CYTOCHROME 35 



goes through the capillaries and enters the veins unreduced. Why? 

 The possibility now presents itself that, cytochrome provides a link 

 in the chain of events which connects the loss of oxygen from the 

 haemoglobin with the actual building up of that gas into the tissue, 

 and that if this link be broken by the potassium cyanide the oxida- 

 tion process in the cell becomes much impaired. Such a conception 

 would present cytochrome as a substance which was always taking 

 up oxygen in the presence of oxidase and imparting it, not as free 

 oxygen, but by some system of double decomposition, to the oxidisable 

 materials in immediate contact with it. 



It may seem to the reader that the claim of cytochrome to pose 

 as a link in the oxidation process of the cell rests on too circum- 

 scribed a foundation, when it is based upon the relation of but one 

 substance, potassium cyanide, to the pigment. Such diffidence is not 

 out of place, and the investigations prompted by it have led to the 

 discovery of some other interesting properties of cytochrome. Let 

 us turn therefore to the consideration of a wholly different class of 

 substances, which, however, resemble potassium cyanide in one re- 

 spect, namely, that they reduce oxidation processes proper to vitality 

 to the point of ultimate extinction. Such substances are general 

 ansesthetics such as alcohol, chloroform, aldehydes, compound ureas, 

 etc. It is remarkable that these, too, abolish the power of cyto- 

 chrome to be alternately oxidised and reduced, and even more re- 

 markable that they effect this immobilisation of the cytochrome in 

 quite a different way from that in which potassium cyanide achieves 

 the same end. The narcotics mentioned do not prevent the oxidation 

 of the reduced cytochrome, on the other hand they prevent the 

 reduction of the oxidised pigment, breaking the link as it were at 

 the other end. Such an hypothesis is one which endows cytochrome 

 with catalytic properties, and which leads one up to the question: 

 Is cytochrome an important catalyst or only one of many which 

 may be in the cell ? Three sets of facts are worth noting with regard 

 to the importance of cytochrome as a catalyst: (1) its distribution 

 as related to the activity of the tissues in which it is found, (2) its 

 claim to be a peroxidase, and (3) its position as a substance which 

 contains iron. Let us discuss these three points. 



1. The distribution and behaviour of cytochrome in relation to the 

 activity of the tissues in which it is found. From numerous illustrations 

 cited by Keilin the following may be quoted in support of his thesis 

 that a general correspondence exists between the activity of a tissue 



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