[Isoenzymes of carbonic anhydrase of an euryhaline fish. Variations related to the osmoregulation].

A chemical study of carbonic anhydrase (EC 4.2.1.1) from the red blood cells and the gills of an euryhaline fish (Anguilla anguilla) is presented. Animals adapted to fresh water were compared to those adapted to sea water. The physiochemical constants of the various molecular formsisolated by chromatography and isoelectric focusing were determined; isoelectric pH, molecular weight, and the Km and V/E of the enzyme dehydration activity were compared. In both red cells and gills of fish adapted in either media various forms were isolated, characterized by different enzymatic kinetics (high- and low- activity forms) but having the same molecular weight (27 250). Some isoenzymes isolated from the gills differed significantly from those isolated from the red cells. Adaptation to fresh water or sea water is accompanied by modifications in the distribution of the isoenzymes in both red cells and gills: adaptation to sea water is characterized by a shift of the molecular forms towards an isoelectric pH higher than pH equals 6. The role of these enzymes is discussed under both a physiological and biochemical point of view in relation to the electrolyte exchange across fish gill. The origin of the different molecular forms of the carbonic anhydrase is discussed in relation to the prevailing theories on this subject.