ASER ROTHSTEIN 



71 



where Km,, is the dissociation constant, (M), (Y), and (MY) the concentrations 

 of free Mn"*"^, free yeast groups and bound ]\In, and fMn the activity coetlficient 

 of ]Mn++. The fM,, was kept constant at a value of 0.52 in many experiments by 

 maintaining a constant ionic strength of 0.03 m/1. Again, as in the case of 

 U02'^^-binding, it was assumed that the ratio of activity coefficients of the 



0.5 



1.0 1.5 2.0 2.5 3.0 3.5 4.0 



BOUND Mn IN M/L x 10^ 



Fig. 4. A mass-law plot of Mn++-uptake by yeast cells. 



soUd phase constituents, Y and MY is constant. This constant is therefore 

 included in the Kmh. Equation j has been cast in the more useful form, 



(MY) ^ f MnY t _ fMn (MY) 

 (M) Kmh KMn 



(4) 



where Yt is the total number of yeast groups. The equation is similar to that 

 used to characterize the binding of cations by proteins (60). If the ratio of bound 

 to free :Mn++, (MY/M), is plotted against bound Mn++, (MY), then according 



to equation 4 a straight line should result with a slope equal to — ^ and an 



I Mn 

 Kmh 



intercept on the X axis equal to Yf Such a plot has been made in iigure 4, 

 using data obtained with Mn**. The points can be conveniently expressed as 

 two straight lines, suggesting that there are at least two species of Mn-binding 

 sites with different affinities for the cation. The species with the greatest affinity 

 is represented by the line with the steep slope. The values of Kmu and Yt calcu- 

 lated from its slope and intercept are 3.6 X 10""^ and i X io~^ m/1. of cells. 

 The steep slope of figure 4 represents the same binding sites as those identified 

 as polyphosphates on the basis of UO2++ studies. This can be established by 

 competition studies with the two cations and it is also indicated by the fact 

 that the concentration of binding sites (i X io~' m/1. cells) is the same as 



