Gastric acid carbonic anhydrase

Carbonate anhydrase (carbonic anhydrase, EC 4.2.1.1) catalyzes the reversible interconversion of C02 and HCO3 (see Sect. 3.7.3). The enzyme is found in erythrocytes, and in kidney and gastric juices where it contributes to the control of the acid-base balance. The esterase activity of carbonic anhydrase is probably due to the similarity between its active site and that of the zinc proteases. A possible physiological role of the esterase activity of this enzyme remains to be established. 

Gastric acid secretion can be pharmacologically inhibited by specific antagonists of the stimulatory receptors (histamine-Hj, muscarine-Mi/Mj, gastrin), by agonists to inhibitory receptors (prostaglandin, somatostatin), by carbonic anhydrase inhibitors, and by HVK -ATPase inhibitors. 

The enzyme carbonic anhydrase facilitates the reaction between carbon dioxide and water to form carbonic acid, which then breaks down to hydrogen (H" ) and bicarbonate (HCOj") ions. This process is fundamental to the production of either acid or alkaline secretions and high concentrations of carbonic anhydrase are present in the gastric mucosa, pancreas, eye and kidney. Because the number of available to exchange with Na" in the proximal tubule is reduced, sodium loss and diuresis occur. But HCOg reabsorption from the tubule is also reduced, and its loss in the urine leads within days to metabolic acidosis, which attenuates the diuretic... 

The mechanism presented for gastric acid production did not reveal the source of protons and chloride ions. Continued production of gastric acid by this mechanism would result in depletion of the chloride ions in the cell and alkalinization of the cell because of the loss of protons. Figure 2.27 shows that the source of chloride ions is the bloodstream and the source of protons is carbonic add, H COj. As detailed in a later section, carbonic acid is produced by the action of carbonic anhydrase, the enzyme that catalyzes the reversible condensation of a molecule of water with a molecule of carbon dioxide. Carbonic acid ionizes to produce a... 

Carbonic anhydrase is an enzyme of central importance to the production of gastric acid. This enzyme, which contains zinc, accelerates the naturally occurring reversible reaction of CO2 with water. Before considering the mechanism of action of the enzyme, note the resonating structure of CO2. The carbon-to-oxygen bonds are polar the molecular structure can be represented by the resonance hybrids shown in Figure 2.54. Note that in two of the three forms shown the central carbon atom has a positive charge. 

Carbonic anhydrase and sodium bicarbonate together neutralize the acids produced by bacterial metabolism of dietary carbohydrate (Fig. 12.2). When salivary carbonic anhydrase is swallowed, it adheres to the mucosal surface of the stomach where it remains active and forms carbonic acid from sodium bicarbonate in the gastric mucosa. A lack of salivary carbonic anhydrase causes acid to remain longer in the stomach, contributing to peptic disease in addition to dental caries (Sect. 15.3.3). 

Carbonic anhydrase B Human erythrocyte 26,600 1 Catalyzes reversible hydration of CO2 needed for CO2 transport, buffering of ECF, gastric and renal acid secretion Zn is catalytic. 

Carbonic anhydrase is assumed to be located at the surface of the membrane of the tubular cells [47, 48]. Carbonic anhydrase is a small zinc protein found in many animal tissues, but its concentration is highest in the kidney tubules cells, the erythrocyte, and some cells of the gastric mucosa. The substrates of the carbonic anhydrase reaction are carbon dioxide and water the product is carbonic acid. The enzyme has been purified from erythrocytes, and its molecular weight is about 30,000. The purified enzyme preparation contains 0.21% zinc, probably 1 atom of zinc per molecule of enzyme. The zinc is tightly bound to the enzyme molecule and cannot be removed by dialysis or electrodialysis. The presence of zinc in the molecule is essential to activity because when zinc is removed from the molecule by extended incubation with 1-10 phenanthroline, the enzyme s activity reflects the zinc content of the preparation. 

When N. U. Meldrum and F. J. W. Roughton found carbonic anhydrase in erythrocytes, they failed to find it in the stomach for reasons that have nothing to do with laboratory work. In late May 1938 R. B. Fisher, persuaded by observations such as those of Brown and Vineberg that the stomach s ability to secrete acid is influenced by the carbon dioxide in the blood, attempted to persuade me to look for carbonic anhydrase in the gastric mucosa. I found it in the first week of June 1938, and the next week I learned Meldrum and Roughton s method for measuring carbonic anhydrase in Cambridge. ... 

After I had studied the secretion of bromide and iodide, it was obvious that the next anion I should study was thiocyanate. I fed my pouch dogs sodium thiocyanate, stimulated them to secrete by subcutaneous injections of histamine, and attempted to collect their gastric juice. I was frustrated by the scantiness of secretion, and eventually I realized that thiocyanate was inhibiting acid secretion. My friend Joseph F. Ross, who was also a fellow in pathology at Rochester, suggested that thiocyanate might be inhibiting carbonic anhydrase. I made the appropriate test and found that the enzyme is indeed inhibited by thiocyanate. 

Figure 1-18. Left, abscissae, concentration of thiocyanate in mM ordinates, ratio of inactive to active carbonic anhydfase in the solution. Center abscissae, concentration of active carbonic anhydrase in arbitrary units in the presence of 1.78 mM thiocyanate ordinates, concentration of inactive carbonic anhydrase in the solution in the same arbitrary units. Right abscissae, concentration of thiocyanate in the oxyntic cells calculated as described in the text ordinates, hydrogen ion concentration in gastric juice as a percentage of uninhibited concentration under maximal histamine stimulation. The dotted line is the regression line calculated from the data. The solid line is the theoretical line derived from the assumptions described in the text. (From Davenport HW. The inhibition of carbonic anhydrase and of gastric acid secretion by thiocyanate. Am / Physiol 129 505-514, 1940.)...

Mann and Keilin quite naturally wanted to compare sulfanilamide with thiocyanate as inhibitors of acid secretion, but because they were biochemists, they had to ask a physiologist who knew how to anesthetize an animal to help them. The result was a brief note in Nature reporting that sulfanilamide in concentrations that largely inhibit carbonic anhydrase in extracts of gastric mucosa have no appreciable effect upon acid secretion, whereas thiocyanate in concentrations having no appreciable effect upon the enzyme does inhibit acid secretion (Fig. 1-19). "... 

Figure 1-19. Gastric acid secretion by cats following a single injection of histamine. Each bar represents the rate of secretion in mols X 10" min" during 1 hour. Thiocyanate and sulfanilamide in the doses indicated were given intravenously before the histamine injection. (Based on data from Feldberg W, Keilin D, Mann, T. Activity of carbonic anhydrase in relation to gastric secretion. Nature 146 651-652, 1940. Reproduced from Enzymes and Drug Action. London Ciba Foundation, 1962, by permission.)...

Discovery of the HCOf -stimulated ATPase revived interest in the possibility that carbonic anhydrase participates directly in secretion of acid. In 1970 Shirley Cross, working for Smith, Kline and French in England, applied the latest version of the cobalt sulfide histochemical method for locating carbonic anhydrase in the rat stomach. She floated frozen sections of gastric mucosa on a solution of cobalt sulfate and sodium bicarbonate. As carbon dioxide evolved from the solution over... 

The carbonic anhydrase theory is mentioned in Gray JS. The formation of acid in the gastric glands. Fed Proc 1 25 5-260, 1942 and in idem. The physiology of the parietal cell with special reference to the formation of acid. Gastroenterology 1 390-400,1943.1 must say, in fairness to John Gray, that he fully understood the words just quoted. 

Davenport HW. The inhibition of carbonic anhydrase and of gastric acid secretion by thiocyanate. Am J Physiol 129 505-514, 1940. 

Davenport HW. In memoriam The carbonic anhydrase theory of gastric acid secretion. Gastroenterology 1 314-315, 1946. 

Lately, extensive research has been devoted to metalloenzymes. " Thus, carbonic anhydrase is a very effective catalyst for conversion of CO2 to carbonic acid (H2CO3), and vice versa This enzyme is found in red blood cells, gastric mucosa, pancreatic cells, and renal tubules. Carbonic anhydrase plays an important role in respiration by influencing CO2 transport in the blood. The enzyme also functions in the formation of hydrochloric acid by the stomach. 

---