TRACE ELEMENTS IN ANIMALS 151 



enzyme, which they regard as practically pure, obtained from 

 the erythrocytes of ox and sheep, contain constantly about 

 0-33 per cent of zinc, but no iron, copper, manganese, mag- 

 nesium or lead. In preparations of the enzyme of varying degrees 

 of purity the enzyme activity is directly proportional to the zinc 

 content, which indicates that the zinc forms part of the enzyme 

 molecule. The enzyme, in fact, appears to be a zinc-protein 

 compound in which the protein in each molecule is combined 

 Avith two atoms of zinc. It may be noted that no other zinc 

 compound is known which has catalytic properties. Keilin and 

 Mann found the concentration of carbonic anhydrase to be 

 about 0-21 g. per 100 ml. of erythrocytes. 



The function, or at any rate one function, of zinc in mammals, 

 is to be found therefore in the action of carbonic anhydrase. In 

 the blood this action accelerates the dissociation of carbonic 

 acid into water and carbon dioxide and so furthers the escape 

 of the latter from the blood into the alveolar spaces. As well as 

 this function in carbon dioxide excretion from the blood, the 

 presence of the enzyme in the parietal cells of the gastric 

 mucosa suggests another function in the stomach, probably in 

 connexion with the formation of hydrochloric acid. 





