CLASSIFICATION 325 



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 &^^ 

 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. 



Caseinogen and vitellin were formerly known as 

 nucleo-albumins, but the term is misleading, owing to 

 the confusion arising with the nucleo-proteins, which are 

 conjugated proteins (see below), and the term nucleo- 

 albumin has for that reason been abolished. 



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. — This group may be divided into three 

 sub-groups. 



1. Chromo-proteins, represented by haemoglobin. 



2. Nucleo-proteins, obtained from blood, chyle and 

 lymph. 



3. Gluco-proteins, represented by mucin. 

 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 derivative, 

 haematin (cf. chlorophyll, p. 236). Similarly, a gluco- 

 protein such as mucin yields a protein and a carbohy- 



