48 MEMBRANE PERMEABILITY AND DRUG ACTION 



action of silver on the invertion of sugar. Curve a shows 

 that the action of silver upon free invertase is immediate, 

 and always reduces the action of invertase. On the other 

 hand, as is shown by curve h, when silver is added to a 

 suspension of yeast cells, low concentrations of silver 

 actually increase the rate of invertion of sugar, and it 

 requires a relatively substantial concentration of sugar 

 to destroy the enzyme action. It is possible that the 

 first action of silver on the cells is to increase their per- 

 meability to sugar, and thus enable more substrate to 

 obtain access to the enzyme than would otherwise be 

 the case. When the concentration of silver is increased, 

 the enzyme itself is affected. These results show that the 

 permeability factor, even in a simple system, may be 

 involved in a relatively complicated manner. In the fol- 

 lowing pages we shall to a large degree ignore compli- 

 cations and deal with permeability problems exclusively. 

 But it is important to remember that in so doing we are 

 indulging in an artificial abstraction. 



Membrane PeTmeability and Drug Structure 



When considering the permeation of drugs into cells, 

 three major questions arise. These are: 



1. Can the structure of the drug be modified without 

 destroying its therapeutic activity? 



2. Do different cells differ in permeability to the same 

 substance ? 



