THE CELL MEMBRANE AS A SITE FOR MERCURIAL ACTION 899 



ToM, and 1.6 mM produces essentially a complete loss of the cell K+ in 

 2 hr.* It is possible from the results with the yeast columns that Hg++ at 

 low concentrations has a specific effect on K+ permeability without depres- 

 sing active transport, and this is borne out in the work with erythrocytes. 

 It was stated that the curve obtained by plotting log(IIg++) against max- 

 imal K+ loss is sigmoid, which fits a "normal distribution" (presumably of 

 susceptibility of different yeast cells to Hg++), and that the loss of K+ is 

 probably an all-or-none phenomenon, this being confirmed by determina- 

 tions of staining by certain dyes in Hg++-treated cells. Although yeast cells 

 undoubtedly show a variation in the sensitivity to Hg++, it is doubtful if 

 the evidence is sufficient to categorize the K+ loss as all-or-none, especially 

 since sigmoid curves of this type (they are not given so one cannot directly 

 evaluate them) are also compatible with graded effects and, in fact, are the 

 commonest relations observed in the actions of most inhibitors on cell me- 

 tabolism or function. There is an increase in general membrane permeability 

 produced by Hg++, as proved by the loss of a variety of substances from the 

 cells and a greater penetration of dyes, and this could be a graded phenom- 

 enon occurring simultaneously with the alterations in K+ efflux, without 

 the need for assuming cytolysis as the necessary concomitant of K+ loss. 

 Hg++ is bound relatively rapidly to yeast cells, the half-time being 2-4 

 min and maximal binding occurring in 15-20 min. Passow and Rothstein 

 (1960) determined the uptake of both Hg++ and Cl~, and found that ini- 

 tially only Hg++ is bound, the Cl~ entering when the concentration of Hg++ 

 is sufficiently high. The binding at low concentrations was thus claimed to 

 represent "binding of Hg++ rather than HgClg." Since the concentration of 

 the Hg++ ion is actually extremely small, it seems more likely that HgClg 

 or other chloride complexes react with the yeast cell wall and membrane, 

 releasing the CI which diffuses into the medium. When the concentration 

 of the mercurial becomes great enough to lead to a significant increase in 

 permeability, Cl~ then enters, either alone or with Hg++. The general con- 

 clusion is that the membrane effect of Hg++ is not specific for K+ but is a 

 more or less nonspecific breakdown of the membrane, caused by the "mol- 



* The approximately 1000-fold difference in sensitivity observed in these two types 

 of experiment deserves some comment and Dr. Rothstein has kindly provided me with 

 the reasons. In the suspension experiments the yeast density was 60 mg/ml and at 

 0.4 mM Hg++ the maximum binding of the metal would be about 7 millimoles/kg 

 of cells. In the column experiments with 600 mg of cells and a flow rate of 5 ml/min, 

 the maximum binding in 30 min at 0.05 mM Hg++ would be only 0.015 millimole/kg. 

 Thus the yeast in the column would be much more readily affected since less of the 

 Hg++ is removed. Second, the suspension experiments measure the steady-state flux 

 and the net loss of K+, whereas the column experiments measure the rate of efflux 

 into K+-free medium. It is therefore difficult to compare the results by the two tech- 

 niques on a quantitative basis. 



