INTRODUCTORY 3 



essential difference between haemoglobin and hsemochromogen ? Anson 

 and Mirsky(9) answer this question as follows: Hsemoglobin is 

 hsematin united with undenatured globin — haemochromogen is united 

 with denatured protein. 



The differences in the globin moiety only carry us a certain way. 

 There are certainly haemoglobins which differ in the haematin moiety 

 — haematin is essentially a compound of iron with porphyrin, and 

 porphjrrin is essentially a compound containing four pyrrol groupings. 

 As there can be many porphyrins, according as the side chains differ 

 slightly, so conceivably there can be as many haemoglobins. Three 

 such have been prepared by Hill and Holden(i2), they are the haemo- 

 globins which correspond to proto-, meso- and haematoporphyrins. 

 There is also one such body which has not been prepared synthetically 

 but which was found in the blood of certain polychaete worms by 

 Ray Lankester(i3) and the true nature of which has recently been 

 recognised by rox(i4). 



Porphyrins may unite with metals other than iron, as has been 

 shown by numerous observers (Laidlaw(i5), Schulz(i6), Milroy(i7)). 

 Knowledge of the metalloporphyrins has been much extended by 

 R. Hill (18), but though he has made many attempts to prepare 

 haemoglobins from metalloporphyrins containing metals other than 

 iron, these have been unsuccessful. Globin will not unite with them : 

 and from the other side, though haemochromogens may be pre- 

 pared by the addition of many substances (hydrazine, nicotine, 

 pyridine) to haematin, globin is the only such material which by its 

 association with oxygen yields a body from which the oxygen is 

 detachable by mere exposure to a vacuum. 



Haemochromogen forms no reversible oxide. Attempts to oxidise it 

 in alkaline solution appear to break it down. An oxide in neutral 

 solution is stable, but it cannot be reduced by a vacuum. It has been 

 studied by Keilin(i9) and is called by him " parahaematin " (formerly 

 it was known as kathaemoglobin). While oxygen does not unite 

 reversibly with haemochromogen, carbon monoxide does. It is possible 

 to determine the curve, as was done by Anson and Mirsky(9), which 

 expresses the equilibrium between CO and haemochromogen. Revert- 

 ing from haemochromogen to haemoglobin, the affinity of CO for the 

 pigment is about 400 times that of oxygen in man, and perhaps 

 100 times that of oxygen in the rabbit — the relative affinities of the 

 two gases differing for the haemoglobins of different animals. 



To the great affinity of CO for haemoglobin, has been attributed 



