METABOLISM OF PROTEINS 589 



back into the blood, and may thus be carried to the liver for conversion 

 into urea. It is not necessary, however, to suppose that all of the 

 nitrogen must perforce make this journey before being changed into 

 urea. There is evidence that all the tissues share to some extent with 

 the liver the power of forming urea just as they share with the liver the 

 power of splitting off NH 2 from the amino-bodies. It may be that 

 the liver surpasses other tissues in its deamidizing power just as it seems 

 to surpass other tissues in its power of transforming ammonium com- 

 pounds into urea. But this does not prevent at least a considerable 

 proportion of the amino-substances absorbed from the intestine from 

 passing into the general circulation. It is of importance to remark 

 that such hydrolytic cleavages as are associated with the splitting of 

 protein into amino-acids, etc., only slightly reduce the available energy 

 of the compounds. In so far as the liberation of the nitrogen from 

 the amino-acids is also accomplished by hydrolytic cleavage (sup- 

 posedly by a ferment desaminase), the residue, relatively rich in carbon, 

 will still be available for yielding to the body by its oxidatien an amount 

 of energy not much less than could be obtained from the original protein. 

 The combination of ammonia with carbon dioxide and the conversion 

 of the carbonate into urea, perhaps through the intermediate stage of 

 ammonium carbamate, does not require any oxidation. Thus, 



/OH /O.NH, /O.NH 4 /NH fi 



, 

 \OH \O.NH 4 \NH 2 \NH 2 



Carbonic acid. Ammonium Ammonium Urea. 



carbonate. carbamate. 



Another way in which some of the urea may be produced is by the 

 direct formation of ammonium carbamate in the katabolism of amino- 

 acids without the preliminary liberation of ammonia. By the loss of a 

 molecule of water the carbamate would then become urea. But if, as 

 there is every reason to believe, a part of the carbonaceous residue is 

 converted into carbo-hydrate, a certain amount of oxidation must 

 occur in the transformation. 



Such compounds as guanin, sarkin or hypoxanthin, xanthin, uric 

 acid, and creatin, used to be cited as among the possible intermediate 

 substances between protein and urea. But while there is now complete 

 evidence that the first three bodies can be and are converted into uric 

 acid, there is nothing at all to indicate that they are stages on the 

 way to urea. Uric acid is, indeed, very closely related to urea, and 

 can be made to yield it by oxidation outside the body. Not only so, 

 but it is, in part at least, excreted as urea when given to a mammal 

 by the mouth and it replaces urea as the great end-product of nitro- 

 genous metabolism almost wholly in the urine of birds and reptiles. 

 But none of these things can be admitted as evidence that in the 

 normal metabolism of mammals uric acid lies on the direct line from 

 protein to urea. Creatin exists in the body in greater amount than 

 any of these, muscle containing from 0-2 to 0-4 per cent, of it; and the 

 total quantity of nitrogen present at any given time as creatin is not 

 only greater than that of the nitrogen present in urea, but greater 

 than the whole excretion of nitrogen in twenty-four hours. But 

 although there are facts which indicate that creatin is an important 

 derivative of the decomposed tissue proteins (p. 597) there is no 

 evidence that it is related to urea formation. 



Summary : Here may be summed up the most probable view of the 

 normal decomposition in the body of the amino-acids, whether they undergo 

 decomposition without being incorporated into the tissue proteins or proto- 



