CONCENTRATION-TIME CURVES FOR INHIBITION 603 



directly interjpretable in terms of any one rate process or mechanism. 

 For example, if n is found to be less than one, various explanations might 

 be found, including (1) reversible processes of inhibition or penetration, 

 (2) competitive inhibition, (3) nonlinearity of internal inhibitor concen- 

 tration relative to the external concentration, and (4) delay in action of the 

 inhibitor on the response measured because of intracellular events subse- 

 quent to the inhibition and leading to the response. This latter factor is 

 probably of major importance when measurements are made of functional 

 or terminal effects of the inhibitors. It is evident that as the inhibitor con- 

 centration is increased, beyond a certain point there will not be a marked 

 reduction in the time required to produce the chosen response, because 

 this time will be mainly dependent on intracellular events whose rates are 

 not directly proportional to the inhibitor concentration or the degree of 

 the primary inhibition. Variations in n could also be produced by adapta- 

 tions of the cells to the inhibitor in long-term experiments, by metabolism 

 of the inhibitor in the cells, by binding of the inhibitor to nonenzymic ma- 

 terial, or by any of the complicating factors considered in this chapter. 

 The expression of inhibition in concentration-time curves or in related 

 equations may eventually be useful when it is combined with a more de- 

 tailed investiga^:ion of the processes involved, but alone it does not lead to 

 reliable conclusions. 



