HEMOGLOBIN AS A CYTOCHROME OXIDASE MODEL 395 



hemoglobin, while in the formation of hemiglobin with ferricyanide 

 at a pH below 7 the globin remains unaltered. We have mentioned 

 above that Darling and Houghton (532) observed, indeed, differences 

 between the hemoglobins produced by these methods. 



These considerations have a very important bearing on the 

 problem of the reaction mechanism of respiratory hematin enzymes. 

 So far very little attention has been paid to the role hydrogen donor 

 or hydrogen acceptor groups in the protein part of these enzyme 

 molecules may play in their reaction mechanism. Nevertheless 

 scattered evidence is available which indicates that here the key to 

 many unsolved problems may be found. Such groups may very well 

 be the mediators of intracomplex reactions, which make it possible 

 for the reactions to proceed without the formation of free radicals. 



6.3.6. Oxidizing Action of Oxygen Liberated from Oxyhemo- 

 globin. Before returning to the respiratory enzymes proper, we 

 consider once more hemoglobin as a model system in a somewhat 

 different aspect. Warburg stressed the important difference between 

 the inactive transport oxygen of oxyhemoglobin and the activated 

 oxygen of the respiratory ferment. While this is perfectly correct, 

 it can be shown that a comparatively mild alteration of the globin, 

 e.g., reversible denaturation by acid or denaturation by pyridine, 

 transforms the oxygen into "active" oxygen. 



It has frequently been observed that ascorbic acid disappears 

 almost instantaneously when a solution of this substance containing 

 oxyhemoglobin or hemolyzed blood is deproteinized with trichloro- 

 acetic acid. Similar observations had been made with adrenaline 

 (390). Some authors {2 18, 390, 8 9 6, 9 6 -If) tried to explain the phenom- 

 enon by assuming adsorption of ascorbic acid or adrenaline to the 

 "acid hematin" precipitate, but others {961,1506,15J^9,1702) found 

 that transformation of oxyhemoglobin into hemoglobin or carboxy- 

 hemoglobin prevented the destruction of ascorbic acid by the ensuing 

 deproteinization. The disappearance of the ascorbic acid was thus 

 recognized as oxidation. 



The phenomenon has been studied more closely by Lemberg 

 {1686, cf. also 1668,1702,1710,1712). Only 40% instead of the 75% 

 of the oxygen expected theoretically is evolved, if an oxyhemoglobin 

 solution is acidified with metaphosphoric acid. The remainder is used 

 up in oxidative reactions. Hydrogen donor groups in the globin are 

 oxidized {cf. also 1308,1309,1963,2218) and a small part of the 



