DISTRIBUTION OF INHIBITORS WITHIN CELLS 411 



an indication of the true concentrations in the enzyme region; the same sit- 

 uation occurs, of course, in the single cell because it is unlikely that any 

 substance will be homogeneously distributed throughout the entire cell. 



A term such as "intracellular concentration" of an inhibitor is usually 

 glibly used, both for convenience and to mask out ignorance of the true situ- 

 ation, and the value is generally arrived at by analysis of a group of cells. 

 One may often ask, "concentration in what?" The most common way of 

 expressing such concentrations is in milligrams per gram or similar weight 

 of inhibitor per weight of cell units. This provides very little information 

 on the concentration in intracellular water. It is easy to correct for the solid 

 content of the cells but one cannot assume that all the inhibitor is free 

 in solution. It is clear that the concentration of the inhibitor in solution 

 may be negligible when the total amount in the cell is large. Plant cells pro- 

 vide a further complication because of the presence of a large vacuole. Since 

 the vacuole can occupy 80-90% of the cell volume, analysis of a plant cell 

 for inhibitor could give very erroneous ideas of the concentration in the 

 protoplasm if the vacuole either accumulates or excludes the inhibitor. 

 Ideally, there should be another word to designate the total amount of a 

 substance in a cell, irrespective of the location or state of the substance. 

 Instead, we shall, where necessary, designate by the term effective concentra- 

 tion the true concentration of a substance in solution in the region where 

 it is exerting its effect. It is the effective concentration of an inhibitor we 

 should know but which is not usually provided by gross analyses. Very 

 often the variation of effective concentration with time will be in the oppo- 

 site direction to that of the over-all concentration. 



The ratio of the effective concentration of an inhibitor in a cell to the 

 concentration outside may not be unity. Deviations from unity have usually 

 been discussed relative to permeability. However, there are several other 

 factors that may play a role and these will be mentioned only briefly because 

 there is little experimental evidence directly bearing on inhibitors. If the 

 inhibitor is an ion, its penetration into a cell implies either the entrance 

 of an oppositely charged ion or the exchange for a similarly charged intra- 

 cellular ion. Anions particularly are distributed extracellularly for the 

 most part. It is likely that fluoride would behave similarly to chloride and 

 bromide, that is that it would be mainly excluded from cells because of 

 the stable anion content of the cells. There may also be reactions within 

 the cell by which the effective concentration of inhibitor is reduced; these 

 could be metabolic degradation, binding to nonenzymic cell components 

 or entry into cell material in which the inhibitor is soluble. In such cases, 

 the effective concentration may be determined by the balance between rates 

 of entry and disappearance, at least over a particular time interval. The 

 possibility of active transport of the inhibitor into the cell in some cases 

 cannot be neglected. Lastly, the effective concentration in the cell is not 



