504 THE BIOCHEMISTRY OF B VITAMINS 



the toxicity of sulfonamide with increasing pH of the medium 124> 198 is 

 parallelled by growth studies with a mutant strain of Neurospora crassa 

 which requires considerably less p-aminobenzoic acid in media at pH 4 

 than at higher pH levels. 202 



The effects of pH on the activities of several sulfonamides, including 

 some which do not ionize, are indicated in Table 7. These data were 

 derived by North ey " from original data supplied by Wyss. Escherichia 

 coli was used for the data below pH 7 and Streptococcus faecalis in tests 

 above pH 7. At pH 7 the results with both organisms agreed so closely 

 that only an average of the results was indicated. The inhibition indices 

 are expressed in terms of the concentration of the sulfonamide to 

 p-aminobenzoic acid, the ratio of the molecular forms of the substances in 

 the medium, and the ratio of the ionic forms in the medium. 



If only the molecular forms penetrate to the site of action, and if the 

 pH within the cell is assumed to be constant over the pH range indicated, 

 the ratio of molecular forms of sulfonamide to p-aminobenzoic acid in 

 the medium would be proportional to the corresponding ratio of ionic 

 forms within the cell. Since this ratio is not constant for the inhibition, 

 it does not appear that the relative activity of sulfonamides can be ex- 

 plained solely on the basis of greater permeability of the molecular form 

 and greater activity of the ionic form. However, the pH inside the cell 

 may be affected more by the pH of the medium than is generally realized. 



From the data presented in Table 7, it is apparent that the sulfonamides 

 are usually most effective at a pH almost equal to their pK a , that at any 

 given pH the most active sulfonamide is usually the one with a pK a ap- 

 proximating that pH, and that non-ionic sulfonamides increase in ac- 

 tivity with increases in pH over the range indicated. Similar results for 

 changes in the activity of sulfonamides with changing pH of the medium 

 have been reported for Mycobacterium tuberculosis. 203 



On the assumptions that (1) only the molecular form of sulfonamides 

 penetrate the cell wall, (2) only the ionic form of sulfonamides acting 

 within the cell inhibit the biological system, (3) that all sulfonamide 

 ions have equal inhibitory activity within the cell, and (4) that the pH 

 within the cells of Escherichia coli is 6, Northey " derived an empirical 

 equation relating activity of a sulfonamide to the fraction ionized in the 

 medium and within the cell. Thus, 



log l/C R = \og Xi (1-XO +7.2573 



where C R is the minimum molar concentration producing the inhibition 

 in medium at pH 7, Xi is the fraction of drug ionized within the cell, X 

 is the fraction ionized in the medium, and the last figure is — log k, where 



