METABOLISM. 359 



Further, some of the amino-acids seem to be necessary for the 

 production of certain definite substances in the body, one of which 

 perhaps is adrenalin. 



Comparatively little is known as to the intermediate stages in the 

 breaking down of protein in the tissues, though the composition of 

 the urine during starvation shows that the end products include urea, 

 uric acid, and creatinine. 



PURINE METABOLISM. 



The nucleo-proteins of the food are broken down in the digestive 

 tract first into nuclein and protein, the nuclein subsequently under- 

 going further digestion with the setting free of nucleic acid, which is 

 absorbed unchanged. Nucleic acid, when hydrolysed, is found to 

 consist of the following bodies: (1) phosphoric acid, (2) purine 

 bases, guanine and adenine, (3) pyrimidine bases, and (4) a carbo- 

 hydrate which is usually a pentose. The same products are yielded by 

 the disintegration of the nucleins present as nucleo-protein in the tissues. 

 The nucleic acids found in the different tissues vary in composition, 

 arid do not necessarily contain all the constituents just mentioned. 



The purine bodies are all derivatives of a substance called purine, 

 C 5 H 4 N 4 , which has the constitutional formula : 



N = CH 



I i 



HC C NH 



N C N 



Purine itself is of purely theoretical interest, but five of its de- 

 rivatives are found in the body, namely : 



Hypoxanthine (monoxy-purine) . . C 5 H 4 N 4 0. 



Xanthine (dioxy-purine) .... C 5 H 4 N 4 2 . 

 Adenine (amino-purine) . . . C 5 H 3 N 4 . NH 2 . 

 Guanine (amino-oxy-purine) . . C 5 H 3 N 4 . NH 2 . 

 Uric acid (trioxy-purine) .... C 5 H 4 N 4 3 . 



After its absorption the nucleic acid taken as nucleo-protein in the 

 food is broken down by a series of enzymes, called nucleases, which 

 are found in many tissues, notably the liver and spleen, first into 

 complex groupings called nucleotides, and then into adenine, guanine, 

 and other bodies. Other ferments subsequently convert adenine and 

 guanine by a process of deamination into hypoxanthine and xanthine. 

 Finally, a third set of enzymes oxidise hypoxanthine to xanthine, and 



