions on E2 and 

 E3 will be only around 14%. Thus some improvement in selectively block- 

 ing E^ can be achieved with two inhibitors and it would be possible to 

 produce even greater specificity with three or more inhibitors if such exist. 



INHIBITION OF BIOLUMINESCENCE WITH SULFANILAMIDE 



AND URETHANE 



A quantitative investigation of the multiple inhibition of bacterial lu- 

 minescence was made by Johnson et al., (1943) and a consideration of the 

 complex results obtained may serve to illustrate the problems and difficul- 

 ties that occur in work with living systems. The results with sulfanilamide 

 and urethane are shown in Table 10-2. By comparing the calculated inhi- 

 bitions in the last colums with the experimental inhibitions, it is observed 

 that antagonism between the inhibitors occurs in every case. 



