Distribution of CAIII in fetal and adult human tissue.

Carbonic anhydrase III (CAIII), an enzyme recently shown by conventional electrophoresis to be muscle specific, has been qualified by "rocket" immunoelectrophoresis. This more sensitive technique has shown that the enzyme is virtually specific to skeletal muscle, where it occurs at a level of 5 mg per g, with trace levels in smooth muscle, cardiac muscle, and lung. In man there does not appear to be any correlation between CAIII levels and the proportion of red and white muscle fibers. The fetal development of CAIII has also been examined using immunoelectrophoresis, and the enzyme can be detected at 11 weeks' gestation. The CAIII level rises gradually up to 25 weeks, and there is then a more dramatic increase to reach approximately half adult level at birth.