The carbonic anhydrase inhibitor in trout plasma: purification and its effect on carbonic anhydrase activity and the Root effect.

The plasma inhibitor of carbonic anhydrase (CA) from rainbow trout was purified using ion exchange chromatography. The inhibitor has a high isoelectric point value (pI>10). SDS-PAGE electrophoresis and gel filtration demonstrated that the inhibitor is a low molecular weight compound of about 6,000 daltons. The plasma inhibitor was more effective against gill CA than against blood CA in vitro, probably reflecting the presence of various CA-isoenzymes in red blood cells and gill tissue. The apparent Root effect, i.e., the impairment of the oxygen binding capacity of hemoglobin in red blood associated with increased blood PCO 2was counteracted by the plasma inhibitor, probably by acting on membrane-bound and/or cytosolic blood CA. This interaction may be of importance in adaptive mechanisms, e.g., during the acidemic phase, when the fish is being acclimated to hypercapnic conditions.