EXCRETION 457 



the metabolism of tissue protein, as well as those stored, undergo a similar 

 chemic change with the formation of urea. This being the case, the muscles 

 must be regarded as a seat of urea formation. 



Uric Acid. Uric acid is one of the constant ingredients of the urine. 

 It is a crystalline nitrogen-holding body closely resembling urea, its formula 

 being C 5 H 4 N 4 O 3 . The total quantity excreted daily varies from 0.2 to i 

 gram. It is doubtful if uric acid exists in a free state in the urine, the indi- 

 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 bases, such as guanin, adenin, etc., which 

 under the action of the enzymes guanase andadenase are combined with water, 

 deaminized, and converted into xanthin and hypoxanthin. Because of the 

 fact that these bodies can also be obtained from a synthesized body termed 

 purin they are known collectively 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 ex- 

 tremely 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. 



Adenin, Guanin, Xanthin, Hypoxanthin. These compounds are also 

 found in urine in small but variable amounts. They are nitrogenized com- 

 pounds derived mainly from the metabolism of the nuclein bodies. 



Kreatinin. This is a crystalline nitrogenous compound closely resem- 

 bling 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 if 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. This acid in combination with sodium and potassium 

 is very generally present in urine, though in small amounts. It ;s 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 



