THE CELL MEMBRANE AS A SITE FOR MERCURIAL ACTION 895 



There are certain aspects of these results which are puzzling and seem 

 to me to be difficult to reconcile with the simple theory presented. Why 

 does the Hg++ uptake cease after the fast phase for the lower initial con- 

 centrations (0.2 mM or below) (Fig. 7-38)? Let us estimate how much 

 Hg++ is taken up by the diaphragm in the fast phase (see accompanying 

 tabulation). If this represents Hg++ bound to membranes and as much as 



9.4 //moles/g of tissue can be taken up, why does uptake stop when so 

 little is bound at the lower concentrations? One would expect all the Hg++ 

 to disappear from the medium when the initial amount of Hg++ is less 

 than that required to saturate the membranes. The maximal Hg++ bound 

 finally at the highest concentration was stated to be about 15 ^/moles/g 

 of tissue. Inasmuch as the plasma membranes cannot contribute more 

 than 1% of the tissue mass, how is it that they can bind over half this 

 amount ? 



The amount of Hg++ diffusing into the extracellular space and existing 

 there unbound must be negligible, since the diaphragms weighed 0.6 g and 

 the total medium volume was 10 ml, so that the extracellular space would 

 be approximately 1 % of the total volume. We have mentioned that Casca- 

 rano and Zweifach (1962) found diaphragm exposed to Hg++ to show evi- 

 dence of dehydrogenase inhibition only in the outer few layers of cells 

 (page 879). Thus Hg++ does not appear to penetrate readily throughout 

 the tissue. One must ask if the fast phase of uptake may be correlated with 

 binding only to the membranes of the outermost layer of cells. A rough 

 estimate of the protein contained in the outermost membranes on both 

 sides of diaphragms, assuming a generous membrane thickness of 200 A, 

 gives 1.5 X 10"* //mole/g of tissue. If all the Hg++ taken up w-ere bound by 

 these membranes, there would be 62,000 Hg++ ions bound per protein 

 molecule (of assumed molecular weight 100,000), and since this value is 

 impossibly large, one must conclude that most of the Hg++ must be bound 

 deeper in the tissue. Demis and Rothstein (1955) assumed that the Hg++ 

 is not bound entirely to the outermost ceUs, but to the plasma membranes 

 throughout the diaphragm. It is difficult to estimate the amount of protein 



