208 PHYSIOLOGICAL CHEMISTRY. 



and by SCHERER'S test. This last consists in the leucin yielding a 

 colorless residue when carefully evaporated with nitric acid on 

 platinum-foil, and this residue when warmed with a few drops of 

 caustic soda gives a color varying from a pale yellow to brown (de- 

 pending on the purity of the leucin), and on further concentrating 

 over the flame it agglomerates into an oily drop which rolls about 

 on the foil. 



Tyrosin, C 9 H U N0 3 , or p. OXYPHENYL-AMIDOPROPRIONIC ACID, 

 HO.C 6 H 4 .C 2 H 3 (NH 2 ).COOH, is derived from most protein sub- 

 stances (not gelatin) under the same conditions as leucin, which it 

 habitually accompanies. It is especially found with leucin in large 

 quantities in old cheese (Tvpos), from which it derives its name. 

 Tyrosin has not with certainty been found in perfectly fresh organs, 

 with the exception, perhaps, of the spleen and pancreas of cattle. 

 It occurs in the intestine in the digestion of albuminous sub- 

 stances, and it has about the same physiological and pathological 

 importance as leucin. 



Tyrosin was prepared by ERLENMEYER and LIPP from p. amido- 

 phenylalanin by the action of nitrous acid. . On fusing with caustic 

 alkali it yields p. oxybenzoic acid, acetic acid, and ammonia. By 

 putrefaction it may yield p. hydrocoumaric acid, oxyphenyl-acetic 

 acid, and p. cresol. 



Tyrosin in a very impure state may be in the form of balls 

 similar to leucin. The purified tyrosin, on the contrary, appears as 

 colorless, silky, fine needles which are often grouped into tufts or 

 balls. It is soluble with difficulty in water, being dissolved by 2454 

 parts water at -j- 20 C. and 154 parts boiling water, separating, 

 however, as tufts of needles on cooling. It dissolves more easily 

 in the presence of alkalies, ammonia, or a mineral acid. It is 

 difficultly soluble in acetic acid. Crystals of tyrosin separate from 

 an ammoniacal solution on the spontaneous evaporation of the 

 ammonia. It is not soluble in alcohol or ether. Tyrosin is known 

 by its crystalline form and by the following reactions : 



PIRIA'S Test. Tyrosin is dissolved in concentrated sulphuric 

 acid by the aid of heat, by which tyrosin sulphuric acid is formed; 

 it is allowed to cool, diluted with water, neutralized by BaC0 3 , 

 and filtered. On the addition of a solution of ferric chloride, the 

 filtrate gives a beautiful violet color. This reaction is impeded by 



