Ultrastructural localization of exogenous carbonic anhydrase activity in ileal absorptive cells of suckling rats.

To test the resolution and reliability of Hansson's histochemical reaction for carbonic anhydrase (CAH) activity at the electron microscopic level, purified exogenous bovine, rabbit, and human B and C CAH was reacted for histochemical activity after uptake by suckling rat ileal absorptive cells and examined microscopically. The cobalt sulfide reaction product was found confined to the apical vacuoles, tubules, and vesicles as small as 30 nm in diameter and was confined within the limiting membrane of the endocytic system. The histochemical technique did not distinguish between the different types or sources of the CAH nor were differences in their enzymatic activity apparent. It was concluded that when the cobalt precipitation technique of Hansson is used to demonstrate exogenous CAH activity it results in the precipitation of the reaction product at or very near the site of the CAH molecule. In addition to the resolution of the technique to demonstrate enzyme activity, this study suggests that the ultrastructural localization of intrinsic CAH activity can be accepted with greater confidence.