PROTEIN METABOLISM 811 



believe therefore that a certain small proportion of the urea, which appears 

 in the urine after the ingestion of protein, is due to this hydrolytic splitting 

 of the arginine contained in the protein molecule. The other moiety of the 

 arginine, namely, the diamino-valerianic acid, probably undergoes the same 

 changes as the other amino-acids, such proportion of it as is not required for 

 the building up of the tissues of the body being deaminised and giving rise to 

 urea and some CHO group in the manner already discussed. 



THE ENDOGENOUS OR TISSUE METABOLISM OF PROTEINS 



On comparing the output of the various nitrogenous excreta given in 

 Folin's Tables quoted above (p. 802), we see that on a low protein diet, when 

 the exogenous or energy metabolism of this foodstuff is reduced to a mini- 

 mum, the only substance which does not undergo simultaneous diminution 

 is the creatinine. Whereas on an ordinary diet free from meat, it accounts 

 only for about 3 per cent, of the total nitrogen output, on the low diet 

 it forms as much as 17 per cent. The conclusion at once suggests itself that 

 creatinine, more than all the other constituents of the urine, must be regarded 

 as an index of the tissue metabolism of protein. Let us see what facts can 

 be adduced in favour of this view. 



Creatinine has the formula : 



NH = C.N(CH 3 ).CH 2 

 NH CO 



and may be regarded as derived by a process of dehydration from creatine 

 .(methyl guanidine acetic acid). 



NH = C.N(CH 3 ).CH 2 COOH 

 NH 2 



It may be formed from this latter substance by boiling for three hours with 

 strong hydrochloric acid. Creatine has long been known as the most 

 abundant nitrogenous extractive in the body. It exists in relatively large 

 quantities in muscle ; and in meat extracts, such as Liebig's, it occurs to the 

 extent of 10 or 12 per cent. It has been calculated that the body of a man 

 at any time contains about 90 gnu. of this substance. On boiling creatine 

 with baryta water, it undergoes hydrolysis with the formation of urea and 

 sarcosine or methyl glycine. 



CH 3 CH 3 



NEL | NFL, 



JC.N.CH 2 COOH + H 2 = >CO + HN.CH 2 COOH 



NH/ NH/ 



Creatine Urea" Methyl glycine 



Owing to the ease with which- this formation of urea from creatine may be 

 brought about outside the body, it was natural that this substance should 

 be regarded as an important precursor of the urea in the urine. The view 



