THE CELL MEMBRANE AS A SITE FOR MERCURIAL ACTION 897 



ing observed by Demis and Rothstein (1955) with 0.6-0.9 mM Hg++ might 

 be due in part to this, the outer layers of cells picking up the Hg++ not 

 only in the membranes but within the cells. One might try to estimate 

 roughly the amount of Hg++ required for saturation of membrane sites by 

 determining the initial concentration so that all the Hg++ is removed from 

 the medium. If one plots as accurately as possible the amount of Hg++ 

 remaining in the medium after the fast phase against the initial concentra- 

 tion, one finds that the nearly linear curve passes almost exactly through 

 the origin. All this shows is that it must require very little Hg++ to saturate 

 the ligands involved in the fast phase uptake. 



It is of some interest to attempt an estimate of the concentration of 

 membrane SH groups in certain cellular suspensions in order to obtain some 

 idea of the order of magnitude. In the experiments of Houck (1942) the 

 suspensions contained 4 x 10^ cells/ml oi Achromobacter fischeri under stand- 

 ard conditions. Achromohacter is a rod with dimensions 0.9 X 1.8 ;/ and 

 thus the surface area of a single cell is around 3.8 X 10~^ cm^. If one as- 

 sumes that the membrane is 200 A thick (which is probablj^ too high), that 

 the membrane is 50% water (since it is perhaps more condensed than the 

 cytoplasm), that the membrane solids include 65% protein (values of this 

 magnitude have been obtained for the membranes of other bacteria), that 

 protein specific gravity is 1.4., that the mean molecular weight of the mem- 

 brane proteins is 100,000, and that there are approximately 10 reactive SH 

 groups on a protein of this molecular weight, one can calculate that the 

 concentration of membrane SH groups is close to 10"^ rcvM. The lowest 

 Hg++ concentration to produce reduction of luminescence was 10~^ m.M, 

 so that even at this lowest concentration the Hg++ was around 100 fold in 

 excess of the membrane SH groups. Of course, ligands other than SH groups 

 may occur in the membrane. This suspension of Achromohacter is fairly 

 dilute relative to most suspensions used, since calculation of total cell vol- 

 ume indicates that the cells occupy 0.046% of the total volume. In more 

 concentrated suspensions, such as are often used, the situation can be quite 

 different. A 10% suspension of human erythrocytes (1.16 X 10^ cells/ml, 

 cell surface area = 1.4 X 10~® cm^, membrane thickness = 82 A, and 10 SH 

 groups/protein molecule of molecular weight 100,000) would be 0.059 mM 

 with respect to membrane SH groups, so that an appreciable amount of 

 Hg++ might be bound by the membranes in this case. In any study relat- 

 ing to a theory of membrane binding of heavy metal ions, it would be well 

 to make some reasonable estimates of the concentration of membrane lig- 

 and groups. Although such calculations cannot be very accurate, the experi- 

 mental results may be of an entirely different order of magnitude, which 

 should impel the investigator to question the validity of the theory. 



