Molecular Pharmacology, Vol 10, 344-348, Copyright © 1974 by the American Society for Pharmacology and Experimental Therapeutics

Kinetics of Complex Formation between Human Carbonic Anhydrase B and Heterocyclic Sulfonamides

¹ Division of Molecular Pharmacology, National Institute for Medical Research, London, NW71AA, England and Department of Pharmacology and Therapeutics, University of Florida College of Medicine, Gainesville, Florida 32601

The kinetics of both the association and dissociation reaction of two heterocyclic sulfonamide inhibitors (S), methazolamide and ethoxzolamide, with human carbonic anhydrase (carbonate hdyro-lyase, EC 4.2.1.1) isoenzyme B (CA) has been studied as a function of pH, using fluorescence stopped-flow instrumentation. The dissociation rate constants are similar for the two inhibitors and are unaffected by pH changes in the range 6.0-10.0. The association rate constants exhibit bell-shaped curves as a function of pH, consistent with the rate being controlled by two ionizing groups. The evidence suggests that one is an ionizing group on the enzyme with a pK near 7.5, which controls the low-pH limb of the curve, while the other function, controlling the high-pH limb, is the ionization of the sulfonamide group of the inhibitor. The data are consistent with the enzyme-drug reaction being either CA + SH or CAH⁺ + S^-.

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ACKNOWLEDGMENT We wish to thank Mr. P. C. Turner for expert technical assistance.