Branchial carbonic anhydrase was purified from silver seabream (Sparus sarba) and antibody against the enzyme was obtained by immunization in rabbits. An assay for quantifying the activity of carbonic anhydrase was developed. Using enzymatic and immunological techniques, the activity, expression and distribution of branchial carbonic anhydrase of silver seabream acclimated to different salinities were studied. Fish gill is one of the most important organs involved in various homeostatic processes. The ability of euryhaline fish to maintain constant internal ionic balance is crucial for the survival of the fish upon change in salinity. The presence of carbonic anhydrase in the chloride cells was suggested to be an important enzyme involved in ion regulation of fish. / In the present study, branchial carbonic anhydrase and erythrocyte carbonic anhydrase were purified from the gill cells of silver seabream with p-aminomethylbenzenesulfonamide-agarose affinity column. They were predominantly cytosolic with a molecular size of 26.6 k Da for branchial carbonic anhydrase and 28.6 k Da for erythrocyte carbonic anhydrase. Investigation of kinetic properties towards the inhibitor acetazolamide has helped determine the inhibition constants (Ki of branchial carbonic anhydrase: 0.54 x 10-9; Ki of erythrocyte carbonic anhydrase: 0.22 x 10-9). The difference in molecular size and inhibition constant towards acetazolamide supported the view that branchial carbonic anhydrase and erythrocyte carbonic anhydrase were two different isozymes. Polyclonal antibody specific to seabream branchial carbonic anhydrase was obtained by immunization in rabbit. The distribution of branchial carbonic anhydrase in the gill of seabream acclimated to different salinities was studied with immunohistochemical method. The enzyme was mainly located at the interlamellar region. The effect of salinity (0, 6, 12, 33, 50 and 70 ‰) acclimation on the expression and activities of branchial carbonic anhydrase has shown a U-shape pattern from freshwater to double-strength seawater on the quantity of seabream branchial carbonic anhydrase. Higher amount of branchial carbonic anhydrase in freshwater was consistent with the current view that the enzyme was actively involved in the ion uptake process through the hydration of carbon dioxide to produce bicarbonate ion and proton for the exchange of chloride and sodium ions, respectively. An interesting finding was obtained with elevated amount of branchial carbonic anhydrase in seabream acclimated to double-strength seawater and the possible role of the enzyme in such extreme environment was discussed. / This study has provided useful information on the properties, localizations and activities of branchial carbonic anhydrase in silver seabream for the understanding of the involvement of the enzyme in salinity adaptation of silver seabream. / Ma, Wing Chi Joyce. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3250. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references (leaves 127-151). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.