ASER ROTHSTEIN 



69 



UY are unknown, l)ul in the mass law equation they are present as a ratio. It is 

 assumed that this ratio is constant. The validity of the assumption is borne out 

 by the constancy of the K calculated from the experimental data (table 2). 

 The calculated constant therefore, includes a term for the activity ratio. 



The cellular sites responsible for UOo"*^ binding are presumed to be located 

 on the cell-surface (55). This conclusion is based on a number of observations. 

 For example, inorganic orthophosphate added to the medium will remove some 

 of the bound UOo"^^ by competitive complex-formation. Yet the cytoplasm of 

 the cell contains a concentration of inorganic phosphate which is 50 times 

 higher. In other words, the uranyl ion is bound in a location where it can be 

 influenced by the extracellular rather than the intracellular concentration of 

 phosphate. Furthermore, the maximum binding capacity of the intact cell is 



Table 2. Mass law constants for binding of uranium by yeast 



sites (Yt) of 2 X 10 * m/I. 



only I X io~^ M/1. whereas the total uranium-binding capacity of the cyto- 

 plasmic contents is over 0.2 m/1. Thus uranyl ion does not diffuse throughout 

 the cell where it can react with the high concentrations of phosphates, bicar- 

 bonate, organic acids, and proteins of the cytoplasm in general. Rather, its 

 interaction with the cell is restricted to an outer zone which contains less than 

 I % of the potential binding groups of the cell. 



The chemical nature of the UO2++ binding sites has been investigated tw^o 

 ways (58). Firstly, the affinities of various anions for UO2++ were compared with 

 the affinity of the yeast cell for UOo"^. Of the different kinds of anions tested, 

 only those containing polyphosphate chains, or those with repetitive phosphate 

 groups (nucleic acids), were similar in affinity to the cellular groups. Secondly, 

 the effect of pH on the stability of uranium complexes was determined. Again, 

 polyphosphates and the cellular groups behaved in an almost identical manner 

 w^hereas other anions behaved differently. It was concluded, therefore, that the 



