468 X. BILE PIGMENT FORMATION, ETC. 



group /8 or 8 to a ^ COH group (Fig. 8). This is a repetition of the process 

 which occurs at methene group a before it is removed {cf. Fig. 3). 



In spite of the transformation of verdohemochrome into azahemin, 

 it is unlikely from these experiments that the conversion of hematin 

 into bile pigment is a reversible process. Neither a conversion of 

 biliverdin into verdohemochrome nor a reconversion of the latter 

 into a porphyrin compound by formic acid or formaldehyde has been 

 observed. 



2.5. Mechanism of Formation of Verdohemochromes 



The mechanism of the formation of verdohemochromes has been 

 thoroughly studied by Lemberg and co-workers (1697,1698). 



Role of hydrogen peroxide. It has been mentioned above that 

 hydrogen peroxide (in high dilution) acting on pyridine hemochrome 

 in the absence of oxygen produces oxyporphyrin hemochrome, which 

 is a precursor of verdohemochrome. The conversion of oxyporphyrin 

 hemochrome to verdohemochrome is probably also caused by hydro- 

 gen peroxide, not by molecular oxygen, although — as we saw above 

 — oxyporphyrin hemichrome is autoxidizable to a substance having 

 the verdohemochrome type of absorption spectrum (1733). It is not 

 yet certain whether the latter reaction leads to verdohemochrome, 

 and in any case the coupled oxidation with ascorbic acid is much 

 faster than the autoxidation. The verdohemochrome formation is 

 partly inhibited by catalase. 



For verdohemochrome formation to occur it is essential that the 

 hydrogen peroxide act on the ferrous heme compound. Oxidation of 

 ferric hematin compounds b}' hydrogen peroxide leads only to decolor- 

 ation. To keep the hematin iron in the ferrous state is one of the 

 functions of the hydrogen donor in the reaction, but not the onl}' one. 

 In the coupled oxidation with some hydrogen donors the hydrogen 

 peroxide may be produced by the autoxidation of the hydrogen donor. 

 With ascorbic acid, under the conditions of the experiment, the 

 autoxidation is, howev^er, too slow to account for the reaction velocity; 

 catalase only partially inhibited this reaction, while the action of 

 added peroxide was completely blocked by catalase. Lemberg and 

 co-workers, therefore, assumed the following reaction mechanism: 



Fe"-+ +0-. ->Fe2+02 

 Fe^+O, 4- H,.\ -^ Fe?+ • H.O, + A 



In the presence of catalase the hydrogen peroxide thus formed in the 



