Amount and distribution of carbonic anhydrases CA I and CA II in the gastrointestinal tract.

The levels of carbonic anhydrase (CA) activity and the amounts of carbonic anhydrase isoenzyme CA I and CA II proteins in the human gastrointestinal tract were determined by kinetic assays and radioimmunoassays. Cellular distribution of carbonic anhydrase activity in the various segments of the gastrointestinal tract were studied by the histochemical method of Hansson and the distribution of CA I and CA II by an immunohistochemical method. The stomach and the colon showed high carbonic anhydrase activity, the jejunum had intermediate activity, and the ileum had low activity. In the stomach CA II was the dominating isoenzyme, whereas the jejunum and the colon contained considerable amounts of both forms. Small amounts of both isoenzymes were found in the ileum. Carbonic anhydrase II immunofluorescence was demonstrated in the surface epithelium and in the parietal cells of the gastric mucosa, and in the epithelium of the jejunal villi. The surface epithelium of the colon contained both CA I and CA II. Carbonic anhydrase I was found in many superficial capillaries in all regions studied. The histochemical method demonstrated enzyme activity also at the cell membranes of gastric chief cells, intestinal crypt cells, and a subpopulation of ileal surface cells. This probably indicates presence of the membrane-bound isoenzyme CA IV.