PROTEINS 



73 



The levo-rotatory form is obtained from proteins. Phenylalanine has 

 been obtained from all the proteins examined except from the pro- 

 tamines and some of the albuminoids. The yield of this body from the 

 decomposition of proteins is frequently greater than the yield of tyro- 

 sine. The crystalline form of phenylalanine is shown in Fig. 23. 



Tyrosine, C6H4(OH)-CH2-CH(NH 2 )-COOH. Tyrosine, one of the 

 first discovered end-products of protein decomposition, is the ammo 

 acid, a-amino-$-para-hydroxy-phenyl-propionic acid or hydroxy phenyl- 

 alanine. It has the following formula. 



H NH 2 



I I 

 ^C C COOH. 



H H 



OH 



The tyrosine which results from protein decomposition is usually levo- 

 rotatory. Tyrosine is one of the end-products of tryptic digestion and 

 usually separates in conspicuous amount early in the process of diges- 



FIG. 23. PHENYLALANINE. 



tion. It does not occur, however, as an end-product of the decomposi- 

 tion of gelatin. 



Tyrosine is found in old cheese, and derives its name from this fact. 

 It crystallizes in tufts, sheaves, or balls of fine needles, which decompose 

 at 095C. and are sparingly soluble in cold (1-2454) water, but much 

 more so in boiling (1-154) water. Tyrosine forms soluble salts with 

 alkalis, ammonia, or mineral acids, and is soluble with difficulty in 

 acetic acid. It responds to Millon's reaction, thus showing the presence 



