THE PRECURSORS OF UREA 675 



constant in quantity and forms about a half to two-thirds of 

 the total urinary purins excreted by the same man on a mixed 

 diet. But the amount of exogenous purin found in the urine is 

 much less than that in the food. Burian and Schur showed that, 

 when a known quantity of a purin was given by the mouth, a 

 man excreted 50 per cent, of it, a rabbit 16 per cent., and a 

 dog only 5 per cent, of the amount introduced. They further 

 found that, with the exception of the methyl derivatives of 

 purin, whichever purin body was given it was excreted in 

 the urine as uric acid. From this they concluded that the uric 

 acid formed from the exogenous purins of the food is largely 

 destroyed in the body. Probably the same is true of the 

 endogenous purins also, but it is much more difficult to demon- 

 strate experimentally. Perhaps the most convincing evidence 

 for this supposition is the fact that many tissues possess ferments 

 which can convert the various purins into uric acid and further 

 break it up. Schittenhelm found that watery extracts of the liver, 

 spleen, lungs, and muscle possessed ferments which converted 

 adenin into hypoxanthin and guanin into xanthin, and then 

 these into uric acid. He found, like previous observers, that 

 the liver, kidney, and muscle possess a ferment or ferments which 

 can break up uric acid. 



It seems pretty clear that the purin N in the urine does not 

 represent the total purin N metabolised in the body, and there 

 can be no doubt that the bulk of the rest of the metabolised 

 purin is excreted as urea in mammals. The older experiments 

 are not conclusive because of the methods of urea estimation 

 used. Salkowski has given uric acid by the mouth to a rabbit 

 and found that the greater part of the absorbed uric acid was 

 excreted as actual urea, a small portion unchanged and another 

 small portion possibly as allantoin. It is not known what pro- 

 portion of the whole urea is thus produced from uric acid, but 

 it is not likely to be large. For, the purin content of food is 

 comparatively small, and the total quantity of nucleo-proteid 

 metabolised in the tissues is likely to be small when compared 

 with that of other proteids. 



It is necessary to consider the chemical changes involved in 

 the breaking up of uric acid. The structure of uric acid suggests 

 that urea might be obtained from it direct, just as from the 

 pyrimidin bases. This can be readily brought about by its 



