392 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



It is usually' overlooked, however, that hydrogen peroxide is 

 formed in the autoxidation of the reduced dye, and that hydrogen 

 peroxide is a powerful former of hem/globin. The objection has-been 

 raised that catalase present in such systems would destroy hydrogen 

 peroxide (24.21), but this is invalid. First, Keilin and Hartree (14-99) 

 have found that hydrogen peroxide formed gradually in small con- 

 centrations can oxidize hemoglobin in the erythrocytes in the presence 

 of an abundance of catalase. Second, a hydrogen donor such as 

 leucomethylene blue may react directly with oxyhemoglobin with 

 formation of an unstable hydrogen peroxide -hemoglobin complex 

 (Lemberg and coworkers, 1708): 



HbOz + H.A -^ HbHoO, + A 

 i 

 HiOH 



The latter type of reaction, which has been shown to be the mecha- 

 nism of the complex oxidation of ascorbic acid with oxyhemoglobin 

 (Chapter X), is incompletely inhibited by catalase. Similarly phenyl- 

 hydroxylamine reacts with oxyhemoglobin (1259). With other hydro- 

 gen donors the first mechanism (oxidation of hemoglobin by hydrogen 

 peroxide formed by autoxidation of the hydrogen donor), the second 

 (direct reaction of oxyhemoglobin with the hydrogen donor), or both 

 may be at work. 



A large number of hydrogen donors (e.g., aminophenols, phenyl- 

 hydroxylamine, phenylhydrazine, hydrazobenzene, ascorbic acid, 

 metabolic products of bacteria and tissues) are thus able to form 

 hemiglobin. These reactions will be discussed further in Chapters X 

 and XI since they cause irreversible as well as reversible oxidation of 

 hemoglobin. 



6.3.5. Autoxidation of Hemoglobin. After having shown in a 

 series of papers (102,1986,2028,2030) that the hemiglobin-forming 

 principle of bacteria consists of reducing substances in the presence 

 of atmospheric oxygen, Neill and collaborators found that autoxida- 

 tion of hemoglobin with formation of hem/globin also proceeds with 

 maximal velocity at a remarkably low partial pressure of oxygen 

 (20 mm. mercury) (2027,2031). They explained this as Brooks did 

 later, by assuming that only hemoglobin reacted with oxygen to form 

 hemi'globin. S{)eaking of oxygen activitation comparable to that 

 obtained with reducing substances, they vaguely suggested a similar- 



