Carbonic anhydrase from spinach leaves. Isolation and some chemical properties

Spinach carbonic anhydrase has been purified by modification and extension of a published method (Pocker, Y., and Ng. J. S. U. (1973) Biochemistry 12, 5127-5134), using (NH4)2SO4 precipitation and chromatography on DEAE-cellulose, agarose, and DEAE-Sephadex. The enzyme so obtained was homogeneous by criteria of both standard and sodium dodecyl sulfate polyacrylamide gel electrophoresis and of constant specific activity throughout the elution profile on DEAE-Sephadex chromatography. The enzyme has an apparent Mr of 212,000 by gel filtration on Sephadex G-200, a Mr of 26,000 by sodium dodecyl sulfate electrophoresis, and each of the subunits contains approximately 1 g atom of zinc. These data and the excellent correlation between the number of lysine and arginine residues per subunit, and the number of tryptic peptides obtained by peptide mapping, suggest that spinach carbonic anhydrase is an octamer consisting of identical or very similar subunits. Its amino acid composition is similar to parsley carbonic anhydrase; both contain large numbers of half-cystine residues relative to erythrocyte carbonic anhydrases. The spinach enzyme is devoid of disulfide bonds. The enzyme is stable around neutrality at -14 degrees, as a suspension in saturated (NH4)2SO4 solution.