412 8. INHIBITOR DISTRIBUTION IN LIVING ORGANISMS 



necessarly linearly proportional to the outside concentration and this 

 factor often makes impossible a quantitative kinetic treatment of inhibition 

 in living material. 



The distribution of inhibitors within cells is as yet an almost totally un- 

 explored field. In the previous chapter some consideration was given to 

 the problems of compartmentalization of enzyme systems. In such systems 

 the possibilities for inhomogeneous inhibitor distribution must also be 

 present. There are more types of membrane in the cell than classically con- 

 ceived but as to how far these membranes impede the diffusion of inhibitors 

 there is no information. It would appear that mitochondrial membranes 

 are more generally permeable than plasma membranes; at least isolated 

 mitochondria are more readily susceptible to inhibitors such as malonate 

 than intact cells and it is likely that large molecules, e.g., diphosphopyridine 

 nucleotide (DPN) and ATP, can penetrate into mitochondria. Whether mito- 

 chondria within the living cell are so readily permeable is not known. Lack- 

 ing evidence one way or the other, one would expect the endoplasmic reti- 

 cular membranes not to present serious barriers to the diffusion of low 

 molecular weight substances, although one must admit the possibility in 

 some cells of a relatively extensive "extracellular" space invaginated 

 within the cell. At present, it must be concluded that the most important 

 hindrance to the diffusion of inhibitors into the cell, or through the cell, 

 is the outer plasma membrane. When inhibitors are unevenly distributed 

 within cells, it is more likely that binding and solubility factors are opera- 

 tive. Some studies recently have attempted to demonstrate inhomogenous 

 distribution of drugs within cells, although such work is subject to great 

 technical difficulties. Interesting results were obtained with quinacrine 

 by analysis of liver cell fractions (Reiner and Gellhorn, 1956). Equilibration 

 of quinacrine with isolated nuclei resulted in a concentration ratio of 200 

 in favour of the nuclei and it is reasonable that the quinacrine was exten- 

 sively bound to nucleic acids. When the quinacrine was administered 

 by various routes to mice at different dosage levels, the distribution between 

 nuclei, mitochondria, and cytoplasmic supernate was quite variable and 

 indicative of relatively nonspecific binding. The binding could not be attrib- 

 uted to deoxyribonucleic acid (DNA) entirely nor to any one protein. How- 

 ever, these results, although of importance in understanding the meaning 

 of cell analyses, probably have very little relevance to the metabolic dis- 

 turbance induced by quinacrine, since all that could be measured was 

 quinacrine bound to mainly nonenzymic material. Since quinacrine is 

 bound quite avidly by cell components, one can only conclude that the con- 

 centration of free quinacrine in solution must have been quite low, since 

 saturation of binding sites was not reached. 



