Biochemistry, localization, and physiology of carbonic anhydrase in the gills of euryhaline crabs

Carbonic anhydrase, found at high levels in crustacean gill, is capable of catalyzing the formation of CO 2 in the forward reaction or the production of HCO 3 − in the reverse reaction. Thus the enzyme may participate in both respiratory and ion regulatory functions. Its subcellular distribution in crab gill epithelial cells suggests that a large proportion of the enzyme is membrane‐bound, where it would be available to pharmacological treatment with membrane‐impermeable carbonic anhydrase inhibitors. These studies suggest that the enzyme plays a central role in CO 2 excretion. Treatment with the permeable inhibitor acetazolamide affects osmoregulation and ion transport in addition to CO 2 excretion, suggesting that the cytosolic form of carbonic anhydrase may provide HCO 3 − for Cl − /HCO 3 − exchange and H + for Na + /H + exchange across the gill epithelium.