Alterations in dihydropteroate synthetase in cell-free extracts of sulfanilamide-resistant Neisseria meningitidis and Neisseria gonorrhoeae.

Extracts from Neisseria meningitidis and N. gonorrhoeae with varying susceptibility to sulfanilamide have been investigated for dihydropteroate synthetase activity. Sulfanilamide was a competitive inhibitor of dihydropteroate synthetase with respect to p-aminobenzoate in extracts from both species. Though the K(m) for p-aminobenzoate was unaffected, the K(i) for sulfanilamide increased and the V(max) decreased as the strains' resistance to sulfanilamide increased. Temperature studies have revealed differences in the dihydropteroate synthetase from N. meningitidis and N. gonorrhoeae. A direct relationship was observed between the minimal inhibitory concentration of sulfanilamide determined in vitro and the ratio of K(i)/K(m). This ratio may be a molecular explanation of sulfanilamide resistance for both N. meningitidis and N. gonorrhoeae.