Carbonic anhydrase, a respiratory enzyme in the gills of the shore crabCarcinus maenas

This paper summarizes investigations on the enzyme carbonic anhydrase (CA) in the gills of the osmoregulating shore crabCarcinus maenas. Carbonic anhydrase, an enzyme catalyzing the reversible hydration of CO2 to HCO3− and H+, is localized with highest activities in the posterior salt-transporting gills of the shore crab- and here CA activity is strongly dependent on salinity. Contrary to the earlier hypothesis established for the blue crabCallinectes sapidus that cytoplasmic branchial CA provides the counter ions HCO3− and H+ for apical exchange against Na+ and Cl−, the involvement of CA in NaCl uptake mechanisms can be excluded inCarcinus. Differential and density gradient centrifugations indicate that branchial CA is a predominantly membrane-associated protein. Branchial CA was greatly inhibited by the sulfonamide acetazolamide (AZ) Ki=2.4·10−8 mol/l). Using the preparation of the isolated perfused gill, application of 10−4 mol/l AZ resulted in an 80% decrease of CO2/HCO3− excretion. Thus we conclude that CA is localized in plasma membranes, maintaining the CO2 gradient by accelerating adjustment of the pH-dependent CO2/HCO3− equilibrium.