Studies on the transport mechanism of sulfonamide compounds in renal tubules. Stopflow analysis applied to the excretion of sulfonamide compounds

Effects of methoxy groups in the NI-substituent of sulfonamides on the pathways of elimination in man. The acetylation-deacetylation equilibrium and mechanisms of renal excretion of sulfisomidine, sulfamethomidine and sulfadimethoxine

Sulfisomidine, sulfamethomidine, sulfadimethoxine and their corresponding N4-acetyl derivatives were administered to man. The percentages of acetylation and deacetylation and those of protein binding, the half-lives of elimination and the apparent and true renal clearance values were measured. No acetylation phenotype could be demonstrated in these compounds. Methoxy substitution in the NI-pyrimidine group of sulfisomidine affects predominantly the renal clearance value and mechanism of the parent compound but has no influence on the renal clearance of the N4-acetyl derivatives. The renal clearance value of sulfisomidine is 232 +/- 33 ml/min, of sulfamethomidine 21.60 +/- 16.4 ml/min and of sulfadimethoxine 10.87 +/- 10.44 ml/min. The renal clearance values of the corresponding N4-acetylsulfonamide derivatives are 314 +/- 91 ml/min, 342 +/- 63 ml/min and 202 +/- 65 ml/min respectively. Tubular reabsorption, caused by methoxy substitution in the NI-pyrimidine ring, lowers the rate of elimination and increases the half-life. The half-life of sulfisomidine is 8.5 +/- 0.5 h, of sulfamethomidine 27.8 +/- 5.3 h and of sufadimethoxine 34.6 +/- 10.4 h.

Acetazolamide inhibition of renal gamma-glutamyl transpeptidase

The effect of acetazolamide (AZ) on renal gamma-glutamyl transpeptidase (EC 2.3.2.2) activity (gamma-GT) was studied with the purified enzyme, subcellular fractions, and in the isolated functioning kidney. Activity of gamma-GT was assessed using either one of two gamma-glutamyl donors, gamma-glutamyl-p-nitroanilide (gamma GpNA) or glutamine, and either the gamma-glutamyl acceptor glycylglycine (Gly-Gly) or methionine (Met). With the microsomal enzyme and beta-GpNA, AZ was shown to inhibit p-nitroaniline (p-NA) formation; however, gamma-GpNA Km remained unchanged (1.8 mM), while the Vmax was reduced significantly, 333 vs 200 mumoles . min-1 . mg-1. Adding Gly-Gly removed AZ inhibition, while AZ elevated the apparent Km from Gly-Gly from 16 to 48; AZ inhibition of gamma-GT activity resulted in a decrease in gamma-glutamyl-Gly-Gly formation consistent with interaction at the gamma-glutamyl acceptor site. With glutamine as the beta-glutamyl donor, AZ reduced NH3 and apparent gamma-glutamylmethionine formation in the purified enzyme in agreement with inhibition at the acceptor site. In the functioning kidney, perfused with 10(-3)M L- or D-glutamine, AZ (10(-3)M) markedly reduced NH3 formation and increased glutamine excretion, results consistent with AZ inhibition of the in situ gamma-GT.