476 X. BILE PIGMENT FORMATION, ETC. 



Keilin and Hartree (1501) have recently found that hydrogen 

 peroxide produced by notatin, the glucose dehj'drogenase of Peni- 

 cillium notatum, formed small amounts of choleglobin as well as 

 hemiglobin from oxyhemoglobin. 



Substances which in the presence of oxygen yield hydrogen peroxide 

 can thus cause the formation of choleglobin or similar substances 

 (cf. Section 6.2.). One of these substances is dithionite, which is 

 frequently used as a reducer of blood pigments. After several '•educ- 

 tions of oxyhemoglobin by dithionite, followed by reoxygenation, an 

 absorption band similar to that of ferrocholeglobin can always be 

 observed. No choleglobin is formed by repeated deoxygenation of 

 oxyhemoglobin by evacuation, followed b}' reoxygenation (691; cf. 

 also 1698). 



For the coupled oxidation of hemoglobin and ascorbic acid, Lemberg 

 and co-workers (1708) showed that the hydrogen peroxide formed by 

 the autoxidation of ascorbic acid can play only a minor role. This 

 autoxidation is a copper-catalyzed reaction; its complete inhibition 

 by specific reagents which combine with copper, such as diethyl 

 dithiocarbamate, did not reduce the rate of the coupled oxidation.* 

 The latter is also not prevented, although it is slowed down, by 

 catalase (cf. also 52,691). The reaction is, therefore, initiated by a 

 direct reaction of oxyhemoglobin with ascorbic acid and its mechanism 

 is similar to that described above for the formation of verdohemo- 

 chrome from pyridine hemochrome. In this connection it is of 

 interest that Kiese has found evidence for the formation of a com- 

 pound of hemiglobin with ascorbic acid (1526; cf. Chapter XI, 

 Section 3.3.2.). 



The coupled oxidation can be formulated as follows: 



Fe2+ + O, -> Fe-+ O2 + H,.\ -► Fe'^+ • H,Oo -^ choleglohiii 



T 



+H2A 



i 



-Fe' 



In this case the Fe-'^02 compound is stable (oxyhemoglobin) and 

 yields hemiglobin directly only at a negligible velocity. Again the 

 major part of the reaction proceeds in the direction (a) so that the 

 formation of choleglobin is completed only after several cycles. The 



*According to recent observations (1609), however, diethyldithiocarbamate does 

 not act on the system merely as copper inhil)itor. 



