Abstract
The inhibitory effect of hydroflumethiazide (HFT) and its metabolite, 2,4-disulfamyl-5-trifluoromethylaniline (DTA) on cyclic AMP phosphodiesterase and the binding of HFT and DTA to carbonic anhydrase was studied in vitro. Significant inhibition of rat kidney low-Km cyclic AMP phosphodiesterase was observed with DTA concentration above 2.5 X 10(-4) mol/l and with HFT concentration above 1 X 10(-4) mol/l. 50% inhibition was observed at a DTA concentration of 1 X 10(-3) mol/l. Binding of DTA and HFT to commercially obtained bovine erythrocyte carbonic anhydrase was demonstrated by equilibrium dialysis. Data were consistent with one class of binding sites. The product of n (number of binding sites) and Kass (association constant) was 5 X 10(5) M for DTA and 3.3 X 10(4) M for HFT at 2 degrees. In human blood in vitro at 37 degrees, the equilibrium erythrocyte/plasma concentration ratio was 18 for DTA and 1.6 for HFT. It is concluded that HFT and DTA have approximately the same potency as cyclic AMP phosphodiesterase inhibitors, whereas DTA is more extensively bound by erythrocyte carbonic anhydrase.
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