72 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



determined by U02"*"'' or by Mn++ uptake. The shallow slope of figure 4 repre- 

 sents a second species of binding sites which have been identified on the basis of 

 indirect evidence as carboxyl groups of proteins. 



Other cations, both bivalent and monovalent, are also bound by the cell sur- 

 face groups (54). In the case of the monovalent cations, it is diflficult to demon- 

 strate the ion-binding directly, because the dissociation constants are rather 

 high. However, in each case the binding can be demonstrated in terms of a 

 competition reaction between Mn++ and the monovalent cation. The latter, if 

 added in increasing concentrations, will displace the former from the cell-surface. 

 Table 3 presents the aifinities of the cell for each of the ions tested, relative 



Table 3. Relative affinity of various cations for yeast surface loci 



Data are based on competition of various cations with Mn++, using Mn^^. Mn concentra- 

 tion was between i and 4 X lo"'' m/1. in each case; yeast concentration was 200 mg/ml. and 

 the pii, 3.5. Concentrations of the cations were sufficient in each case to displace a signifi- 

 cant quantity of Mn++ from the cell. 



* Based on determinations of U0-2+"'" binding by yeast in a previous study (52). 



to that for Mn++, based on the ratios of the mass law constants. It should be 

 noted that the bivalent cations are bound in a much more stable complex than 

 are the monovalent cations. UO2++ forms by far the most stable complex. 

 Although H+ is not shown on the table, it also competes with Mn++. As the pH 

 is reduced, the stability of the Mn-complex is decreased. It is of some interest 

 that there is a i to i competition between the bivalent and monovalent cations. 

 Although extracellular Mn++ or Ca++ equilibrates rapidly with the cell sur- 

 face, there is no exchange with bivalent cations of the cytoplasm in the resting 

 cell. The surface-bound cations, and the cations of the interior of the cell are 

 in two distinct compartments, with only a slow interchange between. The 

 surface-bound cations represent only about 3 to 5% of the total bivalent cations 

 and only 0.5% of the total cations of the cell. 



