PHYSICO-CHEMICAL ASPECTS 17 



the protein and the other colloidal particles dissolved 

 or suspended in it. The difference in pjj between such 

 surface and bulk phases may am,ount to i pH unit or 

 more. Thus the interior of a cell, although its average pn 

 value may be very constant, can in fact present an ex- 

 tremely variable pn in different sub-microscopic regions. 

 This permits of great variations in the degree of enzyme 

 activity, according to the particular sub-microscopic 

 environment with which individual enzyme molecules 

 are associated. 



Very similar effects exist in connection with the con- 

 trol of the reduction of enzyme systems by glutathione. 

 In addition to the "diffusible" SH groups of glutathione, 

 there are other "indiffusible" SH groups permanently 

 fixed to protein molecules. The distribution of glutathi- 

 one between the surface of an enzyme, and the surround- 

 ing bulk phase, is in part determined by the total charge 

 on the protein and the degree of ionisation of the glu- 

 tathione. Hence there is a fairly complex equilibrium 

 determining the distribution of SH compounds between 

 the surface and bulk phases within a cell. Furthermore, 

 it may be important that whereas surface pn is almost 

 independant of the surface SH content, as the result of 

 the operation of electrostatic factors the surface SH con- 

 tent is not independant of pH- 



Dielectric Constant of Cellular Systems. A number of 

 workers have pointed out that the presence of large po- 



