METABOLISM OF PROTEINS 507 



On heating with baryta-water kreatin is decomposed, yielding urea, 

 methylglycocoll or sarcosin, and other substances. It can be prepared 

 synthetically from sarcosin and cyanamid. Thus: 



/NH 2 H.N(CH 3 ) /NH 2 



C==N + | = C^=NH 



CH 2 .COOH \N(CH 3 ) .CH 2 .COOH 



Cyanamid. Methylglycocoll. Kreatin. 



Kreatin is found in considerable amount (p. 741) in muscular 

 tissue, and in traces in other tissues and in blood-plasma. 



Kreatinin can be so readily obtained from kreatin outside the 

 body that it is tempting to suppose that the portion of the kreatinin 

 of the urine which is not formed from the kreatin in the food is 

 derived from the kreatin of the muscles and other tissues, and many 

 theories have been evolved to connect the kreatinin of urine with 

 the kreatin of the muscles. But it is doubtful whether there is any 

 direct connection. The alleged absence of kreatinin from muscle 

 seemed to be opposed to the idea that the kreatin store of the 

 muscular tissue was an important source of urinary kreatinin ; for 

 if a constant transformation of this kind was going on, traces of 

 kreatinin not yet absorbed by the blood might have been expected 

 to be present in the muscles. Recently, however, it has been 

 reported that small quantities of kreatinin do exist in fresh muscle 

 (4 to 8 milligrammes in 100 grammes of tissue), and that when the 

 muscle is allowed to undergo autolysis the kreatinin increases at a 

 very uniform rate at the expense of the kreatin. Added kreatin 

 experiences the same fate as the kreatin originally present, while 

 added kreatinin inhibits the reaction, or even reverses it (Myers and 

 Fine). A parallelism with the conversion of glycogen into dextrose 

 in the liver easily suggests itself, and it is possible that we are here 

 in the presence of a normal reaction which may account for at 

 least a portion of the kreatinin excretion. It is probable that both 

 kreatin and kreatinin can undergo changes in the body, especially 

 in the liver, and it is possible that the products may be further 

 utilized in metabolism. If this were so, the kreatin store of the 

 muscles would acquire new significance as a reserve of useful material 

 with perhaps a long and varied metabolic career before it, and would 

 not constitute merely a temporar}' depot of waste material whose 

 metabolic history was ended, and which was waiting to be excreted. 



However this may be, the constancy of the kreatinin elimination 

 on a meat-free diet (p. 475), and its complete independence of the 

 changes in the total nitrogen excretion, show that it has a different 

 significance in protein metabolism from the urea. Evidence is 

 accumulating that it is especially in the metabolism of the organized 

 or tissue protein that the product eventually excreted as kreatinin 

 arises ; in other words, that it represents especially the nitrogenous 

 waste connected with the wear and tear of the bodily machinery, 

 while urea represents also, and under ordinary conditions of diet 



