Chemical Constituents of the Animal Body. 233 



(c) The Phospho-proteins. The two chief members of 

 this class are caseinogen, the protein of milk and vitellin 

 from eggs. These are characterized by the fact that on 

 treatment with 1 per cent, sodium hydroxide at 3*7 they 

 undergo partial hydrolysis with scission from the molecule 

 of phosphoric acid. They are possibly protein derivatives 

 of this acid. 



(d) TJie hcemoglobins, which undergo scission on treat- 

 ment with dilute acids into a chromatogenic group (hsemin) 

 and a protein (globin). 



III. Derived Proteins. 



(a) The Meta-proteins. On treating proteins with 

 dilute acid or alkali, they undergo change into acid 

 albumin or alkali albumin, which are soluble in excess 

 of alkali or acid. The protein undergoes relatively little 

 chemical change when converted into acid or alkali albumin, 

 both of which substances are included amongst the meta- 

 proteins. 



(b) The Proteases. 



(c) The Peptones. When proteins are partially hydro- 

 lyzed with ferments, or submitted to gentle hydrolysis 

 with acids, products can be obtained in which complete 

 scission of all the peptide linkages has not taken place. 

 Substances which are still polypeptides, but which are not 

 so complex as the original proteins (that is, do not yield 

 so many amino-acids on hydrolysis and have a smaller 

 molecular weight), can be obtained. The proteoses (for- 

 merly called albumoses) and peptones are substances of 

 this description ; the former can be precipitated from 

 aqueous solutions when they are saturated with zinc or 

 ammonium sulphate, whereas the latter cannot. The amino- 

 acids, peptones, proteoses, and proteins bear more or less 

 the same kind of relationship to one another as do glucose. 



