44 METABOLISM, NUTRITION, AND DIET 



major part of the urea eliminated, other organs or tissues are capable of 

 forming it to a limited degree. 



Formation of Uric Acid. The relation which uric acid and urea bear 

 to each other in different animals, as we have seen, is still obscure. The 

 fact that they exist together in the same urine makes it seem probable that 

 they have different origins. The entire replacement of one by the other, as 

 of urea by uric acid in the urine of birds, serpents, and many insects, shows 

 their close relationship. But although it is true that uric acid on oxidation 

 yields urea, this is not evidence that uric acid is an antecedent of urea in the 

 nitrogenous metabolism of the body. The chemical structure of the uric 

 acid shows it has a nucleus of purin, and therefore is a close relative of 

 adenin, guanin, hypoxanthin, xanthin, theobromin, caffein, etc. The nu- 

 cleins on cleavage yield members of this group, hence may be looked to as 

 the primary source of uric acid in man. Uric acid, according to Chittenden, 

 has a double origin endogenous from nuclear metabolism, and exogenous 

 from metabolism of foods rich in nuclear and other purin compounds. In 

 man, at least, the uric acid is to be ascribed to these two sources. 



Operative experiments on birds tend to show that the final step in uric- 

 acid formation takes place chiefly in the liver, for on the removal of this organ 

 ammonium compounds, i.e., lactates, accumulate in the blood. 



Hippuric Acid, Creatinin, etc. The hippuric acid found in the urine 

 is derived in part from aromatic constituents of vegetable diet which can be 

 transformed into benzoic acid in the body. It is derived in part also from 

 the phenyl propionic acid formed in the intestinal putrefaction of protein. 

 Hippuric acid is formed from the union of benzoic acid with glycocol 

 (C 2 H 5 NO 2 +C 7 H 6 O 2 =C 9 H 9 NO 3 +H 2 O): this union, in dogs, takes place 

 under experimental conditions in the kidneys themselves, but in other 

 animals the synthesis will occur after nephrectomy. 



The source of the nitrogenous extractives of the urine is chiefly from the 

 metabolism of the nitrogenous foods and tissues, but we are unable to say 

 whether these nitrogenous bodies have merely resisted further decomposition 

 into urea, or whether they are the representatives of the decomposition of 

 special tissues, or of special forms of metabolism of the tissues. There is, 

 however, one exception, and that is in the case of creatinin. This represents 

 not only the creatinin which enters the body in ordinary flesh food, but is a 

 nitrogenous w r aste which Folin regards as a measure of muscle metabolism. 

 The creatinin eliminated is almost a constant quantity in a given individual, 

 irrespective of the quantity of protein in the diet. Koch has shown some 

 relation of creatinin excretion to the amount of lecithin in the food. 



THE METABOLISM OF FATS. 



Fats, with carbohydrates, are the direct source of most of the energy 

 manifested by the body, a fact demonstrated by numerous observations. 



