THE DISTRIBUTION OF CARBONIC-ANHYDRASE TYPE-I AND TYPE-II ISOZYMES INLAMPREY AND TROUT - POSSIBLE COEVOLUTION WITH ERYTHROCYTE CHLORIDE BICARBONATE EXCHANGE

The subcellular distribution and kinetic properties of carbonic anhydr ase were examined in red blood cells and gills of the lamprey, Petromy zon marinus, a primitive agnathan, and rainbow trout, Oncorhynchus myk iss, a modern teleost, in relation to the evolution of rapid Cl-/HCO3- exchange in the membrane of red blood cells. In the lamprey, which ei ther lacks or has minimal red cell Cl-/HCO3- exchange, there has been no compensatory incorporation of carbonic anhydrase into the membrane fraction of either the red cell or the gill. Carbonic anhydrase activi ty in red cells is exclusively cytoplasmic, and the single isozyme dis plays kinetic properties typical of the type I, slow turnover, isozyme . In the red blood cells of the trout, however, which possess high amo unts of the band-3 Cl-/HCO3- exchange protein, the single carbonic anh ydrase isozyme appears to be kinetically similar to the type II, fast turnover, isozyme. It thus appears that the type I isozyme present in the red blood cells of primitive aquatic vertebrates was replaced in m odern teleosts by the kinetically more efficient type II isozyme only after the incorporation and expression of a significant amount of the band-3 exchange protein in the membrane of the red cell.