92 RESPIRATORY MECHANISMS 



The role of carbonic anhydrase in CO2 transport. In 1928 O. 

 Henriques studied the reaction H 2 C0 3 ^ C0 2 + H 2 0, which 

 is a necessary link in the bicarbonate formation in the tissues 

 as well as in the C0 2 elimination from bicarbonate at the 

 respiratory surfaces. He pointed out that according to the 

 kinetic studies of Faurholt (1924) this reaction is a compara- 

 tively slow process which could not possibly reach an equilib- 

 rium in the second or so during which a blood particle is in 

 contact with respiring cells and alveolar air respectively. 

 He found that C0 2 is given off from blood, but not from a 

 simple bicarbonate solution, with extreme rapidity, and he 

 brought forward evidence to show that the C0 2 in the blood 

 was not in the main present as bicarconate but in a complex 

 combination with haemoglobin. Later investigations, sum- 

 marized by Roughton (1935), have shown, however, that the 

 bicarbonate concept is substantially correct, but the reaction 

 is speeded up so as to attain equilibrium in a fraction of a 

 second by an enzyme, carbonic anhydrase, present in the 

 corpuscles. The enzyme was isolated in a highly concentrated 

 form by Meldrum and Roughton (1934). It is present in the 

 red cells of vertebrates, but not related to the haemoglobin, 

 and has been demonstrated also in many tissues, both in 

 vertebrates and invertebrates. The enzyme may perform 

 an important function in the tissues by catalysing the reaction 

 C0 2 + H 2 —» H 2 C0 3 , but it is a puzzling fact that it is 

 absent in many cases where one would expect it to be useful, 

 e.g., in many muscles. In certain animals (Lumbricus, Am- 

 phioxus) it seems to be completely lacking (Brinkman, 1933; 

 H. van Goor, 1937). A fairly high concentration is found in 

 the gills of many marine invertebrates (Loiigo, Limulus, Horn- 

 arus, according to Ferguson, Lewis, and Smith, 1937) and 

 fishes (Leiner, 1938), and this seems easy to understand, but 

 Leiner found even more in the pseudobranchia of several 

 fishes where the respiratory function which Leiner postulates 

 is extremely doubtful, because these structures do not come in 

 contact with the surrounding medium and are supplied with 

 arterial blood which goes on to the ipsilateral eye. The 



