674 THE PRECURSORS OF UREA 



found that from 70-95 per cent, of its nitrogen was excreted 

 in the urine as urea. Since, as we have seen, arginin is split 

 into urea and ornithin, and each of these contains half of the 

 arginin-nitrogen, it is clear that a large part of the ornithin- 

 nitrogen must be excreted as urea. Thompson has since shown 

 that ornithin injected intravenously reappears in the urine in 

 part at any rate as urea. 



(c) The Purin and Pyrimidin Bases. These bodies are pro- 

 duced during the autolysis of glandular organs ; they are also 

 obtained by the hydrolysis with acids of nucleo-proteids, and are 

 known to be derived from the nucleic- acid portion of those 

 proteids. 



The pyrimidin bases, cytosin, thymin, and uracil, as their 

 formulae show, have an obvious relation to the purin bodies. 

 For purin can be synthesised from pyrimidin, consequently the 

 pyrimidin ring is contained within the purin ring. 



N = C.NH NH-CO NH-CO NH - CO 



We know nothing about their production in the body, but it is 

 possible that they represent a stage in the disintegration or 

 synthesis of the purin bases. Thymin and uracil, as their formula3 

 suggest, have been synthesised from urea, and might possibly 

 yield urea in the body, but as yet we know nothing of the fate 

 of their nitrogen. 



The purin bases, adenin, guanin, hypoxanthin, and xanthin, 

 although they occur plentifully during the autolysis of tissue 

 nucleo-proteids, are largely replaced in the urine by another 

 purin base, uric acid. The total purin N in the urine may not 

 amount to more than 1 to 3 per cent, of the whole urinary N. 

 It has two origins, the purins of the food and the disintegration 

 of nucleo-proteid in the body. The possibility of uric acid also 

 having a synthetic origin in mammals as in birds need not be 

 considered here. By placing a man on a purin-free diet it is 

 found that the endogenous purin N in the urine rapidly becomes 



