338 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Now, lysatinin occurs also in the body as one of the products of the con- 

 tinued action of trypsin on proteids (see p. 303). It is possible, therefore, that 

 by further hydrolysis this substance, when it occurs, is converted to urea, and 

 that normally a part of the urea arises from proteids by this process. 



Uric Acid and Xanthin Bodies. — Uric acid, which has the formula 

 CgH 4 X 4 3 , is found constantly, but in relatively small quantities, in human 

 urine and in the urine of mammals generally. The total quantity in the urine of 

 man under normal conditions varies from 0.2 to 1 gram every twenty-four hours. 

 In the urine of birds and reptiles it forms the chief nitrogenous constituent. In 

 these animals it takes the place physiologically of urea in mammalia in that it 

 represents the main end-product of the metabolism of the proteids in the body. 

 It i» evident that at some point in the process the metabolism of the proteids 

 in mammalia differs from that in birds and reptiles, since in the one urea, 

 and in the other uric acid, is the outcome. Uric acid occurs in such small 

 quantities in mammals that its place of origin has been investigated with dif- 

 ficulty. Among birds and reptiles uric acid represents the chief nitrogenous 

 excretion of the urine, taking the place physiologically of urea in the mam- 

 malia. As in the case of urea, it has been shown that in birds uric acid 

 originates in the liver. Extirpation of the kidneys in these animals leads 

 to an accumulation of uric acid in the blood and tissues. Removal of 

 the liver, on the contrary, causes a decrease in the excretion of uric acid and 

 an increase in the ammonia contents of the urine. It may be concluded, 

 therefore, that in birds uric acid is formed in part in the liver from ammonia 

 compounds (ammonium lactate). Reasoning from analogy, we should sup- 

 pose that in the mammalia uric acid has a similar origin, but experiments 

 fail to support this view. When a mammal is fed with ammonium lactate 

 or urea there is no increase in the excretion of uric acid. Within recent 

 years a new hypothesis has been advanced by Horbacewsky, and consider- 

 able experimental evidence has since given material support to his views. 1 

 According to Horbacewsky, uric acid may be regarded as a specific end- 

 product of the nucleins contained in the nuclei of cells, and is formed by an 

 oxidation of a grouping in the nuclein which may also give rise to other 

 members of the class of so-called alloxuric bases, such as xanthin, hypo- 

 xanthin, or adenin. Feeding a man with food rich in nucleins — the thymus 

 gland, for instance — leads to a marked increase in the excretion of uric acid, 

 and feeding with one, at least, of the alloxuric bases, hypoxanthin, gives a 

 similar result. On this view uric acid should give an indication of the 

 extent of the katabolism or disintegration of the cell-nuclei, especially 

 perhaps in the lymphoid tissue. It is probable, however, that the actual 

 amount of uric acid excreted in the urine does not represent truly the entire 

 amount formed in the body. When uric acid is fed to an animal it does not 

 all reappear in the urine, indicating that this substance may undergo metab- 

 olism in the body to a limited extent, its nitrogen appearing probably as 

 urea. Possibly, therefore, some of the uric acid normally produced in the 

 body undergoes a similar fate, only a portion escaping in the urine. 



1 Minkowski : Archivfur ezperimenteUe Pathologie wnd Pharmakoloffie, Bd. 41, S. 375. 



