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In the present paper evidence is adduced to show (a) that bovine carbonic anhydrase acts as an esterase on a wide variety of phenyl and naphthyl acetates, (b) that various esters as well as aldehydes, aldehyde hydrates, and ketones act as inhibitors of the enzymecatalyzed hydrolysis of p-nitrophenyl acetate, and (c) that hydrase and esterase activity of the enzyme are related. The position of the nitro group, p > m > o, and the size of the acyl residue in nitrophenyl esters were found to greatly influence the rate of the enzymatic hydrolysis. With p-nitrophenyl acetate as substrate, the inhibitory potency was shown to follow the order: pnitrophenyl trimethylacetate, p-nitrophenyl «-hexanoate > o-and /«-nitrophenyl acetate > phenyl acetate > naphthyl acetate » methyl, ethyl, «-propyl, and «-butyl acetates; p-nitrophenyl cinnamate and benzoate showed no inhibition at saturation. The aliphatic acetates were weak inhibitors, K, ~0.5 m, whereas the aryl acetates were weak to moderately strong inhibitors, A", from 10-2 to 10-4 m. Lineweaver-Burk plots characterize these esters as competitive inhibitors. Furthermore, the binding of one of the above ester molecules to carbonic anhydrase is shown to be sufficient to prevent the binding of p-nitrophenyl acetate to the same In recent investigations it has been found that besides the reversible hydration of C02 (eq 1) carbonic anhydrase catalyzes two general reactions, given by eq 2 and 3. The first of these embodies the physiological signif-

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