THE METABOLISM OF PROTEIN 663 



ways, mainly by forming inert compounds with other substances, par- 

 ticularly with glycocoll, sulphuric acid or glycuronic acid. The com- 

 pound thus formed is then excreted in the urine. 



Hippuric Acid, Glycocoll is used mainly to detoxicate the benzoic 

 acid which results from the oxidation of the aromatic substances pres- 

 ent in large quantities in vegetable food and fruit (particularly in cran- 

 berries). Some benzoic acid may also be produced by the breakdown 

 of the aromatic group of the protein molecule; phenylalanine, for ex- 

 ample, gives rise to benzoic acid by bacterial decomposition. The com- 

 pound formed is hippuric acid, this name indicating that it is present in 

 large quantities in the urine of the horse, as it is also in the urine of 

 all herbivorous animals. 



Hippuric acid is benzoyl-glycine (C 6 H 5 .CO.NH.CH 2 COOH), and it 

 can readily be produced in the laboratory by bringing together benzoyl 

 chloride with glycocoll, thus: 



C 6 H 5 .CO : Cl + H HN . CH 2 COOH = C 6 H 5 CO . NH . CH 2 COOH + HC1. 

 (benzoyl chloride) (glycocoll) (hippuric acid) 



Under ordinary dietetic conditions only a trace of hippuric acid is 

 present in the urine of man, but much larger quantities, 2 grams a day 

 for example, may appear when the diet contains a large proportion of 

 fruit or vegetables. It is not known to undergo any characteristic varia- 

 tions in disease. The benzoic acid which is contained in certain canned 

 foods as preservative also combines in the body with glycocoll, so that 

 any toxic effect which it might produce is practically negligible. There 

 is certainly no very evident reason why canned foods containing benzoic 

 acid should be tabooed, for in so far as the benzoic acid is concerned, they 

 can be no more toxic than a diet composed largely of vegetables and 

 fruit. 



This detoxication of benzoic acid requires the presence in the organ- 

 ism of a constant supply of glycocoll, which, it will be recalled, 

 is the lowest in the series of amino acids, being aminoacetic acid 

 (CH 2 NH 2 COOH). It is present in greatest amount in the protein of the 

 connective tissues. It is said, however, that not more than from 2 to 

 3.5 per cent of glycocoll is available in the proteins of the body. Al- 

 though this amount of glycocoll would amply suffice to detoxicate the 

 benzoic acid produced by the metabolism of the food in carnivora, it 

 is quite inadequate for this purpose in the case of herbivora, and the 

 question naturally presents itself as to where the glycocoll in these 

 animals comes from. It is said, for example, that of the total nitrogen 

 excretion in herbivora 50 per cent may appear as glycocoll under cer- 

 tain conditions. These facts along with those gained by observations on 



