HIPPURIC A CID. 60 1 



in many fruits and in the cortical parts of most plants. Vegetable food 

 greatly increases, therefore, the excretion of hippuric acid. 



But the vegetable aromatic compounds are not the sole source of the 

 urinary hippuric acid. In the decomposition of proteids, which occurs 

 in the bowel, aromatic residues split off. Precursors of benzoic acid 

 (mainly, perhaps, phenylpropionic acid) are thus formed, and after 

 oxidation they appear in the urine as hippuric acid. The metabolism 

 of the tissue proteids themselves, moreover, may yield precursors of the 

 same kind, so that even in starvation hippuric acid does not wholly dis- 

 appear from the urine. 



This dual origin (from aromatic precursors in the diet chiefly, but 

 likewise from proteid metabolism) is found also in the case of the other 

 aromatic constituents of the urine (p. 605). 



Upon a mixed diet the excretion of hippuric acid in human urine 

 amounts to about 07 grms. per diem ; upon a diet rich in fruits it may 

 be raised to three or four times this. In herbivora the quantity is much 

 larger ; the urine of cattle, for instance, often contains as much as 

 2 per cent., though, as might be expected, that of sucking calves only 

 contains small amounts. 



Properties. It forms four-sided prismatic crystals ending in two or 

 four facets, and often grouped in clumps (Fig. 53), of which the melting 

 point is about 187. It is but slightly soluble in cold water or alcohol ; 

 but both these solvents dissolve it easily when hot. It is soluble in 

 acetic ether, but not so in most other organic liquids. If heated to 240 

 it decomposes, benzoic acid subliming out and a reddish residue being left 

 behind. When first heated at this temperature a hay-like odour is given 

 off, which is succeeded by that of prussic acid. When boiled with strong 

 hydrochloric acid it splits up into its components, benzoic and amido- 

 acetic acids. The growth of the Micrococcus urece can bring about this 

 decomposition, so that stale specimens of urine often contain benzoic in 

 place of hippuric acid. Taken to dryness with nitric acid, it yields an 

 odour of nitrobenzene. 



Solutions of hippuric acid react acid to litmus, and even when very 

 dilute they impart a violet colour to congo-red. By the use of the latter 

 indicator Briicke proved the absence of the free acid from the urine. It 

 is present always as salts. It forms salts with bases, but does not 

 combine with acids. Its iron compound is insoluble in hot water, and 

 may be employed in separating the acid from its solutions. 



Isolation and estimation. The method of Bunge and Schmiedeberg a 

 consists in making an alcoholic extract of the urinary solids, evaporating off 

 the spirit, dissolving the residue in water, and, after acidifying with hydro- 

 chloric acid, shaking up repeatedly with successive quantities of acetic-ether. 

 On evaporating the latter, impure crystals of the acid are obtained, the 

 impurities being removed by treatment with petroleum-ether, in which 

 hippuric acid is insoluble. 



Tests. The substance is recognised by its crystalline form, by its 

 melting point, by its behaviour on heating, and by the formation of its 

 insoluble iron compound when neutral ferric chloride is added to its 

 solutions. 



In addition to hippuric acid, minute quantities of its homologue 

 phenaceturic acid (phenylacetylglycin), C 6 H 5 .CH 2 CO NH.CH 2 .COOH, 

 1 Arch. f. exper. Path. u. Pharmakol., Leipzig, Bd. vi. S. 235. 



