Inhibition by anions of human red cell carbonic anhydrase B: physiological and biochemical implications

The hydration rate of CO2 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 CO2 is generally more (up to 24 times) sensitive to inhibition by anions and sulfonamides than the dehydration of HCO3-, probably reflecting different mechanisms. The sensitivity of enzyme B to anion inhibition also depends upon the substrate, being much greater for CO2 than for certain esters. On the basis of the very low catalytic activity of B for CO2 in the presence of physiological concentration of chloride, and the fact that carbonic anhydrase C is effective for CO2 hydration (in the presence of chloride) at a rate 340 times greater than that of CO2 output from tissues, it appears that the biological role of enzyme B is not that of a carbonic anhydrase.