Red blood cells from several members of the cat family have been found to contain only carbonic anhydrase (EC 4.2.1.1) isozymes of the "high activity" (carbonic anhydrase II class) in red blood cells. 2. Two carbonic anhydrase II type isozymes have been isolated from red cells of the domestic cat. Kinetic and structural characterisation shows that the two isozymes have identical composition except for a probable difference of one amide group. The general characteristics of cat red cell carbonic anhydrase are similar to other mammalian carbonic anhydrase II isozymes, in particular the dog carbonic anhydrase II. 3. A carbonic anhydrase I (low activity) isozyme has been isolated and characterised from cat caecal mucosa. This carbonic anhydrase I is similar to the carbonic anhydrase I extracted from canine red cells. Comparative studies showed that whereas members of the dog family have carbonic anhydrase I and carbonic anhydrase II in red cells the cat family has only a carbonic anhydrase II type isozyme, however, cat liver and spleen homogenates were also found to contain carbonic anhydrase I identical to that found in caecum.
