Kinetics and inhibition of carbonic anhydrase-catalyzed hydrolysis of 2-hydroxy-5-nitro-alpha-toluenesulfonic acid sultone. Comparison with reactions of other substrates.

The catalytic activities of human red cell carbonic anhydrase (EC 4.2.1.1) isozymes B and C for the hydrolysis of 2-hydroxy-5-nitro-alpha-toluenesulfonic acid sultone have been compared with their activities towards three other substrates. The substrate specificity (measured as kcat/Km) for either isozyme decreases in this order: CO2 greater than 2-hydroxy-5-nitro-alpha-toluenesulfonic acid sultone greater than acetaldehyde greater than p-nitrophenyl acetate. Unlike CO2 hydration, enzyme B is slightly more active towards sultone hydrolysis than C. Despite these widely differing activities of both isozymes with regard to different substrates, the inhibition constants for anion and sulfonamide inhibitors are nearly independent of the substrate used. This suggests that the binding sites of these substrates in the enzyme are the same or nearly the same. Earlier studies on 2-hydroxy-5-nitro-alpha-toluenesulfonic acid sultone from this and other laboratories had underestimated both the intrinsic activity and the susceptibility to anion inhibition of human carbonic anhydrase B. We now find that this was due to the use of acetonitrile as the substrate solvent, which is often contaminated with cyanide, a powerful inhibitor of carbonic anhydrase. The inhibition of human carbonic anhydrase B by several industrial batches of acetonitrile agrees completely with the spectrophotometrically determined cyanide content of these batches.