Current status of membrane-bound carbonic anhydrase.

We have studied the kinetic properties, and susceptibility to inhibition, of cytoplasmic and membrane carbonic anhydrase from dog kidneys, and attempted to place the data in the context of earlier work on this subject. The cytoplasmic enzyme thus far seems the same as human red cell carbonic anhydrase C, on the basis of kinetics, inhibition, amino acid composition and immunochemistry. On the other hand, the membrane enzyme is quite a different protein from either the cytoplasmic, or human red cell B or C. This enzyme is found in both luminal (brush border) and antiluminal (basolateral) fractions, and there appear no differences between the two. The turnover number (kcat) lies between those of B and C, and susceptibility to sulfonamide inhibition is two to 135-fold less than for the cytoplasmic enzyme, depending on the drug used. The usual difference is about fivefold. The KI for acetazolamide against the membrane enzyme is 10(-7) M, so that at the renal concentrations achieved at the usual in vivo doses (approximately 20 mg/kg) or used in current in situ perfusion work (both 10(-4) M) the enzyme is 99.9% inhibited. A striking difference between the membrane carbonic anhydrase and cytoplasmic or red cell B or C is its resistance to inhibition by halions. At 0.5 M chloride, there is no effect, whereas for the other three types inhibition ranges from 70%-99%. The membrane renal enzyme is also immunologically distinct from the other three types. The membrane enzyme has activity in its native state, but can be solubilized without loss of activity by treatment with Triton or sodium dodecyl sulfate. The actions of the renal carbonic anhydrases are depicted in a scheme that takes into account the protolysis of water, the attraction of H+ and of OH (and HCO3-) to the luminal and antiluminal membranes respectively, and the catalytic hydration and hydroxylation of CO2.