The carbonic anhydrase of the Chinese crabEriocheir sinensis: Effects of adaption from tap to salt water

In the present investigation we studied the carbonic anhydrase (CA) in various tissues of Chinese crabEriocheir sinensis which were acclimated to different salinities (0, 10, 20, 30‰). We found only negligible CA activity in haemolymph, heart, hypodermis, antennal gland, leg muscle and digestive gland, irrespective of the acclimation medium. However, high amounts of CA activity were found in the gills. In the case of the posterior gills, a strong dependence on the acclimatization of the animals was demonstrated; the highest activities were found in those adapted to tap water. To investigate the cellular distribution of the CA in the posterior gills, the additional enzyme activities were measured in all fractions of a differential centrifugation of the gill homogenate: Na⁺/K⁺-ATP'ase (a marker for the plasmamembrane); lactate dehydrogenase (LDH; as marker for the cytosol); and succinate dehydrogenase (SDH; as marker for mitochondria). Independent of the acclimation salinity (0 or 36‰ salinity), we found about 70% of CA associated with the highest level of the Na⁺/K⁺-ATP'ase in the second 100 000 g pellet (membrane fraction), while only 15% were found in the cytosolic fractions (associated with highest levels of LDH). We conclude that the carbonic anhydrase of posterior gills of the Chinese crab is mainly membrane-bound. Furthermore, the activity of CA shows a strong dependence on the salinity of the water in which the crabs were kept.