256 Biological Chemistry. 



THE GLYCO-PROTEINS. 



The chemistry of this group of substances is still very 

 indefinite. The principal members of the class are the 

 cartilaginous substances and the mucins, and they are 

 characterized by the fact that they yield on hydrolysis 

 substances which reduce Fehling's solution. Cartilage 

 yields a substance known as chondroitin sulphuric acid,* 

 of which the constitution has not yet been determined. 



THE PHOSPHO-PROTEINS. 



The two chief members of this group are caseinogen, 

 the principal protein of milk, and vitellin, the protein of 

 egg-yolk. They are both characterized by the fact that 

 on treatment at 37 with 1 per cent, sodium hydroxide a 

 scission of phosphoric acid takes place. This is in marked 

 contrast to the other conjugated proteins which contain 

 phosphorus, namely, the nucleo-proteins. It may be recalled 

 that the part of the molecule containing the phosphorus, 

 namely, the nucleic acid, is stable towards alkalis, but is 

 readily decomposed by acids. By investigating the action 

 of alkalis, it is possible, therefore, to distinguish between 

 a phospho-protein and nucleo-protein. Caseinogen may be 

 obtained from skimmed milk by precipitation with dilute 

 acetic acid. It is an acid substance which is soluble in 

 solutions of ammonia and caustic alkalis, from which it 

 can be precipitated by the addition of acetic acid. 



THE HAEMOGLOBINS. 



These are proteins (globins) which are supposed to be 

 of a histone-like character, in combination with a chroma- 

 togenic group. They form the chief constituent of the 

 red blood corpuscles. They are capable of forming com- 



* Since the text of this book was prepared, a considerable amount of work 

 on this substance has been carried out by Levene 



