602 



THE CHEMISTRY OF THE URINE. 



are occasionally found in human urine. Its origin and significance are 

 analogous to those of the more abundant substance. 



When benzoic acid or its precursors are administered to birds, they 

 are excreted as ornithuric acid, which is an analogous conjugated com- 

 pound of benzoic acid with diamidovalerianic acid. 



(h) Amido- acids. These, in simple unconjugated form, are seldom 

 found in normal urine. Under certain pathological conditions leucine 

 and tyrosine appear in considerable quantities. The elimination of these 

 substances is especially associated with conditions in which a rapid 

 destruction of the hepatic tissue has occurred ; thus they are found in 

 acute yellow atrophy of the liver, and, to a less extent, in phosphorus- 

 poisoning. 



When these amido-acids are given by the mouth in moderate 

 quantity, and under conditions of normal health, their nitrogen is 

 excreted wholly in the form of urea. If, however, tyrosine be adminis- 

 tered in very large amounts, it may be excreted in part as tyrosine- 

 hydantoin, in which it exists as a conjugate compound with urea ; 1 and 

 at the same time other aromatic constituents of the urine are increased 

 in quantity by derivation from its aromatic nucleus. Only when the 

 normal hepatic functions are in abeyance does the unaltered amido-acid 

 itself appear. 2 



When present in urine, leucine and tyrosine are usually found 

 together. If in large quantity, they may, though very rarely, form 

 a deposit; at other times they may be seen under the microscope 

 when a drop of the urine is evaporated. 



In general, however, they must be 

 separated by special means. The 

 leucine may be dissolved, by means of 

 hot alcohol, from the residue obtained 

 by evaporating the urine, and when 

 the alcoholic extract cools it separates 

 as a greasy mass, which under the 

 microscope will be seen to consist of 

 minute spheroids with concentric mark- 

 ings interrupting a radiated structure. 

 To demonstrate the presence of tyrosine, 

 the urine is first precipitated with 

 basic acetate of lead, the filtrate from 

 the lead precipitate treated with sul- 

 phuretted hydrogen and again filtered. 

 On thorough concentration and cool- 

 ing of the lead-free filtrate, the tyrosine 



separates out in characteristic acicular prisms, which are mostly combined 



into sheaves or stars (Fig. 54). 



FIG. 54. Leucine and Tyrosine. 



Cystine 3 is another amido-acid, but it is at the same time a sulphur- 

 containing substance, differing in its metabolic significance from leucine 

 and tyrosine. 



1 Jaffe, Ztschr.f. physioL Chem., Strassburg, 1883, Bd. vii. S. 306. 



2 According to the recent observations of Ulrich, lencine and tyrosine are always to be 

 found in normal urine, though in small quantity, Centralbl. f. PhysioL, Leipzig n. "Wien, 

 1897, Bd. xi. S. 12. 



3 Of. Baumann, Ztschr. f. physioL Chem., Strassburg, 1884. Bd. viii. S. 299 ; also 

 Brensinger, ibid., 1892, Bd. xvi. S. 552. 



