592 METABOLISM, NUTRITION AXD DIETETICS 



the urine. The increase is mainly due to the production of uric 

 acid from the nuclein substances of the food. But this is not the 

 only source of the uric acid, since extracts of the thymus gland 

 containing only traces of nucleins or nucleic acid cause, when in- 

 jected, a characteristic increase in the uric acid excretion, just as 

 the entire gland does when taken by the mouth. And during the 

 period of increased nitrogen excretion occasioned by a meal contain- 

 ing protein, the increase in the uric acid occurs particularly in the 

 hours immediately following the ingestion of the food, and does not 

 last so long as the increase in the urea. Now, the nucleins of the 

 food are comparatively little affected during the earlier stages of 

 digestion (Hopkins and Hope). Whether in mammals any portion 

 of the uric acid comes from amino-acids is still in doubt, but there 

 are facts which indicate that a fraction of it may do so. We may 

 conclude, therefore, that in the mammal, as well as in the bird, a 

 portion of the uric acid, although certainly a far smaller portion in 

 the mammal, is derived from bodies other than the nuclein substances 

 of the food that is io say, from the nuclein substances of the tissues 

 contained particularly in the cell-nuclei and probably from the 

 ordinary proteins of both food and tissues. The portion derived 

 irom the proteins may be assumed to be that small fraction which 

 has already been spoken of as synthetically formed. 



Metabolism of the Nucleic Acids and Purin Bases. Our know- 

 ledge of the metabolism of the nucleo-proteins and nucleins has 

 been greatly augmented in recent years. When nucleo-protein is 

 digested by gastric juice, a certain amount of protein is easily split 

 off and hydrolysed to peptone and the other ordinary products 

 of proteolysis. An insoluble residue of nuclein remains. This is 

 acted upon with difficulty by gastric juice, although eventually an 

 active juice will split it up also. By the action of pancreatic juice, 

 or by heating with dilute acids, it is more easily hydrolysed, yielding 

 a further quantity of protein along with nucleic acid. This second 

 fraction of protein, which is split off with so much more difficulty 

 than the first, undergoes proteolysis in the usual way. The result- 

 ing amino-acids no doubt take their place in the general metabolism 

 precisely like the amino-acids derived from ordinary proteins, and 

 yield the same end-products. As regards the nucleic acid (or rather 

 acids, since different nucleo-proteins contain different nucleic acids), 

 pancreatic juice is practically inert, although succus entericus can 

 effect a partial hydrolysis. For their complete decomposition more 

 drastic treatment is required namely, heating with hydrochloric 

 acid in a sealed tube. Thus treated, nucleic acids yield a number 

 of components, out of which they may be assumed to be built up, 

 as the proteins are built up out of amino-acids, etc. The charac- 

 teristic components are purin bases (adenin, C 5 H 3 N 4 .NH 2 ; guanin, 

 C 6 H,N 4 O.NH 2 ; hypoxanthin, C 6 H 4 N 4 O; and xanthin, C 5 H 4 N 4 O 2 ); 



