482 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The Change of Oxyhcemoglobin into Methcemoglobin. 



When oxyhsemoglobin is converted into methaemoglobin a volume 

 of oxygen equal to that originally combined with the oxy haemoglobin 

 is evolved. Methaemoglobin is usually said to contain the same 

 amount of oxygen as oxyhaemoglobin ; it is probably in a different 

 state of combination, as it cannot be removed by submitting the 

 methaemoglobin to a vacuum as in the case of oxyhaemoglobin. This 

 change is represented as follows : 



/ ^ 



Hb/ | + 2 = Hb^ + 2 . 



\o 



The evolution of oxygen in the conversion of oxyhaemoglobin into 

 methaemoglobin can be readily observed : 



An equal volume of water is added to a little defibrinated blood 

 in a test tube and warmed to 50. An equal volume of potassium 

 ferricyanide solution is mixed with this solution. If the tube be in- 

 clined for a short time, the evolution of bubbles of oxygen will be seen. 



The later work of Letsche and of Buckmaster upon the action of 

 hydrazine on oxyhsemoglobin and methaemoglobin shows that met- 

 hrumoglobin contains half as much oxygen as is present in oxyhaemo- 

 globin. Methaemoglobin has more probably the formula Hb=O and 

 the conversion of oxyhaemoglobin into methaemoglobin may be 



> 



Hb( | + O = Hb^O + O s 

 \O 



or 



/OH 



Hb< + O 2 = Hb=O + H a O + O 2 . 

 \OH 



Haematin. 



Haemoglobin is decomposed by the action of dilute alkalies or of 

 dilute acids into haematin and globin ; also by digestion with pepsin 

 and hydrochloric acid. Haematin is insoluble in water, but soluble in 

 acids or alkalies, giving solutions which are known as acid haema- 

 tin and alkaline haematin. In alkaline solution haematin can be 

 reduced by Stokes' reagent or ammonium sulphide to haemochromo- 

 L^cn, or reduced alkaline haematin. These changes can also be seen 

 with a solution of defibrinated blood. 



Acid Hcematin. 



A quarter of a volume of 33 per cent, acetic acid is mixed with 

 some diluted blood (1:5) and warmed in a water-bath to 40 or 50 for 

 5-10 minutes. The solution becomes brown. On diluting a portion 

 with water and examining with a spectroscope an absorption band in 

 the red between C and D will be seen. This is characteristic of acid 

 haematin; it resembles the absorption spectrum of methaemoglobin. 

 Us position depends on the amount of acid present in the solution; 

 it is nearer C with more acid. 



