1274 TYROSINE. 



which means the acid is fixed as a hippurate of calcium. It is 

 then filtered, the filtrate concentrated to a small bulk and 

 treated when cold with hydrochloric acid in slight excess ; this 

 decomposes the calcium salt, liberating hippuric acid, which 

 separates out at once owing to its comparatively slight solubil- 

 ity. It is then purified by several recrystallizations from boil- 

 ing water, but it is extremely difficult to obtain it colourless. 



When rapidly separated out from its aqueous solutions, as 

 in the above method of its preparation, it assumes the form of 

 fine needles. By slower crystallization it yields long four-sided 

 prisms or columns with pyramidal ends ; these are frequently 

 arranged in groups and present a semitransparent, milky ap- 

 pearance. 



When pure they are odourless and of a somewhat bitter 

 taste. They require 600 parts of water for their solution at 0°, 

 are very readily soluble in hot water, also in alcohol and to a 

 less extent in ether. They are conveniently insoluble in 

 petroleum-ether, in virtue of which hippuric acid can be readily 

 separated from benzoic acid, which is soluble in this reagent. 

 Its solutions redden litmus-paper. ■ 



Apart from the characteristics already stated the acid may 

 be recognized by the following reactions. When gently heated 

 in a small tube the acid does not at once sublime as does ben- 

 zoic acid, but melts and solidifies ao-ain on cooling. If more 

 strongly heated it melts as before but is now decomposed, 

 yielding a sublimate of benzoic acid accompanied by an odour 

 like that of new hay, while oily red drops are observed in the 

 tube. When treated with boiling nitric acid (see above mb 

 benzoic acid) and evaporated to dryness, the residue, on being 

 heated, yields the marked and characteristic odour of nitro- 

 benzene. 



3. Tyrosine. C 9 H n N0 3 . [OH . C 6 H 4 . CH 2 . CH(NH 2 ) . 

 COOH.] (Para-oxyphenyl-a-amidopropionic acid.) 



This substance always accompanies leucine, though less in 

 amount, as a product of the pancreatic digestion of proteids, 

 but not of gelatin, also as a product of their putrefactive 

 decomposition as well as of the action of boiling mineral acids 

 and alkalis. It is also perhaps found normally in small quan- 

 tities in the pancreas and its secretion and in the spleen, and 

 traces have been described as obtained from various tissues of 

 the body. It is normally absent in urine but makes its appear- 

 ance together with leucine in this excretion in several diseased 

 conditions of the liver, notably acute yellow atrophy, also in 

 phosphorus poisoning ; there is however some conflict of opinion 

 as to its constancy in such cases. It is also present in not 

 inconsiderable quantities, along with leucine, in many plant 

 tissues. 



