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 CO2 in the forward reaction or the production of HCO3- 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 CO2 excretion. Treatment with the permeable inhibitor acetazolamide affects osmoregulation and ion transport in addition to CO2 excretion, suggesting that the cytosolic form of carbonic anhydrase may provide HCO3- for Cl-/HCO3- exchange and H+ for Na+/H+ exchange across the gill epithelium.