Carbonic anhydrase inhibitors. Zonisamide is an effective inhibitor of the cytosolic isozyme II and mitochondrial isozyme V: solution and X-ray crystallographic studies.

The antiepileptic drug zonisamide was considered to act as a weak inhibitor of the zinc enzyme carbonic anhydrase (CA, E.C. 4.2.1.1) (with a KI of 4.3 microM against the cytosolic isoenzyme II). Here the authors 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 soln. of 15 min, a KI of 10.3 microM has been obtained. However, when the incubation between enzyme and inhibitor was prolonged to 1 h, the obtained KI was of 35.2 nM, of the same order of magnitude as that of the clin. used sulfonamides/sulfamates acetazolamide, methazolamide, ethoxzolamide and topiramate (KIs in the range of 5.4-15.4 nM). The inhibition of the human mitochondrial isoenzyme hCA V with these compds. has been also tested by means of a dansylamide competition binding assay, which showed zonisamide and topiramate to be effective inhibitors, with KIs in the range of 20.6-25.4 nM. The x-ray crystal structure of the adduct of hCA II with zonisamide has also been solved at a resoln. of 1.70 .ANG., 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 no. of strong van der Waals interactions (<4.5 .ANG.) with residues Gln92, Val121, Phe131, Leu198, Thr200, Pro202.