1. 1. Carbonic anhydrase (carbonate hydro-lyase, EC 4.2.1.1) has been purified from erythrocytes of hagfish (Myxine glutinosa). A single form with low specific CO₂ hydration activity was isolated.

The purified carbonic anhydrase appeared homogeneous judging from polyacrylamide gel electrophoresis and gel filtration experiments. The protein has a molecular weight of about 29 000, corresponding to about 260 amino acid residues. This molecular weight is in accordance with other vertebrate carbonic anhydrases with the exception of the elasmobranch enzymes, which have M_r 36 000–39 000.

2. 2. The molecular weight obtained for hagfish carbonic anhydrase indicates that a carbonic anhydrase with M_r approx. 29 000 is the ancestral type of the vertebrate enzyme rather than, as in sharks, a heavier carbonic anhydrase molecule.

3. 3. The circular dichroism spectrum may indicate a somewhat different structural arrangement of aromatic amino acid residues in this enzyme than in the mammalian carbonic anhydrases.

4. 4. The enzyme is strongly inhibited by acetazolamide and also to a lesser extent by monovalent anions.

5. 5. Zn²⁺, which is essential for activity, appears, contrary to other characterized carbonic anhydrases, less strongly bound in the active site of the enzyme.