Testosterone‐induced, sulfonamide‐resistant carbonic anhydrase isozyme of rat liver is indistinguishable from skeletal muscle carbonic anhydrase III

Three isozymes of carbonic anhydrase, CA I, CA II and CA III, have been characterized from the tissues of a variety of vertebrate species. CA I (low-activity, sulfonamide-sensitive form) has been found mainly in red cells but also occurs in other tissues (e.g., rumen epithelium, caecum, colonic mucosa, pituitary gland, ciliary body), CA II (high-activity, sulfonamide-sensitive form) is found in red blood cells and in a majority of other tissues, and CA III (low-activity, sulfonamideresistant form) has been reported to occur largely in red skeletal muscle; cf. [l-7]. Although all 3 isozymes catalyze the reversible hydration of CO*, hydrolyze certain ester linkages, and are selectively inhibited by heterocyclic sulfonamides such as acetazolamide (Diamox), their relative activities and degrees of sulfonamide inhibition can vary considerably. For example, the CA II isozymes have the highest CO2 hydrase and esterase (toward p-nitrophenyl acetate) activities and the highest affinities for sulfonamides, followed by the CA I isozymes, while the CA III isozymes exhibit markedly lower CO2 hydrase and esterase activities and are only weakly inhibited by sulfonamides [l-4]. Early studies on carbonic anhydrase activity in rat liver homogenates [8,9] showed that its CO* hydrase activity was poorly inhibited by acetazolamide compared to the strong inhibition of rat red cell carbonic anhydrase (presumably CA I and CA II) activities. A partial characterization of a carbonic anhydrase isolated from livers of adult male rats [IO] showed that the specific CO* hydrase activity of this form was -1% that of the high-activity CA II isozyme purified