34 HEMOGLOBIN 



There is still the possibility, however, that one cytochrome consists 

 of not more than two haemochromogens : we have seen how these 

 two spectra can be made from turacin or haematin according as it is in 

 a state of colloid suspension or of true solution. A mixture of these 

 two might easily give a three-banded spectrum, involving, say, the 

 lines b, c and so much of d as was involved in y and x. This would 

 leave only a and z to be accounted for by a second body. But we 

 are still in the region of speculation where there is as yet no certain 

 knowledge. 



Here we must leave the constitution of cytochrome until further 

 information is forthcoming, and turn to its function — a field no less 

 exciting than that of its constitution or occurrence. 



What cytochrome effects in the ceU, appears to be bound up with 

 the fact that the pigment itself is oxidisable when the cells containing 

 it are exposed to air and reducible by reducing agents of the cells. 

 To understand its function we must first grasp certain facts con- 

 cerning the conditions under which the cjrtochrome acquires and parts 

 with its oxygen. Of these the first is the influence of potassium 

 cyanide upon the oxidation of the pigment. If bee's muscle or yeast 

 be examined whilst precautions are taken to keep the air away from 

 the material, as by putting a coverslip over it, the cytochrome will be 

 reduced by the activity of the material itself. If now it be treated 

 in the reduced condition with a very dilute (N 110,000) solution of 

 potassium cyanide the spectrum of the reduced pigment becomes fixed, 

 so that subsequent exposure to air does not wipe out the bands and 

 therefore may be presumed not to oxidise the cytochrome. The action 

 of potassium cyanide leads to one interesting phenomenon, namely, 

 by invoking its aid the bands of oxyhaemoglobin and cytochrome 

 may be obtained in the same solution. Under ordinary circumstances 

 they cannot exist together because the oxyhaemoglobin band can 

 only persist in the presence of a certain partial pressure of free 

 oxygen, which if present oxidises the cytochrome. If, however, a 

 mixture of cytochrome and haemoglobin, both in the reduced con- 

 dition, be treated with potassium cyanide and then shaken with air, 

 the oxygen introduced will oxidise only the haemoglobin, thus giving 

 the oxyhaemoglobin bands without abolishing those of the cytochrome. 



The reader should grasp the importance of the above observation. 

 Potassium cyanide is of all reagents one of the most lethal. The 

 text-book statement is that the cyanide prevents oxidation in the 

 tissues. The fact is that in animals poisoned with cyanide the blood 



