INHIBITION OF RECOMBINANT PNEUMOCYSTIS-CARINII DIHYDROPTEROATE SYNTHETASE BY SULFA DRUGS

Forty-four sulfa drugs were screened against crude preparations of rec ombinant Pneumocystis carinii dihydropteroate synthetase. The apparent Michaelis-Menten constants (K-m) for p-aminobenzoic acid and 7,8-dihy dro-6-hydroxymethylpterin pyrophosphate were 0.34 +/- 0.02 and 2.50 +/ - 0.71 mu M, respectively. Several sulfa drugs, including sulfathiazol e, sulfachlorpyridazine, sulfamethoxypyridazine, and sulfathiourea, in hibited dihydropteroate synthetase approximately as well as sulfametho xazole, as determined by the concentrations which cause 50% inhibition and/or by K-i. For all sulfones and sulfonamides tested, unsubstitute d p-amino groups were necessary for activity, and sulfonamides contain ing an N-1-heterocyclic substituent were found to be the most effectiv e inhibitors. Folate biosynthesis in isolated intact P. carinii was ap proximately equally sensitive to inhibition by sulfamethoxazole, sulfa chlorpyridazine, sulfamethoxypyridazine, sulfisoxazole, and sulfathiaz ole. Two of these drugs, sulfamethoxypyridazine and sulfisoxazole, are known to be less toxic than sulfamethoxazole and should be further ev aluated for the treatment of P. carinii pneumonia.