p-AMINOBENZOIC ACID 501 



analogue or that the analogue combines irreversibly with the enzyme, 

 or that some other such phenomenon prevents a competitive reversal by 

 p-aminobenzoic acid. 



Mechanism of Action of Sulfonamides and Correlation of Activities with 

 Physical Properties and Structure 



Attempts to correlate the inhibitory activities of the N 1 -substituted 

 sulfonamides and related compounds with structure or some physical 

 property of the compounds have resulted in several different theories as 

 to the mode of action of these substances. However, the theory of com- 

 petitive inhibition of the utilization of p-aminobenzoic acid as an essen- 

 tial metabolite 1 explains more effectively the data which have accumu- 

 lated. 



Application of the Michaelis-Menten equations adapted to the rate of 

 an inhibited reaction was made by Wyss 192 to demonstrate the com- 

 petitive nature of the relationship between p-aminobenzoic acid and 

 sulfanilamide for Escherichia coli (p. 455) . A similar treatment was ap- 

 plied to the inhibition index by Wood, who suggested that the variations 

 in the bacteriostatic activity of the different sulfonamides might be the 

 result of differences in affinities for the enzyme involved in the function- 

 ing of p-aminobenzoic acid. 108 



Sulfanilamide does not displace any appreciable amount of p-amino- 

 benzoic acid from cells of Streptococcus hemolyticus, 193 and there is no 

 appreciable binding of sulfanilamide labelled with radioactive sulfur in 

 the cells of Escherichia coli. ld4 Thus, p-aminobenzoic acid appears to be 

 converted into a coenzyme form, and very little of the total p-amino- 

 benzoic acid of cells exists in a combination from which it is displaced by 

 sulfonamide. This is further indicated by the observation that bacteria 

 are capable of undergoing a definite, limited number (six or seven) of 

 cell divisions in the presence of any effective drug concentration, re- 

 gardless of the inoculum employed. 195, 19C After inhibition is obtained, 

 p-aminobenzoic acid is reported to exert its effect immediately under 

 certain conditions. 158 * m 



Correlation of Activities with Ionization of Sulfonamides. The influence 

 of pH on the inhibitory activity of sulfonamides and on the ability of 

 p-aminobenzoic acid to prevent the toxicity of these drugs was first indi- 

 cated by Lwoff and co-workers. 198 The amount of sulfanilamide necessary 

 to prevent the reproduction of the flagellate, Polytomella caeca, was five 

 times greater at pH values below 3.1 than at values above 5.5. The 

 change in inhibitory activity of sulfanilamide appeared to occur only 

 between these two points. The ratio of sulfanilamide to p-aminobenzoic 

 acid at which complete reversal of the inhibition is obtained increases 



