460 9. INHIBITION IN CELLS AND TISSUES 



roneous and an unjustifiable extension of the original concept. For this 

 reason it may be well to mention some of the mitigating circumstances that 

 restrict the complete applicabilty of this concept. (1) Reactions which 

 are pacemakers in certain states of a cell may not be when conditions are 

 changed. The concentration of substrates in the medium outside the cell 

 may determine which step is limiting and it is evident that the type of 

 substrate that is available may be of primary importance. Alterations of 

 intracellular ion concentrations or pH can likewise shift a pacemaker; it 

 is possible that in magnesium deficiency, enolase would be a limiting step 

 in glycolysis. (2) The ability of multienzyme systems to maintain their 

 steady state in the presence of an inhibitor is to a great extent dependent 

 on the possibility of intermediates rising sufficiently in concentration. This 

 is not always possible for a number of reasons: the intermediate may be 

 metabolized along another pathway when its concentration begins to rise, 

 it may diffuse away from the reaction region, or it may begin to inhibit 

 some other enzyme at a higher concentration. Thus inhibition of respiration 

 by fluoride can be quite marked, despite the fact that enolase is normally 

 not a pacemaker in glycolysis, but it may well be that phosphoenolpyru- 

 vate cannot accumulate to maintain the original rate. (3) We have seen 

 in Chai^ter 7 that there are many types of multienzyme systems in which 

 buffering against inhibition is present but far from complete. In cyclic 

 and feedback systems, inhibition on any step usually produces some in- 

 hibition of the over-all i)rocess, and in regenerative systems, such as elec- 

 tron transport, the constancy of total regenerant concentration imposes 

 a restriction on the adaptability to inhibition. Therefore, for inhibition to 

 be exhibited the inhibitor need not attack the rate-determining enzyme, 

 if such exists. (4) Sometimes a metabolic pathway is considered as if iso- 

 lated from the other reactions to which, in the living cell, it is closely re- 

 lated. The over-all rate of glycolysis may well depend on reactions outside 

 the main sequence. For example, inasmuch as the glycolytic rate does de- 

 pend on the availability of ATP, AJDP. and inorganic phosphate, enzymes, 

 such as ATPase, can indeed be factors in determining the rate, and inhibition 

 of ATPase, by elevating ATP and reducing ADP and P, levels, might pos- 

 sibly modify glucose utilization. One does not know as yet the kinetic im- 

 portance of such enzymes in the over-all pattern of metabolism, so often 

 it is somewhat premature to designate i^acemakers. (5) The factor of time 

 must also be considered. If inhibition is suddenly achieved with respect to 

 a single step in meta1)olism, it may require some time for the system to 

 regain a steady state, assuming it can (see Chapter 7). Thus during this 

 period of temi)orary inhibition the over-all rate will be suppressed, even 

 though the enzyme attacked is not a pacemaker, and during this interval 

 secondary changes may take place so that a return to the original state is 

 made impossible. 



