870 PHYSIOLOGY 



haemochromogen. The same change is further observed when alkaline 

 haematin, made by the action of alkalies on ordinary blood, is treated with 

 reducing agents such as ammonium sulphide. Since this substance, hsemo- 

 chromogen, is responsible for the respiratory functions of the haemoglobin, 

 i. e. the power of its molecule to form unstable compounds with oxygen, its 

 preparation merits fuller consideration. 



Hcemochromogen is prepared by the action of caustic alkalies on 

 haemoglobin in the absence of oxygen. For this purpose a test-tube con- 

 taining a solution of sodium or potassium hydrate is placed in a bottle with 

 two necks containing a solution of haemoglobin, care being taken not to 

 spill a'ny of the alkaline solution. Hydrogen is then passed through the 

 larger bottle until the haemoglobin is entirely reduced and all the air is 

 replaced by hydrogen. The bottle is then inverted so as to mix its contents 

 with the caustic alkali, when haemochromogen is formed and can be recog- 

 nised by its characteristic colour and spectrum. The haemochromogen in 

 solution has a cherry-red colour, and when sufficiently diluted shows two 

 well-marked absorption bands identical with those given by reduced alkaline 

 haematin (Fig. 374, 7). Of the two absorption bands which are situated 

 between D and E, that nearest to D has very sharply defined borders ; the 

 position of the two absorption bands may be given in terms of. their wave- 

 lengths as follows : A 567 to 547 and I 532 to 518. The band nearest D is 

 given by haemochromogen solutions diluted until there is only one part 

 of the pigment in 25,000 parts of water, so that the formation of reduced 

 alkaline haematin is an even more delicate test for blood than the spectrum 

 of oxy haemoglobin itself. When CO-haemoglobin is treated in the same way 

 with alkali in the absence of oxygen, a body CO-haemochromogen is formed 

 which contains exactly the same volume of CO in combination as the original 

 CO-haemoglobin. This fact, combined with the possibility of reducing 

 ordinary alkaline haematin by the action of ammonium sulphide or Stokes's 

 fluid, indicates that the group of atoms which in haemoglobin is responsible 

 for taking up oxygen or carbon monoxide gas passes unchanged into the 

 haemochromogen molecule. Haemochromogen therefore represents an iron- 

 containing coloured radical which can combine with protein bodies to 

 form haemoglobin, and is responsible for the oxygen-combining powers of the 

 latter. We may assume therefore that oxyhaemoglobin and CO-hsemoglobin 

 contain oxy haemochromogen and CO-haemochromogen respectively. 



Hcematoporphyrin. If haemoglobin, haematin, or haemin be mixed with 

 concentrated sulphuric acid, it dissolves forming a purple-red solution. 

 On pouring this solution into a large quantity of water, hsematoporphyrin 

 is thrown down in the form of a brown precipitate. In order to prepare 

 haematoporphyrin, pure crystallised haemin is added to a saturated solution 

 of hydrobromic acid in glacial acetic acid. The whole is allowed to stand 

 for three or four days and then thrown into distilled water. The resulting 

 mixture is filtered and the haematoporphyrin thrown down by careful 

 neutralisation of the hydrobromic acid with caustic soda. Haemato- 

 porphyrin is easily soluble in alkalies and somewhat less readily so in acids, 



