1. Three forms of the zinc-containing enzyme carbonic anhydrase (EC 4.2.1.1) were isolated from the erythrocytes of the rat and two forms from the dorsolateral prostate of the rat. Several additional minor components were observed but not isolated. Separation of the isoenzymes was achieved by ion-exchange chromatography, polyacrylamide-gel electrophoresis and isoelectric focusing. 2. The general properties of the isolated isoenzymes, their molecular weights and their contents of zinc were closely similar. As catalysts of the hydration of carbon dioxide, however, they were distinctly different. The two most abundant isoenzymes of the erythrocytes, which were found in equal proportions, differed 70-fold in specific activity, whereas the isoenzymes of the dorsolateral prostate were similar to one another and resembled the high-activity component of the erythrocytes. The inhibition of the latter by acetazolamide (5-acetamido-1-thia-3,4-diazole-2-sulphonamide) was mainly competitive, whereas in identical conditions the low-activity erythrocyte component and the dorsolateral prostate isoenzymes were non-competitively inhibited. 3. The use of chloroform–ethanol to remove haemoglobin from the rat haemolysate was found (a) to bring about changes in the kinetic properties of the soluble isoenzymes and (b) to cause the appearance of an additional isoenzyme. 4. The actions were compared of the inhibitors acetazolamide, 1,1-dimethylaminonaphthalene-5-sulphonamide and ethoxzolamide (6-ethoxybenzothiazole-2-sulphonamide) on the hydrolysis of p-nitrophenyl acetate catalysed by the isoenzymes. 5. The low-activity erythrocyte isoenzyme was an efficient catalyst of the hydrolysis of β-naphthyl acetate whereas the high-activity forms were much less active towards this ester. Neither of the isoenzymes present in the dorsolateral prostate catalysed this reaction. 6. Carbonic anhydrase in the rat dorsolateral prostate accounts for no more than 5% of the unusually high content of zinc in this organ.

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