Inhibition by Anions of Human Red Cell Carbonic Anhydrase B: Physiological and Biochemical Implications

Abstract

The hydration rate of CO 2 catalyzed by human red cell carbonic anhydrase B is 92 percent reduced by the normal concentrations of chloride and bicarbonate in red cells. This reflects a general sensitivity of this reaction to halides and other anions, up to 87 times greater than the effect on red cell carbonic anhydrase C. The catalytic hydration of CO 2 is generally more (up to 24 times) sensitive to inhibition by anions and sulfonamides than the dehydration of HCO 3 -, probably reflecting different mechanisms. The sensitivity of enzyme B to anion inhibition also depends upon the substrate, being much greater for CO 2 than for certain esters. On the basis of the very low catalytic activity of B for CO 2 in the presence of physiological concentration of chloride, and the fact that carbonic anhydrase C is effective for CO 2 hydration (in the presence of chloride) at a rate 340 times greater than that of CO 2 output from tissues, it appears that the biological role of enzyme B is not that of a carbonic anhydrase.