METABOLISM OF PROTEINS 581 



ferment is concerned, and the known mechanism of similar reactions 

 in the body scarcely permits the physiologist to acquiesce in any 

 other explanation. It must not be forgotten that the urinary 

 constituents which must come into contact with the ferment when 

 the kidney is crushed may injure or inhibit the enzyme. In 

 herbivora hippuric acid cannot normally be detected in the blood; 

 it is present in large quantities in. the urine ; it must therefore be 

 manufactured in the kidney, not merely separated by it. In certain 

 animals, as the dog, the kidney is the sole seat of the production 

 ol hippuric acid. But in the rabbit and the frog some of it must 

 also be formed in other tissues, for after extirpation of the kidneys 

 the administration of benzoic acid causes hippuric acid to appear 

 in the blood. It has, indeed, been recently shown that when the 

 rabbit's liver is perfused with blood containing benzoic acid, hip- 

 puric acid is produced. The benzoic acid required for the normal 

 excretion of hippuric acid comes mainly from substances of the 

 aromatic group contained in vegetable food, but a small amount is 

 produced in the body, since hippuric acid does not entirely dis- 

 appear from the urine in starvation. 



The differences which may exist in the metabolism of different 

 groups of animals is well illustrated by the fact that in birds, when 

 benzoic acid is given in the food, it unites not with glycin, but with 

 ornithin, a derivative of arginin, forming not hippuric acid, but 

 ornithuric acid (dibenzoyl-ornithin) . A much more important 

 instance of such a difference will be seen when we come to consider 

 the formation of urea and uric acid. 



The method by which the presence and the production of glyco- 

 coll in the body are demonstrated by coupling it with benzoic acid, 

 and so saving it from decomposition and bringing it to excretion, 

 can also be applied to other amino-acids. If instead of fishing 

 with the bait benzoic acid we fish with a bait caUed brom-benzol or 

 bromo-benzene (C 6 H 5 .Br), a substance derived from benzol by 

 the substitution of an atom of bromine for an atom of hydrogen, 

 we capture the amino-acid cystein in the form of a compound 

 called mercapturic acid, produced by the union of brom-benzol with 

 cystein and acetic acid, with oxidation and loss of water. In other 

 words, when this substance is administered, mercapturic acid is 

 excreted in the urine, the cystein, which is very unstable and readily 

 changes into cystin (p. 360), being thus preserved from decom- 

 position. Another instance in which amino-acids (tyrosin and 

 phenylalanin), which would normally be decomposed and so escape 

 detection, come to the surface by being excreted in the urine, has 

 a'ready been alluded to in connection with alkaptonuria (p. 483). 

 In this condition, which seems to have no serious significance so 

 far as the well-being of the patient is concerned, not only does the 

 taking of food containing the aforesaid amino-acids lead to an 



