Phosphanilic acid inhibits dihydropteroate synthase.

Intact cells of Pseudomonas aeruginosa were more susceptible to phosphanilic acid (PA) than cells of Escherichia coli. In cell extracts, the dihydropteroate synthases of P. aeruginosa and E. coli were about equally susceptible to inhibition by PA. These results suggest that cells of P. aeruginosa are more permeable to PA than cells of E. coli. Although a weak inhibitor, PA acted on dihydropteroate synthase in the same manner as the sulfonamides with which PA is structurally related. Inhibition of E. coli by PA in a basal salts-glucose medium was prevented by p-aminobenzoic acid (pABA). However, pABA did not protect P. aeruginosa from PA under these conditions, possibly because pABA itself exhibited an inhibitory effect. PA also appeared to have a second mode of action. The mechanism was not elucidated.