THE PROTEINS 



below). Acid metaprotein may be changed into alkali metaprotein, 

 but the reverse change cannot take place, since the alkali splits off 

 some of the loosely combined nitrogen and sulphur. If alkali meta- 

 protein be prepared by the action of strong alkali on egg white, a 

 substance known as " Lieberkuhn's jelly " is formed. Acid meta- 

 protein is formed in peptic, alkali metaprotein in tryptic, digestion. 



Proteoses and Peptones. These occur in the first stages of protein 

 cleavage (see table below), and are more fully dealt with under Diges- 

 tion (p. 386). 



The proteoses are usually divided into primary and secondary. 

 They are not coagulated by heat, are very slightly diffusible, 

 give a pink colour with the biuret test, and a white precipitate 

 with salicyl-sulphonic acid, which disappears on heating and reappears 

 on cooling. Primary proteoses (except a form known as hetero- 

 proteose) are salted out by half-saturation with ammonium sulphate, 

 secondary proteoses by full saturation (see table bcl^w, also p. 386). 



Peptones are characterized by their ready diffusibility through 

 parchment membranes. They also give a characteristic pink colour 

 with the biuret test. They are not coagulated by heat, and are not 

 precipitated by nitric or salicyl-sulphonic acid. They are precipitated 

 by alcohol and picric acid. 





