CLASSIFICATION 427 



ammonia, but only small amounts of arginine and histi- 

 dine, and no lysine. 



Glutelins and gliadins are the chief protein constitu- 

 ents of the substance known as gluten. 



Sclera -proteins. — This term is synonymous with the older term 

 albuminoid, and includes substances of skeletal origin, 

 such as keratin from hair, horn, etc., gelatin, elastin, and 

 silk fibroin. 



No representative of this class has as yet been found 

 among vegetable proteins. 



Phospho-proteins. — This group, which is probably not repre- 

 sented in the vegetable world, contains such substances 

 as caseinogen and vitellin, obtained from milk and egg 

 yolk respectively. The phosphorus of these proteins is 

 in intimate organic combination with the protein mole- 

 cule, and is not contained in the " prosthetic group " 

 (see below) as in the case of the nucleo-proteins, which 

 are composed of proteins with the phosphorus-containing 

 nucleic acids. 



The phospho-proteins are insoluble in water, but 

 soluble in alkalis. 



The phospho-proteins resemble the nucleo-proteins in 

 their solubilities, but they differ from them in their be- 

 haviour on hydrolysis ; they yield at first a so-called 

 pseudo- or para-nuclein, corresponding to the formation of 

 a nuclein from a nucleo-protein, but whereas a nuclein 

 on further hydrolysis yields nucleic acid, and ultimately 

 purine bases, the pseudonuclein yields no corresponding 

 pseudonucleic acid, but on the other hand is broken up 

 by baryta water into phosphoric acid, but gives no purine 

 bases. 



Conjugated Proteins. — Conjugated proteins are characterized 

 by the fact that on hydrolysis they break up, yielding 

 a true protein and a substance of a different nature, for 

 which Kossel has proposed the name " prosthetic " group. 

 Thus, for example, a chromo-protein like haemoglobin 

 breaks up into globin (a protein) and a pyrrole deriva- 

 tive, haematin (cf. chlorophyll, p. 323). Similarly, a 



