270 PRINCIPLES OF GENERAL PHYSIOLOGY 



lysine, nor tryptophane, but an excess of glutamie acid ; gelatine contains neither 

 tyrosine nor tryptophane, but an excess of glycine. The animal, however, utilises 

 tin- nitrogen of zein to the extent of 80 per cent., and that of gelatine to 50 or 60 

 per cent. This is shown by the fact that, instead of the extra nitrogen given 

 appearing in the urine, as would happen if it were not utilised for repair, only 20 

 per cent, or 40 per cent, respectively is excreted. On the other hand, when zein is 

 given, even in considerable excess over maintenance need, no evidence is obtained 

 of the formation of new body tissue ; whereas, if casein is given, 20 to 25 per cent. 

 increase in body protein results. It seems evident that the repair processes are 

 of a different character from those of growth. The processes of cell wear and tear 

 and their repair do not appear to involve the destruction and resynthesis of an 

 entire protein molecule. 



In the investigation of the endogenous nitrogen metabolism, the importance of 

 creating has been pointed out, so that a few words as to its chemical nature are 

 advisable. It may be looked upon as a substituted guanidine, in that one of the 

 NH 2 groups is replaced by methyl-glycine. Thus : 



Guanidine is 



/NH 2 

 H-N = C 



\NH 2 

 Methyl-glycine or sarcosine is 



H N CHXOOH 



CH 3 



Creatine is 



/NH, 



H N=C 



CH ;! 



COOH 



When boiled with dilute acids, it loses a molecule of water and is converted into 

 creatinine, an internal anhydride, with basic properties, since the COOH group 

 has disappeared : 



NH 



H N = C 



\ \ 



N CH 2 -CO 



CH 3 



Creatinine is converted again into creatine in alkaline solution (see Bunge-Plinimer, 

 1907, pp. 153-155). 



The method used for estimation is that of Folin (1904), which depends on the colour 

 reaction of creatinine with alkaline sodium picrate, as described by Jafle. 



As micleiiuf are important constituents of the cell nucleus, it is to be expected 

 that their metabolism would be chiefly of the endogenous kind. Before discussing 

 the question, the chemical nature of these substances must be indicated. As 

 already described, their characteristic group is the purine nucleus, the chemistry of 

 which has been completely worked out by Emil Fischer (1882-1906). It may be 

 regarded as a fusion of the pyrimidine and imin-azole rings, thus : 



N C= | Nl CG= 



C N v | | 



=C C >C= =02 Co N7\ 



C N/ | )C8= 



N C- N3 C4-NIK 



Pyrimidine Ring. Imin-azole Ring. Purine Nucleus. 



It will be noticed that in the purine nucleus the two component rings have two 

 . carbon atoms in common. 



For convenience, each constituent of the purine nucleus is numbered, as in the 

 formula. Purine itself is represented as : 



