The American Journal of Physiology

Davenport and Fisher (1938) and Davenport (1939, 1940) have shown that there is a high concentration of carbonic anhydrase in the parietal cells and a lower concentration in the cells of the surface epithelium of the gastric mucosa of rats, cats and dogs. They expressed the belief that the enzyme is part of the mechanism which secretes hydrochloric acid. Davenport argued that carbon dioxide is formed in the cells and that under the influence of carbonic anhydrase it is rapidly hydrated to carbonic acid. The carbonic acid ionizes giving hydrogen ions and bicarbonate ions. By some unknown means the hydrogen ions are concentrated and secreted while the bicarbonate ions pass into the blood to replace the chloride ions which accompany the hydrogen ions in the secretion. If carbonic an-hydrase in the gastric mucosa were inhibited the chain of reactions would be interrupted, and the mucosa would be unable to secrete acid. This argument is now supported by the observations that carbonic anhydrase is inhibited by thiocyanate ions, that the secretion of acid by the gastric mucosa is inhibited by thiocyanate ions and that the degree of inhibition of the secretion is that which is to be predicted from a knowledge of the nature of the inhibition of carbonic anhydrase by thiocyanate. METHODS. Carbonic anhydrase was determined by the method of Meldrum and Roughton (1933). The activity was calculated according to the method of Meldrum and Roughton and expressed as enzyme units E, but no correction was applied to bring the calculated activity to 15OC. Consequently the unit used in this work is some two to three times smaller than that of Meldrum and Roughton. All observations were made at atmospheric pressure and, unless otherwise stated, at OOC. The deter-minations of activity were reproducible to within 1t5 per cent, and the 505