370 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



DERIVATIVES OF PROTEINS. 



Proteins are hydrolysed into their constituent amino acids by 

 boiling with concentrated mineral acids or alkalies for 6-10 hours. 

 This hydrolysis of the complex protein takes place in several stages, 

 products intermediate between the large protein molecule and the 

 amino acid molecule being formed. These are metaprotein, proteoses, 

 peptones and polypeptides. In the formation of metaprotein only a 

 comparatively small change in the protein molecule occurs. Proteoses 

 and peptones are formed by the breaking up of the large molecule 

 into several large complexes, each of which is gradually hydrolysed into 

 smaller complexes, from which the amino acids are finally formed. 



The metaproteins, the proteoses and the peptones still possess 

 some of the properties of a typical protein, but the amino acids do 

 not The polypeptides are intermediate between the peptones and 

 amino acids; some resemble the peptones, others the amino acids. 

 In all probability peptone is a mixture of polypeptides. 



METAPROTEINS. 



The most typical metaproteins are formed from the mixture of 

 albumin and globulin of egg-white or of blood serum, but they are 

 also formed from the other proteins. 



Preparation. 



Metaprotein is most easily prepared by the action of acid or 

 alkali upon proteins. It is formed fairly rapidly at 60 and higher 

 temperatures, more slowly at 37. 



(a) By Acid. 



(i) Egg-white or serum is mixed with 10 times its volume of 0*4 

 per cent, hydrochloric acid and kept in an incubator at 37 for at least 

 24 hours. 



(ii) Egg-white or serum is mixed with one-third of its volume of 

 glacial acetic acid and allowed to stand. The mixture sets to an 

 opaque jelly. The opacity is due to the coagulation of the protein by 

 the strong acid. On diluting with water the jelly dissolves leaving 

 coagulated protein. 



(6) By Alkali. 



(i) Egg-white or serum is mixed with 10 times its volume of cri 

 per cent sodium hydroxide and kept at 37 for about 18 hours. 



(ii) Egg-white or serum is mixed with about one-third of its 

 volume of 2N sodium hydroxide. On standing it sets to a transparent 

 jelly (Lieberkiihn's jelly). The jelly dissolves on diluting with water. 



Small quantities may be rapidly prepared by adding about one 

 quarter of the volume of dilute acid or alkali to 10 or 20 c.c. of an 

 egg-white solution and keeping in water at 40-50 for 10-15 minutes. 



