BIOLOGICAL SIGNIFICANCE OF HEMOGLOBIN 189 



oxygen. What then is the determining factor? Evidently it is in 

 some way associated with the protein portion of the haemoglobin 

 molecule. But even when the issue has been narrowed to the protein 

 portion, the precise cause of the reversibility of the reaction with 

 oxygen is remarkable; hsematin forms many compounds with protein, 

 and many compounds with bodies similar to protein, but it is only 

 compounds with one class of protein bodies — the globins — which can 

 behave in this way. Indeed we can narrow the issue still further, 

 for the compounds of hsematin and denatured globin do not form 

 reversible compounds with oxygen; nor does methsemoglobin, which 

 differs from haemoglobin by containing iron in the ferric state (Conant). 

 The essential point, however, is that the globin compound of hsematin 

 possesses the power of undergoing thp transformation necessary to 

 effect the lability of the oxygen — no other compounds of hsematin 

 do so. 



Not that the analogues of hsemochromogen are insufficiently 

 numerous. Many nitrogenous substances exist with which hsematin 

 makes compounds on the haemochromogen level, but Robin Hill was 

 unable to make a haemoglobin analogue out of any but the globin com- 

 pound, though he has tried quite a number. 



Why the oxygen in haemoglobin should be easily dissociated whilst 

 that in haemochromogen is not, remains for the present, a mystery. 

 A further mystery is furnished by the fact that the general analogy 

 between the oxygen and carbon monoxide compounds of haemoglobin 

 breaks down at this point. Carbon monoxide dissociates from haemo- 

 globin much less readily than oxygen, nevertheless CO forms a com- 

 pound with haemochromogen that breaks down in the presence of a CO 

 vacuum whilst the oxygen compound does not. That at least is the 

 way in which the matter is presented in the hterature, but we do not 

 know really what the "CO compound of haemochromogen" is. It was 

 assumed by Anson and Mirsky to be a body made by the addition 

 of CO to haemochromogen. In that case it seems to bear no real 

 analogy to anything which is formed on the oxidation of haemo- 

 chromogen, for this latter reaction appears to be associated with the 

 splitting of the haematin from the globin. It may be that CO can 

 combine with the C34H3o04N4FeOH without tearing it away from the 

 globin, and it may be that this is the reason why the CO compound 

 is reversible — we do not know. Here we must be content simply to 

 confess our ignorance, and point out that before we have a complete 

 understanding of haemoglobin we must discover why globin, of all 



