770 PHYSIOLOGY 



THE FATE OF THE AROMATIC AND OTHER CYCLIC 

 GROUPS IN THE PROTEIN MOLECULE 



A typical protein such as can be utilised as a complete food-stuff con- 

 tains, in addition to the amino-acids of the fatty series, a number of other 

 nitrogenous derivatives of cyclic compounds, including benzene, indol, pyrrol, 

 and iminazol. Substances such as gelatin, from which some of these 

 groupings are absent, cannot, as we have seen, entirely replace protein in the 

 food. So far we are acquainted with three compounds of the aromatic 

 series among the products of disintegration of the protein molecule. These 

 are tyrosine, phenylalanine, and tryptophane. Since these substances 

 are also contained in the protein constituents of the tissues we may assume 

 that, after they have been set free by the digestive hydrolysis of proteins, they 

 are absorbed and built up again with the other amino-acids in appropriate 

 groupings. Like these they are susceptible of complete oxidation in the 

 body, so that they can contribute to the supply of energy. Any one of these 

 substances, administered with the food or subcutaneously, is entirely de- 

 stroyed, with the production of urea, carbon dioxide, and water. In this 

 respect they present a marked contrast to almost all other compounds of 

 the aromatic series. In these we find that the benzene ring is extremely 

 stable, so that, although changes may occur in its side-chains, the benzene 

 ring itself appears intact in the urine, and is not broken up in the body. 

 Thus benzoic acid, benzylalcohol, and phenyl propionic acid, when 

 administered, are passed in the urine as hippuric acid (benzoyl glycine). 

 Indol and skatol, which are closely allied to tryptophane, undergo oxida- 

 tion in the body without further modification and appear in the urine as 

 conjugated aromatic sulphates. 



Some light is thrown on the conditions of breakdown of these aromatic 

 bodies by the study of a rare disorder in metabolism, which may occur in 

 certain families and is known as alcaptonuria. In this condition, which is 

 congenital and lasts throughout life, the urine darkens considerably when 

 made alkaline and exposed to the air. It has the power of reducing Fehling's 

 solution, so that the presence of sugar might be suspected. On analysis 

 the peculiarities of the urine are found to be due to the presence in it of a 

 substance known as homogentisic acid. This is dioxyphenyl acetic acid. 



CHgCOOH 



The amount of this substance in the urine bears a constant ratio to the 

 nitrogen excreted. It does not disappear during starvation, and is much 

 increased on a large protein diet. It must therefore be derived from the 

 disintegration of proteins both exogenous and endogenous. If tyrosine or 

 phenylalanine be administered to patients affected with this disorder, both 

 substances are quantitatively converted into homogentisic acid. The ratio 



