Abstract
A spin-labeled sulfonamide [1-oxy1-4-(p-sulfamylbenzoyl)-amino- 2,2,6,6-tetramethyl-piperidine] has been synthesized and its interaction with bovine erythrocyte carbonic anhydrase B studied by means of electron spin resonance. When free in solution, this sulfonamide exhibited a sharp three line spectrum due to hyperfine interaction between the unpaired electron and the nitrogen nucleus of the nitroxide group. However, in the presence of bovine carbonic anhydrase B, the spectrum of the spin-labeled sulfonamide resembled the powder spectrum of the nitroxide free radical. This indicated that when the sulfonamide binds to carbonic anyhydrase the nitroxide group is immobilized by interaction with the enzyme binding site. Ethoxyzolamide (a drug which has a very high affinity for the active site of carbonic anhydrase) specifically displaced the sulfonamide spin label from its binding site on carbonic anhydrase. These results suggest that the active site of bovine carbonic anhydrase B is a narrow crevice similar to that already described for human carbonic anhydrase C.
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