Immunochemical evidence for a unique GPI-anchored carbonic anhydrase isozyme in human cardiomyocytes

To clarify the controversial question of cell-specific distribution of carbonic anhydrase (CA) in the heart, endothelial cells and cardiomyocytes were isolated from porcine and human hearts and were characterized with cell-specific markers. CA activity was found in the microsomal fraction of both cell types. It was shown by Triton X-114 phase separation that both cell types possess a membrane-bound form of CA. These CAs share the same mechanism of membrane-anchoring via glycosylphosphatidylinositol (GPI), which excludes identity with transmembrane isoforms CA IX or CA XII. Western blotting analysis of human microsomes with anti-human CA IV antibodies revealed a marked difference in immunoreactivity. Endothelial CA activity resulted in 11-fold stronger CA IV bands compared with identical amounts of myocytic CA activity, indicating that cardiac endothelium and cardiomyocytes possess immunologically distinct forms of CA. We conclude that in human hearts CA IV is associated with the endothelium, whereas most of the CA in myocytes is not identical with one of the known CA isozymes. This suggests that cardiomyocytic CA is a novel isozyme.