428 9. INHIBITION IN CELLS AND TISSUES 



The activity and susceptibility to inhibition of an enzyme depend on 

 the state of the enzyme and since there are several possible reasons why 

 the states of isolated and intact enzymes should differ, it is reasonable to 

 anticipate that an exact correlation between the behavior of an enzyme 

 in the cell and the behavior of the same enzyme in an extract or in a pu- 

 rified state would seldom be possible. Some of the factors responsible for 

 these different states of an enzyme may be outlined. 



(I) Different chemical compositions of the media 



A. Inorganic ions (including pH) 



B. Organic substances (both low molecular -weight compounds, such as glu- 

 tathione, and high molecular weight compounds, such as proteins and nu- 

 cleic acids). 



(II) Different physical states of the enzyme. 



A. Complexes of the enzyme with other substances, such as lipids, in the cell, 

 or a variety of possible compounds released from their normal sites during 

 extraction 



B. Presence of the enzyme at an interphase or in a molecular layer 



(III) Different temperatures 



(IV) Different states or concentrations of the enzyme's substrate 



^4. Enzyme may be part of an organized multienzyme system so that its sub- 

 strate may be immediately available in a high local concentration or acti- 

 vated form 



B. Enzyme may be separated from its substrate by membranes that reduce 

 the substrate concentration in the region of the enzyme 



(V) Different concentrations of the inhibitor used 



The last two categories have been discussed in the previous chapters. The 

 effects of pH on enzyme inhibition will be treated in Chapter 14 and the 

 effects of temperature, ions, ionic strength, and buffers in Chapter 15; 

 for the present we may assume that these factors are often of significance. 

 Some inquiry must now be made into the differences in enzyme environ- 

 ment within the cell and in the media generally used for the investigation 

 of isolated enzymes. 



CHEMICAL COMPOSITION OF THE MEDIA 



It is, of course, impossible at the present time to duplicate exactly intra- 

 cellular conditions in the study of isolated enzymes, since our understanding 

 of the normal enzyme environment in the cell is limited. The usual media 

 employed in the characterization of enzymes contain only a buffer in ad- 



