474 TEXT-BOOK OF PHYSIOLOGY. 



cations being that it is combined with sodium and potassium in the form of a 

 quadriurate. The urates when in excess are frequently deposited from the 

 urine as a brick-red sediment, the color being due to their combination with 

 the coloring-matter uroerythrin. When pure, uric acid crystallizes in the 

 rhombic form, though it assumes a variety of forms. Uric acid 'was long 

 regarded as a product of general protein metabolism. This view has been 

 abandoned. At present it is believed that it is a cleavage product of nuclein, a 

 constituent of all cell nuclei. In the metabolism of nuclein a protein and 

 nucleic acid are formed, from the latter of which uric acid is derived. Nu- 

 cleic acid when decomposed yields a series of bases, such as xanthin, hypo- 

 xanthin, adenin, guanin, etc. Because of the fact that these bodies can also 

 be obtained from a synthesized body termed purin they are known collect- 

 ively as the purin bases. Though there is a close relationship between uric 

 acid and the purin bases, it has been impossible experimentally to derive one 

 from the other. When hypoxanthin, however, is given internally it is 

 oxidized and converted into uric acid. It is extremely probable, therefore, 

 that uric acid is an oxidation product of one or more of the purin bases. 



It is probable, however, that not all of the uric acid eliminated is derived 

 from the nuclein of tissue-cells and their decomposition products, the 

 purin bases. Some of it is undoubtedly derived from the nucleins contained 

 in foods. The uric acid eliminated is therefore partly endogenous and 

 partly exogenous in origin. 



There is some evidence that not all the uric acid produced in the body is 

 excreted as such, but that a portion perhaps one-half is changed to urea. 



Xanthin, hypoxanthin, guanin, etc., are also found in urine in small 

 but variable amounts. They are nitrogenized compounds derived mainly 

 from the metabolism of the nuclein bodies. 



Kreatinin is a crystalline nitrogenous compound closely resembling 

 kreatin, one of the constituents of muscle tissue. The amount excreted 

 daily is about i gram. The origin of kreatinin is not very clear. It is 

 probable, however, that as kreatin is capable of transformation into kreatinin 

 a certain portion is derived from the kreatin contained in the meat con- 

 sumed as food. But as kreatinin is steadily excreted though in less amounts 

 on a diet from which meat is excluded it is certain that this portion at least 

 must have some other source containing nitrogen, and the inference is that 

 it is one of the end-products of the protein metabolism that is taking 

 places in tissues generally and more particularly in muscle tissue. 



Hippuric acid in combination with sodium and potassium is very gener- 

 ally present in urine, though in small amounts. It is more abundant in the 

 urine of the herbivora than the carnivora. In man the amount excreted 

 daily is about 0.7 gram, though the amount may be raised by a diet of 

 asparagus, plums, cranberries, etc., and by the administration of benzoic and 

 cinnamic acids. There is evidence that hippuric acid is formed in the 

 kidney from benzoic acid, its precursors, or related bodies. Various com- 

 pounds of this class are found in vegetable foods, a fact which may account 

 for the increase in the excretion of hippuric acid on a vegetable diet. 



Indol, skatol, phenol, cresol, products of the putrefactive changes in the 

 derivatives of protein are present in variable amounts, associated with 

 potassium sulphate (see page 454). These compounds are known as the 



