| Abstract: | Pig muscle carbonic anhydrase III (carbonate hydro-lyase, EC 4.2.1.1) has been isolated and purified to homogeneity with chromatographic techniques. It has been found to be a 30 kDa protein displaying the same three activities (CO2 hydratase, acetate esterase, p-nitrophenyl phosphatase) previously described for the rabbit muscle isoenzyme, including the phosphatase activity not seen in the erythrocyte isoenzymes. The turnover numbers of the three activities are of the same order of magnitude as previously reported for rabbit muscle carbonic anhydrase III. Km and Vmax for the pig muscle CO2 hydratase activity were found to be 83 mM and 6000 s-1, respectively. The extinction coefficient at 280 nm (1 cm light path) is 22.2 for a 1% solution. Five half-cystine residues determined by performic acid oxidation are free for reaction with p-mercuribenzoate but only four are accessible to titration with dithiobisnitrobenzene. The amino acid composition of the pig muscle isoenzyme III has a high level of homology compared with that of rabbit and bovine muscle carbonic anhydrases III. |
