CYTOCHROME ^ 33 



whilst the concentration of cytochrome, especially in the muscles, increases. The 

 thoracic muscles of the pupa, and still more of the imago ready to hatch, contain a 

 great proportion of cytochrome in addition to a little haematin. In the same way the 

 free haematin of the endosperm of grass seed or of the reserve tissues of other plants 

 can store material for the formation of the haemochromogen and of the cytochrome 

 appearing in the cells which are carrying on an active metabolism (4). 



The fact is that haematin is a substance of very wide distribution. 

 The reason why its presence has been unsuspected is that it is not 

 at all easily seen, not being a body with sharp spectral bands. Our 

 new knowledge of hsemochromogen, however, teaches us how to turn 

 the inconspicuous hsematin into the conspicuous hsemochromogen — 

 that is done by the addition of pyridine in the presence of a reducer. 

 Indeed haematin may be demonstrated in the presence of cytochrome 

 by converting the haematin into CO-haematin and then adding 

 pyridine. Haematin is present in quite ordinary articles (3), (4) of 

 vegetable diet — in wheaten flour and oaten meal for instance. It 

 seems remarkable enough that mankind, in what he has been pleased 

 to call the staff of life, has for countless centuries been eating, all 

 unknowingly, the outstanding constituent of his blood. 



Something remains to be said about the nature of the trans- 

 formation from haemochromogen to cytochrome. It takes place 

 in the plant; it can be made to take place in the test-tube by 

 the alternate reduction and oxidation (with ferricyanide) of the 

 haemochromogen and it is not dependent upon the protein since the 

 spectra of the analogous nicotine and pyridine cytochromes can be 

 made from the haemochromogens in a similar way. The nature of 

 the change from the haemochromogen which contains natural haematin, 

 to the cytochrome analogue, or possibly the three cytochrome ana- 

 logues, is still obscure. 



To commence with, there is but one natural haematin, and there 

 appear to be three haemochromogens in cytochrome. The easiest 

 assumption is that one haematin is attached to three different nitro- 

 genous moieties; this explanation is negatived by the fact that 

 cytochrome may be obtained in which the nitrogenous moiety is in 

 every case pyridine or in every case nicotine. It must be then that 

 the haematin itself is altered. In Chapter ii it was shown that the 

 formation of almost any number of porphyrins was possible in theory. 

 If therefore the porphyrin in haematin could be altered at all by 

 successive oxidations and reductions, it might as easily be altered 

 into three other porphyrins as one. 



