Purification and properties of a polymorphic high activity equine erythrocyte carbonic anhydrase

A polymorphic form of the high activity or C-type of horse erythrocyte carbonic anhydrase has been isolated. It has been designated C2 and differs from the usual C1 form by having a cysteine replacement for arginine at residue 180. This second cysteine, unlike the other, is highly reactive. Isolation of the C2 isozyme by the usual methods results in most of it forming a mixed disulfide with glutathione and this product designated as C3 has an increased anodic mobility. The enzymatic activity and immunologic reactivity of both the C2 and C3 components are the same as for the usual C1 form of the enzyme. The C2 form can be stabilized by alkylation and the carboxamidomethyl derivative has been isolated in crystalline form.