156 INTERMEDIATE METABOLISM 



Example : Acetic acid is completely oxidised in the body. 

 Theoretically, either formic acid or oxahc acid might be an 

 intermediate compomid. But oxaUc acid on injection is 

 excreted michanged, whereas formic acid is oxidised. The 

 oxidation of acetic acid therefore takes place thus — - 



CH3COOH -> HCOOH -> CO2 + H2O 



rather than thus — 



COOH 



CH3C00H-> I ->C02 + H.O. 



COOH 



3. Achninistration in Excess. — When a substance is 

 injected in excess of the amount which can be completely 

 oxidised it often appears in the urine in an incompletely 

 oxidised form. 



Example : Xanthine administered in small quantities 

 to most animals is converted into allantoine. Adminis- 

 tered in excess it appears partly as allantoine, partly as 

 uric acid. Uric acid is therefore an intermediate stage. 



4. Perfusion ami Digestion with Tissue-Pulps. — By this 

 means have been proved the conversion of ammonia into 

 urea by the hver and many other reactions. 



5. PatJiological Method. — When an abnormal substance, 

 A, is excreted owing to a pathological condition, if the 

 administration of a substance B, leads to increase in the 

 amount of A, the inference is drawn that B is converted 

 into A, and that the same change may occur under normal 

 conditions, but is masked omng to the complete oxidation 

 of A. 



Exam])le : Administration of certain amino-acids leads 

 in diabetes to an increase in the amount of glucose excreted. 

 The body therefore possesses the power of converting 

 protein into carbohydrate. 



An interesting instance of this method is found in the 

 abnormahty known as alcaptonuria. In this condition 

 homogentisic acid is excreted by the kidney, and the corre- 



