The Haem-Globin Linkage 



185 



mesoporphyrin to apohaemoglobin and to carboxyhaemoglobin. These 

 curves are most probably also related to a differential carbamate equilibrium 

 curve which can be prepared by subtracting the points of the curves of 

 Figs. 5a and 5b of Stadie and O'Brien (1937) for ferro- and oxyhaemoglobin 

 (species unspecified). 



A new interpretation of the differential titration curve is suggested — (1) that 

 the alkaline loop represents the attachment of propionate groups to imida- 

 zolium side-chains; (2) that the acid loop mirrors the difference between 



Fig. 5. Comparison of specific increment curve (upper) for attacliment of nickel 



mesoporphyrin to apohaemoglobin with the differential titration curve (lower) 



of German and Wyman (1937) for ferroHb-oxyHb (upper curve) prepared by 



deducting increment with COHb from increment with apoHb). 



the detachment of the haem iron, in the 'essentially covalent' (and stronger) 

 linked oxyhaemoglobin and the 'essentially ionic' ferrohaemoglobin. As the 

 hydrogen ion concentration increases, the group bonding the iron in ferro- 

 haemoglobin will be liberated, that in oxyhaemoglobin will still be held by the 

 stronger bond. The maximum difference occurs at about pH 5-4 after which 

 the 'covalent' bond begins to break and the extent of ionization of the two 

 groups becomes the same at pH 4-3. Preliminary studies (O'Hagan, unpub- 

 lished) show very marked differences in the stability of carboxyhaemoglobin 

 and ferrihaemoglobin in the pH range 4-6. If the above interpretation is 

 correct, a more pronounced acid loop should be found for the differential 

 titration of carboxyhaemoglobin-ferrohaemoglobin. 



The experiment with apomyoglobin shows that a much stronger basic 

 group or groups is bonding the propionate(s) as evidenced by the greater height 

 of the curve in the pH range 5-9 and the major shift (30 m/t) in the position 



