Carbonic anhydrase III in liver and muscle of male rats purification and properties.

Cytosolic carbonic anhydrases CAI, CAII, and CAIII from liver, and CAII, and CAIII from muscle of adult male Sprague-Dawley rats were purified to homogeneity. CAIII from liver and muscle had the same amino acid composition and were immunochemically similar. Their kinetic properties at 0 degrees C were also similar. Km(CO2) was 4 mM and kcat 3x105 s(-1). Ki was 0.4 and 0.2 M for acetazolamide and NaCl, respectively. Both CAIIIs ran as single bands on SDS-electrophoresis and high-speed centrifugation, with a mol wt of 29.3 kDa. Their hydrodynamic properties suggest that CAIII is a compact, nearly spherical molecule. It contained 0.9 M zinc per M protein. In both tissues isoelectric focusing identified neutral and acidic isoforms with pIs near 7.0 and 6.3, respectively. These forms were immunologically identical and had the same amino acid composition and mol wts. The acidic forms probably represent subspecies of CAIII in different states of oxidation. CAIII is the major soluble protein in rat liver and muscle. Its function is probably to protect proteins of these tissues from oxidation catalyzed by iron-containing degradation products of haemoglobin and myoglobin. Liver CAI and CAII and muscle CAII were identical to CAI and CAII of rat erythrocytes.