188 



1. MALONATE 



Table 1-27 



Erythrocyte Permeabilities and Molecular Properties of Dicarboxylate 



Anions " 



Anion 



Relative 

 permeability 



Ionic 

 volume 



Ionic 

 length 



(A) 



(H,B) 



(HB-) 



" The permeabilities were determined in bovine erythrocytes by Giebel and Passow 

 (1960). The values given here for the relative permeabiHties are the reciprocals of the 

 entrance rate half-times (ti/^) multipHed by 100. Tlie calculations of the ionic volumes 

 and lengths are approximate and usually depend on the configuration of the ions. 

 The concentrations of HjB and HB are given for a total concentration of 1 M. 



different than those given by Giebel and Passow, must be considered as 

 only relative values, neglecting hydration and special configurations of the 

 ions. Their experiments were run at a pH of 7.35 so the concentrations of 

 the undissociated and singly dissociated forms of the acids are given for 

 this pH. The ionization constants are sometimes quite different from those 

 assumed by Giebel and Passow and are those in Table 1-2. There is certainly 

 little or no general correlation between permeabilities and the concentra- 

 tions of either H2B or HB". Succinate, for example, penetrates one-thirty 

 eighth as rapidly as malonate and yet (H2B) for succinate is 23 times higher 

 than for malonate. This does not necessarily invalidate the assumption 

 that for a single substance the unionized forms penetrate more rapidly than 

 the ionized, but it shows that there are other factors which are quite im- 

 portant. There is also no correlation with the ionic length and it is unlikely 

 that one would be expected. However, there is some correlation with ionic 

 volume, leading Giebel and Passow to suggest that the dicarboxylate anions 

 penetrate through pores in the membrane, whereas the monocarboxylates 

 pass through the lipid phase of the membrane. They calculate the pore 

 radius to be between 3.8 and 4.5 A. If these ions pass through the pore 

 channels in the extended form, which is likely, there are two major factors 

 which may contribute to the permeability: the cross-sectional area perpen- 



