EXCRETION 441 



in nucleins e.g., the thymus or substances containing purin 

 bases e.g., hypoxanthin in meat. When the amount of pro- 

 tein in the food is greatly reduced, the absolute quantity of uric 

 acid is diminished, but the proportion of the total nitrogen of the 

 urine eliminated as uric acid is increased, since the ' endogenous ' 

 uric acid (p. 507) still continues to be formed and excreted. 



The purin bases (sometimes called the nuclein bases, the alloxuric 

 bases, or the xanttiin bases) are a group of substances allied to uric 

 acid, and including, besides xanthin itself, hypoxanthin, guanin, 

 adenin, and other bodies. They exist in very small amount in urine, 

 but, like uric acid, are increased in amount by the ingestion of 

 nuclein-containing substances. The greater part of the purin bases 

 produced in the body is transformed into uric acid ; it is only 

 the untransformed residue which appears in the urine. An interest- 

 ing fraction of the purin bases in the urine which is not related to 

 the nuclein metabolism is composed of the so-called heteroxanthin, 

 derived from caffeine in the coffee and tea, /-methylxanthin, derived 

 from theobromine in the cocoa, and paraxanthin, derived from theo- 

 phyllin in the tea, consumed as beverages. 



The purin bodies are so called because they, and uric acid also, are 

 all to be considered chemically as derivatives of a substance called 

 purin (C 5 H 4 N 4 ), which, however, has itself never been discovered 

 in the body. Their empirical formulae are as follows : 



Purin - C 5 H 4 N 4 . 



Hypoxanthin - C 5 H 4 N 4 O Monoxypurin } g ^ 



Xanthin C 5 H 4 N 4 O 2 - Dioxypurin I <B 



Adenin C 5 H 3 N 4 .NH 2 Amino-purin | j 



Guanin - C 5 H 3 N 4 O.NH 2 - Amino-oxypurinJ 



Uric acid - C 5 H 4 N 4 O 3 - Trioxypurin. 



Hippuric acid (C 9 H 9 NO 3 ) occurs in considerable quantity in the 

 urine of herbivora (Practical Exercises, p. 486) ; in the urine of 

 carnivora and of man only in traces ; in that of birds not at all. Its 

 amount is much more dependent on the presence of particular sub- 

 stances in the food than that of the other organic constituents of 

 urine. Anything which contains benzoic acid, or substances which 

 can be readily changed into it (such as cinnamic and quinic acids), 

 causes an increase of the hippuric acid in urine. In fact, one of the 

 best ways of obtaining the latter is from the urine of a person to 

 whom benzoic acid is given by the mouth ; the sweat may also in 

 this case contain a trace of hippuric acid. Chemically, it is a con- 

 jugated acid formed by the union of benzoic acid and glycin. Thus : 



C 7 H 6 O 2 + C 2 H 5 NO 2 = C 9 H 9 NO 3 + H 2 O. 



Benzoic acid. Glycin. " Hippuric acid. Water. 



Benzoic acid, therefore, meets glycin in the body, and combines with 

 it, as fatty acids meet glycerin and combine with it. But while a 

 minute amount of free glycerin has been found in the plasma, no 

 free glycin has been detected in the normal blood or tissues. Glycin, 

 however, has been demonstrated in the blood and ascitic fluid in 

 cases of nephritis. 



Amino-acids. The only amino-acid hitherto detected with cer- 

 tainty in normal urine is glycin. 



Oxalic acid is always present, although in' very small amount. 

 Some of it comes from the oxalates of the food, but a portion of it 



