The antiepileptic drug zonisamide was considered to act as a weak inhibitor of the zinc enzyme carbonic anhydrase (CA, EC 4.2. 1.1) (with a K, of 4.3 μ M against the cytosolic isozyme 11). Here we prove that this is not true. Indeed, testing zonisamide in the classical assay conditions of the CO2 hydrase activity of hCA II, with incubation times of enzyme and inhibitor solution of 15 min, a K-1 of 10.3 μ M has been obtained. However, when the incubation between enzyme and inhibitor was prolonged to 1d h, the obtained K-1 was of 35.2 nM, of the same order of magnitude as that of the clinically used sulfonamides/sulfamates acetazolantide, methazolamide, ethoxzolamide and topiramate (K(1)s in the range of 5.4-15.4 nM). The inhibition of the human mitochondrial isozyme hCA V with these compounds has been also tested by means of a dansylamide competition binding assay, which showed zonisamide and topiramate to be effective inhibitors, with K(1)s in the range of 20.6-25.4 nM. The X-ray crystal structure of the adduct of hCA 11 with zonisamide has also been solved at a resolution of 1.70 A, showing that the sulfonamide moiety participates in the classical interactions with the Zn(II) ion and the residues Thr199 and Glu106, whereas the benzisoxazole ring, is oriented toward the hydrophobic half of the active site, establishing a large number of strong van der Waals interactions (< 4.5 A) with residues Gln92, Vall2l, Phe131, Leu198, Thr200, Pro202. © 2005 Elsevier Ltd. All rights reserved.
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