The Nature of Carbonic Anhydrase From Plant Sources

Abstract

Carbonic anhydrase activity in leaves of plants is low compared with that in animal tissues. In the absence of cysteine the activity of crude leaf extracts was readily lost. A method of purification of the enzyme from crude leaf extracts is described. The most active prepn. had a catalytic activity about 50-60 times that of the crude leaf extract. During purification the enzyme was inactivated by dialysis in the absence of cysteine, by Pb acetate, acetone, and by high concns. of ammonium sulfate. The partially purified product contained 0.056% Zn, not removable by dialysis. The enzyme is heat-labile. The relation of activity to pH is described; opt. activity occurs within the range pH 6-8. Activity of the enzyme prepn. is inhibited only by relatively high concns. of cyanide, sulfocyanide, azide, and Na sulfide. Sulfanilamide in relatively high concns. does not innibit activity. Activity of the prepn. is strongly inhibited by p-chlormercuribenzoate and by Na arsenite; their inhibitory effects are reversible, reactivation occurring by the addition of simple thiol compounds such as cysteine and reduced glutathione. Polarographic investigation suggests the presence of sulfydryl groups in the enzyme prepn. The enzyme prepn. from plant sources differs from that from animal sources in its inactivation during dialysis in the absence of cysteine, its inactivation by Pb acetate, acetone, and ammonium sulfate, its relative in-sensitivity to inhibitors of metal proteins and to sulfanilamide, and in its strong and reversible inactivation by sulfydryl inhibitors. It is suggested that carbonic anhydrase from plant sources is a different enzyme from that occurring in animal tissues.