500 10. EFFECTS OF MORE THAN ONE INHIBITOR 



Nevertheless, one factor must be borne in mind and that is that many 

 multienzyme systems in the cell are prboably not in a steady state, or that 

 at least periods occur when a steady state does not exist. In such nonsteady 

 states the above reasoning may not accurately apply. Also the introduction 

 of two inhibitors may produce an interval of more severe depression before 

 a new steady state can be reached. The complex effects observed upon tis- 

 sue function or growth may be the result of actions during a nonsteady 

 state period. Cellular metabolic pathways rarely are completely closed sys- 

 tems so that concentrations of intermediates may be determined by factors 

 other than the relative rates of the enzyme reactions. The operation of an 

 isolated monolinear chain in the cell is lorobably very uncommon, inasmuch 

 as most intermediates can be formed or metabolized by reactions not 

 in the chain. 



In view of the conclusions of this section, it is perhaps necessary to at- 

 tempt an explanation for the previously mentioned potentiation observed 

 in the assumed simple sequential inhiljition of E. coli growth by tyrosine 

 and /?-hydroxyphenylalanine (Beerstecher and Shive, 1947). Actually, a 

 metabolic response was not measured here but only the final result on the 

 complex process of growth. In other words, activity in a metabolic mono- 

 linear chain was not determined but only the end result of disturbances in 

 such a chain. The growth rate may depend very critically on the utilization 

 rate of phenylalanine, so that up to a certain degree, inhibition could be 

 exerted without a significant effect on the growth, but a marked depression 

 could result from a rather small further increment in the metabolic inhi- 

 bition. Or in this particular case, the major factor could be the amount of 

 abnormal /?-hydroxyphenylalanine that is utilized. Depression of the syn- 

 thesis of phenylalanine may augment the competitive action of /?-hydroxy- 

 phenylalanine but the growth response need not be a quantitative measure 

 of the depression of phenylalanine utilization. 



TWO INHIBITORS ACTING ON VARIOUS MULTIENZYME 



SYSTEMS 



Opportunities for potent metabolic depression with two inhibitors often 

 occur when the metabolism involves two pathways. Convergent chains 

 (Eq. 7-22) offer simple examples of this. Inhibition of either of the converg- 

 ing limbs (El or Eg) will depress d{C)ldt less than the individual enzymes 

 and never completely. The inhibitions of the formation of C are given by: 



Inhibition of E^ only: i,^ = ii/(l + r) (10-23) 



Inhibition of E, only: /,^ = rislil + r) (10-24) 



Inhibition of E^ and E3: it^ ^ = ih + rh)l{l + r) (10-25) 



where r is the ratio between v^ and ^'j, ?', and i^ are the inhibitions on Ej 



