432 9. INHIBITION IN CELLS AND TISSUES 



ditions. Thus the common incorporation of sucrose (usually 250 milf) 

 into mitochondrial media may help maintain a steadier metabolic rate but 

 may also alter the biochemical properties of the component enzymes. 

 Likewise, the use of EDTA in mitochondrial suspensions should probably 

 be discontinued because it not only removes calcium and other metal ions 

 from solution, but exerts more direct effects to disturb the normal electron 

 transport and phosphorylation (Yang, 1960). 



The purpose of this section is to emphasize the striking discrepancies 

 between conditions within the cell and in the artificial media used for en- 

 zyme study. Some of these differences are inherent in the technical impos- 

 sibility of reproducing complex intracellular conditions, but some are also 

 due to the general lack of interest in investigating enzymes physiologically. 

 This state of affairs introduces uncertainty into the interpretations of 

 inhibition in the cell on the basis of work on isolated enzymes. Uncertainty 

 of this sort was expressed long ago by Linderstr0m-Lang and Holter 

 (1931) but the problem still remains unsolved. In later chapters, corre- 

 lations between in vitro and in vivo results will be made because at pre- 

 sent there is no alternative, but it should always be remembered that the 

 conclusions must be provisional until data from more physiological studies 

 are available. 



DISTRIBUTION AND STATE OF ENZYMES IN CELLS 



Enzymes are not distributed homogeneously throughout a cell but are 

 concentrated in certain compartments or subcellular structures. This may 

 be of great importance in cellular inhibition since the accessibility of the 

 inhibitor to the enzyme can determine the effect produced. The manifold 

 reactions of the cell could not be carried out simultaneously in an organ- 

 ized manner without a heterogeneous arrangement of enzymes. Cori 

 (1956) has stressed the role of metabolic compartments in controlling many 

 enzymic reactions. There are many instances known where a substrate and 

 enzyme exist in the cell without reaction and, in some cases at least, this 

 is probably due to spatial separation, which may be broken down when the 

 cell is activated functionally. If enzymes are unavailable to their sub- 

 strates, it is likely that they are protected from certain inhibitors. Com- 

 partmentalization, in addition, can modify enzyme kinetics by introducing 

 diffusion factors, and hence alter the response of these systems to inhibitors 

 as already discussed in Chapter 7. Finally, one should be cognizant of the 

 possibility of an inhibitor directly or indirectly affecting the intercompart- 

 mental barriers and in this way changing metabolism without any neces- 

 sary effect on the enzymes involved. 



