572 12. RATES OF INHIBITION 



critical approach in the following words: "The study of the kinetics of 

 drug action is, therefore, of a qualitative rather than a quantitative nature. 

 It is perfectly easy to obtain attractive curves relating time and action, 

 and the ease with which these curves can be fitted by simple formulae is 

 very striking. The difficulties begin when an attempt is made to analyse 

 the factors that must be concerned with the delay." Such a statement is 

 equally valid when applied to the study of inhibition and one must be par- 

 ticularly aware of the dangers of drawing conclusions as to the fundamental 

 mechanisms of inhibition from the fit of the experimental data to some 

 mathematical formulation. Unfortunately many different assumptions as 

 to the mechanism give essentially the same predicted behavior in most 

 cases. It is possible to distinguish between these different mechanisms only 

 by a more direct investigation of the penetration and distribution of the 

 inhibitors and the nature of the metabolic alterations. 



Relation of Inhibition Kinetics to the Changes Measured 



Consideration must first be given to the relationship between the primary 

 inhibition and the processes whose changes are determined in any study 

 of the rates of inhibition in complex systems. If the inhibitor acts on some 

 enzyme within the cell, ideally the activity of this susceptible enzyme would 

 be determined as it changes with time, but this is usually difficult if not 

 impossible. Unless the inhibition is irreversible, or very slowly reversible, 

 it is not feasible to extract the enzyme from the tissue at intervals to de- 

 termine the course of the inhibition, so that, if the activity cannot be directly 

 measured within the cells, one must resort to indirect procedures. Most 

 commonly tested is some metabolic process dependent upon the susceptible 

 enzyme (e.g., respiration or glycolysis), but such measurements will not 

 provide an accurate course of the enzyme inhibition unless the rate of the 

 over-all process is immediately and quantitatively related to the activity 

 of this enzyme. In some cases, such as the depression of respiration resulting 

 from the inhibition of cytochrome oxidase, a satisfactory correlation may 

 exist, but in general the metabolic changes will lag behind the enzyme 

 inhibition to varying degrees, and indeed frequently there will be no di- 

 rect or simple relationship at all. Especially when the uptake of some sub- 

 strate or the formation of a metabolic product is determined, the kinetics 

 will often relate to factors other than those of the enzyme inhibition. Com- 

 monly some biochemical or physiological (functional) change only secon- 

 darily dependent on the susceptible enzyme is used as a measure of the 

 development of inhibition and here it is evident that only a very fortuitous 

 set of circumstances will allow quantitative interpretation. The many prob- 

 lems of correlating enzyme inhibition with the functional depression of 

 cells have been discussed in Chapter 9 with regard to final inhibitions but 

 the same problems arise, perhaps accentuated, in kinetic studies. The 



