672 METABOLISM 



The free base may then be split off from the nucleosides by specific 

 hydrolyzing enzymes (7) (8) (9 1 ) (10). 



The joint action of these enzymes leads to the formation of oxypurines, 

 xanthine and hypoxanthine, which are oxidized to uric acid by xanthine- 

 oxidase (11). 



In man and the anthropoid apes uric acid is the end product of the 

 above changes, but in other mammals most of the uric acid is further 

 oxidized into allantoine. It has also been found, except in man and the 

 chimpanzee, that extracts of organs such as the liver, are capable of 

 decomposing uric acid into allantoine. The identification of these specific 

 enzymes is sought by a determination of the free amino-purine bases 

 and the phosphoric acid produced by allowing an aqueous extract of 

 the tissue in question to act on nucleic acid (of yeast)* at body tempera- 

 ture. Another portion of the digested mixture is then hydrolyzed by 

 means of boiling sulphuric acid and the constituents again determined. 

 From the results it is often possible to draw conclusions as to the exact 

 nature of the enzymes present. 



The most remarkable outcome of this work has been to show that 

 the distribution of the enzymes is not the same in the tissues and organs 

 of different animals. Very briefly, some of the most important results 

 that have so far been obtained are as follows: Gastric and pancreatic 

 juices do not contain a trace of any of the enzymes. Intestinal juice, 

 on the other hand, contains a nuclease capable of splitting the poly- 

 nucleotides into mononucleotides. The two pyrimidine nucleotides split 

 off do not undergo further change, but the purine nucleotides are con- 

 verted into nucleosides (the enzyme being designated "nucleotidase")- 

 Extract of the intestinal mucosa, besides having the same action as the 

 intestinal juice, can also decompose the purine, but not the pyrimidine 

 nucleosides, into carbohydrate and purine groups (specific action of 

 "nucleosidase"). A similar action is produced by extracts of kidney, 

 heart muscle, and liver. Blood serum, hemolyzed blood, and extract of 

 pancreas, on the other hand, are capable of carrying the decomposition 

 only as far as the mononucleotides. 



Eegarding the other enzymes mentioned in the above list, it is im- 

 portant to note that they appear at different stages in embryonic develop- 

 ment, and that their distribution varies considerably in different species 

 of adult animal, the spleen, liver, thymus, and pancreas containing them 

 most abundantly. The distribution of enzymes in the organs of the 

 monkey resembles that in the lower animals considerably more than it 

 does that in man. Some remarkable facts have come to light regarding 

 guanase and adenase, particularly that guanase is deficient in the organs 



*Yeast nucleic acid is used because it is less resistant to disintegration than thymic nucleic acid. 



