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Carbonic anhydrase,a respiratory enzyme in the gills of the shore crabCarcinus maenas
Authors:K Böttcher  D Siebers  S Sender
Institution:(1) Biologische Anstalt Helgoland, Notkestr. 31, D-22607 Hamburg, Germany;(2) Abt. Vegetative Physiologie; Konstanty, Medizinische Hochschule Hannover, Gutschow Str. 8, D-30625 Hannover, Germany
Abstract: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.
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