SULFHEMOGLOBIN 497 



reconversion into protoporphyrin derivatives by acid and alkali and 

 the fact that the extra sulfur is not removed in these reactions. 



It appears more likely that the heme group is bound to a thiol- 

 histidine by a sulfur linkage between the heme iron and the imidazole 



-Fe 



^ C^ 



Fig. 12. Structure of sulfhemoglobin {?). 



ring, which replaces the normal iron-imidazole linkage in hemoglobin 

 (Fig. 12); or sulfur may be added to a methene bridge double bond. 



7.7. Myosulfhemoglobin 



From myooxyliemoglobin a sulfhemoglobin is formed even faster than 

 from oxyhemoglobin. The formation of sulfhemoglobin cannot, therefore, 

 be due to interaction between the heme groups in hemoglobin. Kiese and 

 Kaeske {1527) found the absorption band of myosulfhemoglobin and that of 

 its carboxy compound in the same position as those of sulfhemoglobin and 

 its carboxy compound. This is in contrast to the difference between hemo- 

 globin and myohemoglobin derivatives observed for the choleglobins and 

 pseudohemoglobins. Michel {IDJ^o), however, reports an absorption band at 

 617 vciji (tmM = 17) for myosulfhemoglobin. i.e., at somewhat shorter wave- 

 lengths than that of sulfhemoglobin. Ferricyanide and mercuric chloride 

 oxidize myosulfhemoglobin to the ferric state; the band disappears and can be 

 restored by reduction with dithionite. Like myohemoglobin, myosulfhemo- 

 globin is far more stable toward alkali than sulfhemoglobin. 



7.8. Distinction between Sulfhemoglobin 

 and Compounds with Similar Absorption Spectra 



Sulfhemoglobin, hemzglobin, ferrihemalbumin (methemalbumin), 

 and ferrocholeglobin all have absorption bands in the orange between 

 620 and 635 m^u which cannot be distinguished from one another with 

 a hand spectroscope, although the Hartridge reversion spectroscope 

 allows the distinction between the bands of heniiglobin and sulfhemo- 

 globin. It is preferable to rely for differentiation on reactions of the 

 pigments, in which case a hand spectroscope can be used. The 



