Carbonic anhydrase inhibitors: Cloning, characterization, and inhibition studies of the cytosolic isozyme III with sulfonamides Original Research Article

The cytosolic human carbonic anhydrase (hCA, EC 4.2.1.1) isozyme III (hCA III) has been cloned and purified by the GST-fusion protein method. Recombinant pure hCA III had the following kinetic parameters for the CO2 hydration reaction at 20 °C and pH 7.5: kcat of 1.3 × 104 s−1 and kcat/KM of 2.5 × 105 M−1 s−1, being a slower catalyst for the physiological reaction as compared to the genetically related cytosolic isoforms hCA I and II. An inhibition study with a library of sulfonamides and one sulfamate, some which are clinically used compounds, is reported. hCA III is less prone to be inhibited by these compounds as compared to hCA I and II for which many low nanomolar inhibitors were detected earlier. The best hCA III inhibitors were prontosil, sulpiride, indisulam, benzolamide, aminobenzolamide, and 4-amino-6-chloro-benzene-1,3-disulfonamide which showed KIs in the range of 2.3–18.1 μM. Clinically used compounds such as acetazolamide, methazolamide, ethoxzolamide, dorzolamide, brinzolamide, topiramate, zonisamide, celecoxib, and valdecoxib were less effective hCA III inhibitors, with affinities in the range of 154–2200 μM. This is the first study in which low micromolar hCA III inhibitors are reported.