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

To clarify the controversial question of cell-specific distribution of carb onic anhydrase (CA) in the heart, endothelial cells and cardiomyocytes were isolated from porcine and human hearts and were characterized with cell-sp ecific markers. CA activity was found in the microsomal fraction of both ce ll types. It was shown by Triton X-114 phase separation that both cell type s possess a membrane-bound form of CA. These CAs share the same mechanism o f membrane-anchoring via glycosylphosphatidylinositol (GPI), which excludes identity with transmembrane isoforms CA IX or CA XII. Western blotting ana lysis of human microsomes with anti-human CA IV antibodies revealed a marke d difference in immunoreactivity. Endothelial CA activity resulted in Ii-fo ld stronger CA ni bands compared with identical amounts of myocytic CA acti vity, indicating that cardiac endothelium and cardiomyocytes possess immuno logically distinct forms of CA. We conclude that in human hearts CA IV is a ssociated with the endothelium, whereas most of the CA in myocytes is not i dentical with one of the known CA isozymes. This suggests that cardiomyocyt ic CA is a novel isozyme.