Characterization of carbonic anhydrase and anion exchange in the erythrocytes of bowfin ( Amia calva), a primitive air-breathing fish

Abstract

The purpose of this study was to investigate the characteristics of carbonic anhydrase (CA) and the Cl −/HCO 3 − exchanger (Band 3; AE1) in the erythrocytes of bowfin ( Amia calva), a primitive air-breathing fish, in order to further understand the strategies of blood CO 2 transport in lower vertebrates and gain insights into the evolution of the vertebrate erythrocyte proteins, CA and Band 3. A significant amount of CA activity was measured in the erythrocytes of bowfin (70 mmol CO 2 min −1 ml −1), although it appeared to be lower than that in the erythrocytes of teleost fish. The turnover number ( K cat) of bowfin erythrocyte CA was intermediate between that of the slow type I CA isozyme in agnathans and elasmobranchs and the fast type II CA in the erythrocytes of the more recent teleost fishes, but the inhibition properties of bowfin erythrocyte CA were similar to the fast mammalian CA isozyme, CA II. In contrast to previous findings, a plasma CA inhibitor was found to be present in the blood of bowfin. Bowfin erythrocytes were also found to possess a high rate of Cl −/HCO 3 − exchange (6 nmol HCO 3 − s −1 cm −2) that was sensitive to DIDS. Visualization of erythrocyte membrane proteins by SDS-PAGE revealed a major band in the 100 kDa range for the trout, which would be consistent with the anion exchanger. In contrast, the closest major band for the membranes of bowfin erythrocytes was around the 140 kDa range. Taken together, these results suggest that the strategy for blood CO 2 transport in bowfin is probably similar to that in most other vertebrates despite several unique characteristics of erythrocyte CA and Band 3 in these primitive fish.