J54 LIFE ; OUTLINES OF GENERAL BIOLOGY 



According to Folin's theory of protein metabolism, already 

 referred to, the amino or — NH^ groups of amino-acids are trans- 

 formed into ammonia, and this ammonia is converted into urea, 

 which in man is the principal nitrogenous waste- product. Within 

 eight or ten hours of a meal, nearly the whole of the nitrogen in the 

 protein consumed is excreted m the form of urea. This urea, whose 

 nitrogen is directly derived from the amino-acids of the food, is 

 called exogenous; but there is also a much smaller excretion of 

 endogenous urea, due to the break-up of the proteins which actually 

 form part of the living tissues, and are always suffering a certain 

 amount of wear and tear. 



Urea, with its formula 



/NHa 



co( 



^NH, 



is the simplest and in man the most important in amount of the 

 nitrogenous waste-products, but it is by no means the only one. 

 Thus, of undoubted importance, though somewhat puzzling in 

 origin, is creatinine, 



/NH-CO 

 HN=C( I 



^N CH, 



I 

 CH3 



which appears to be almost entirely endogenous, since the amount 

 excreted seems almost wholly independent of the amount of nitrogen 

 in the food. Creatine, a closely related product, occurs in considerable 

 amount in the muscles (which always contain various nitrogenous 

 "extractives" of this type), yet it is by no means certain that the 

 creatinine of the urine is derived from this. Another important 

 waste-product, notably in Invertebrates, is uric acid, 



NH CO 



I I 



CO C--NHv 



I ii ;co 



NH C— NH-^ 



of which part is derived from the food and part is derived from 

 the breakdown of the nucleo-proteins, which, as we have seen, 

 contain substances of the pyrimidine type. The pyrimidine ring 



-N C= 



I I 



=c c 



I !! 



-N C- 



