CARBONIC-ANHYDRASE, A RESPIRATORY ENZYME IN THE GILLS OF THE SHORE CRAB CARCINUS-MAENAS

This paper summarizes investigations on the enzyme carbonic anhydrase (CA) in the gills of the osmoregulating shore crab Carcinus maenas. Ca rbonic anhydrase, an enzyme catalyzing the reversible hydration of CO2 to HCO3- and H+, is localized with highest activities in the posterio r salt-transporting gills of the shore crab - and here CA activity is strongly dependent on salinity. Contrary to the earlier hypothesis est ablished for the blue crab Callinectes sapidus that cytoplasmic branch ial CA provides the counter ions HCO3- and H+ for apical exchange agai nst Na+ and Cl-, the involvement of CA in NaCl uptake mechanisms can b e excluded in Carcinus. Differential and density gradient centrifugati ons indicate that branchial CA is a predominantly membrane-associated protein. Branchial CA was greatly inhibited by the sulfonamide acetazo lamide (AZ) K-i = 2.4 . 10(-8) mol/l). Using the preparation of the is olated perfused gill, application of 10(-4) mol/l AZ resulted in an 80 % decrease of CO2/HCO3- excretion. Thus we conclude that CA is locali zed in plasma membranes, maintaining the CO2 gradient by accelerating adjustment of the pH-dependent CO2/HCP3- equilibrium.