THE PRECURSORS OF UREA 677 



urine in man on a meat-free diet. He found, like Macleod, that 

 the excretion was still considerable, about 1*5 grm. a day, and that 

 this endogenous kreatinin was constant for the individual and 

 quite independent of the total nitrogen of the food. In different 

 individuals the total amount of endogenous kreatinin excreted 

 appears to be determined by the body weight and by the fatness 

 or thinness of the person, the excretion per kilo of body weight 

 being less in fat than thin persons. It seems probable that hard 

 muscular exercise which increases the kreatin content of muscle 

 also increases the excretion of kreatinin in the urine. From these 

 observations it seems clear that some of the kreatin or kreatinin 

 of muscles leaves them unchanged and is excreted as kreatinin. 

 But considering the quantity present in the muscles as a whole, it 

 would seem a priori likely that, as in the case of uric acid, the 

 excretion of kreatinin does not represent the total formation, and 

 that kreatin leaves the muscles also in some other form. It does 

 not seem likely that kreatin leaves the muscles partly as urea, for, 

 quite apart from the question whether the muscles can form urea 

 at all, they contain but little, in fact less than the blood. From 

 analogy we should imagine that muscles would contain a ferment 

 capable of splitting off the amido -nitrogen of these substances as 

 ammonia ; but there is no evidence in favour of such an idea 

 except the fact that muscles appear to produce ammonia in large 

 quantities, as is shown by the estimation of the ammonia content 

 of blood going to and from masses of muscles and of the muscles 

 themselves. 



On the other hand kreatin, as its formula shows, 



NH 2 CH 3 



I I 

 NH = C-N-CH 2 .COOH 



although chemically not unlike arginin, is peculiar in that it con- 



tains the group ( | L which the body apparently cannot easily 



\ N / 



deal with. Thus glycin is converted in the body into urea, but 

 its methyl derivative, sarcosin, is excreted unchanged. Ordinary 

 proteids do not yield kreatin on autolysis, possibly because they 



do not possess the necessary ( | j group, but myosin does yield 



\ N / 



