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 omithurie 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 
O O O 
and tyrosine. 
1 Jade, Ztschr. f. physiol. Ghem., Strassburg, 1883, Bd. vii. S. 306. 
2 According to the recent observations of Ulrich, leucine and tyrosine are always to be 
found in normal urine, though in small quantity, Centralbl, f. Physiol., Leipzig u. Wien, 
1897, Bd. xi. S. 12. 
3 Cf. Baumann, Ztschr. f. physiol. Chem., StrassbuTg, 1884. Bd. viii. S. 299; also 
Brensinger, ibid., 1S92, Bd. xvi. S. 552. 
