Purification, Molecular Properties and Ontogeny of Carbonic Anhydrase Isozymes. Evidence for A, B and C Isozymes in Avian and Mammalian Tissues

Carbonic anhydrase A from sheep and chicken red skeletal muscles and carbonic anhydrases B and C from chicken intestine and red cells respectively have been isolated in a high state of purity by affinity chromatography. The subunit and native molecular weights of sheep carbonic anhydrase A determined by sodium dodecyl sulphate/polyacrylamide gel electrophoresis and Sephadex gel filtration respectively were both approximately 34000. In contrast, carbonic anhydrases A, B and C from chicken exhibited a molecular weight determined by sodium dodecyl sulphate electrophoresis of 30000. The amino acid composition of sheep muscle carbonic anhydrase A was distinct from the B and C isozymes, particularly in basic amino acid content, which was significantly higher for the A isozyme (38 cf. 24–29/mol). The zinc content of sheep carbonic anhydrase A was found to be 0.81 g atom Zn/mol. The specific activities of chicken carbonic anhydrase isozyme differed significantly according to the ratio A:B:C = 1:6:47. In addition, these isozymes differed in their affinities for sulphonamide. Chicken carbonic anhydrase A was approximately 100 times less sensitive to acetazolamide inhibition than the chicken C isozyme. Ontogenetic studies in cats showed carbonic anhydrase C to be the only form of the enzyme present in mid-term fetal animals, with carbonic anhydrase A appearing in muscle in the late stages, and carbonic anhydrase B appearing in caecum within 2 days of birth. The evidence is consistent with previous proposals for a third locus encoding carbonic anhydrase in mammalian and avian species. This isozyme (A) is a monomeric, zinc metalloenzyme, which differs from the extensively studied B and C isozymes of carbonic anhydrase in terms of catalytic efficiency, amino acid composition, molecular weight (in sheep), time of ontogenic appearance, sulphonamide inhibition and tissue distribution.