186 PHYSIOLOGICAL CHEMISTRY 



tion of nucleic acid. Thus the nucleoproteins are the ultimate 

 source. This fact increases interest in uric acid, since evidently it 

 is a product of the breaking down or catabolism of nuclear ma- 

 terial. The purine bases, adenine and guanine, are constituents of 

 nucleic acid. In the body these compounds are oxidized to 

 hypoxanthine and xanthine, and these latter substances to uric 

 acid. These purines may come from the breaking down of nu- 

 clear material in the food, or in the tissues themselves. This 

 fact may be demonstrated, for if glandular material such as 

 sweet breads (pancreas) is fed, there is an increase in the uric 

 acid of the urine. Since this uric acid has an origin outside the 

 tissues, and comes from material which probably is at no time a 

 constituent of the cells, it is called exogenous uric acid, i. e., com- 

 ing from without. It is interesting that all of the ingested 

 purines are not recovered in the urine as uric acid. Evidently a 

 portion either is not absorbed, or is destroyed in the body. If 

 an animal is kept upon a purine free diet, uric acid does not dis- 

 appear from the urine; from 0.3-0.5 grams a day still is 

 excreted. This must come from the nuclear material of the tissues 

 themselves, and hence is called endogenous uric acid, i.e., coming 

 from within. There is a possibility that at least a portion of 

 this uric acid comes from the nuclear material of dead bacteria 

 in the intestine, but it is generally considered to arise mainly in 

 the catabolism of the tissue nucleins. 



The transformation of the purines, adenine and guanine, into 

 uric acid involves several steps. Quite recently this process has 

 been carefully studied, and enzymes have been found in vari- 

 ous tissues which possess the property of carrying out all the 

 intermediate stages of the process. 



In most mammals an enzyme uricase occurs which has the 

 power of destroying uric acid, allantoin being one of the chief 

 products. This enzyme has not been found in man, however, 

 and it is probable that man has lost the power of destroying uric 

 acid. To be sure, if uric acid is given to a human being by 

 mouth, only a small portion of it appears in the urine. This 

 fact is still unexplained. Perhaps intestinal bacteria are respon- 

 sible for its destruction. 



