Subcellular distribution of carbonic anhydrase activity in the gills of the blue crab, Callinectes sapidus

Abstract

AbstractDifferential centrifugation and density gradient ultracentrifugation were used to separate a basal–lateral membrane fraction from the cytoplasm in the gills of the blue crab, Callinectes sapidus. For both anterior and posterior gills of animals acclimated to 35 or 12 parts per thousand salinity the basal–lateral membrane fraction of the gill possessed significant levels of carbonic anhydrase activity. Membrane‐associated carbonic anhydrase (CA) activity was uniformly distributed between anterior and posterior gills, and it was only slightly sensitive to changes in salinity. This activity was not significantly altered by freeze‐thawing or sonication, but it was virtually eliminated after treatment with detergent. Stability of CA activity stored in vitro was not influenced by the presence of reducing agents. Protein‐specific CA activity in the membrane fraction was equal to or greater than that in the cytoplasmic fraction in all cases. Cytoplasmic CA activity in the posterior gills was highly sensitive to environmental salinity, increasing over 10‐fold. These data show that, in addition to a large cytoplasmic pool of CA activity, there is also a significant amount of CA activity associated with the basal–lateral membrane. The membrane associated CA is potentially available to catalyse the dehydration of hemolymph HCO3− to CO2 and thus facilitate CO2 excretion, while the role of cytoplasmic CA in ion regulation is confirmed.