Chemical Constituents of the Animal Body. 261 



precipitated by comparatively low dilutions of the salts 

 (for example, when the liquid is one-third saturated), 

 whereas others are only precipitated when their solutions 

 are completely saturated with the salt. By taking advan- 

 tage of these facts, it is possible to accomplish a partial 

 (but only partial) separation of the proteoses from one 

 another. The salt is added to the digest to a known 

 degree of saturation (for example, one-third or one-half) ; 

 the precipitate of albumoses is filtered off, more salt is 

 then added to the filtrate (for example, to the degree of 

 two-thirds saturation), a second fraction of albumoses is 

 then produced, whilst from the filtrate, by a similar pro- 

 cedure of gradual increase in the degree of saturation, 

 further fractions can be produced. It will be seen that 

 the process of separation of products produced by the 

 partial hydrolysis of proteins by means of ferments is an 

 empirical one, which does not lead to the separation of 

 pure chemical entities.* 



There are two chemical methods which may be employed 

 for determining the degree of degradation of a protein, 

 both of them due to the fact that the scission of a peptide 

 linkage sets free an amino and a carboxyl group. This 

 is illustrated by the equation representing the hydrolysis 

 of glycyl glycine 



CH 2 (NH 2 )-CO NH CH 2 COOH + H a O = 2CH 2 (NH 2 ) COOH 



for whereas glycyl glycine contains only one amino group 

 and one carboxyl group, the two molecules of glycine 

 obtained by the hydrolysis contain two such groups. 



Now when a molecular equivalent of glycyl glycine is 

 treated with nitrous acid, only two atoms of nitrogen are 

 evolved 



CH,(NH 2 ) CO NH-CH 2 -COOH + HO NO 



= N 2 + CH 2 '(OH).CO-NH CH 2 COOH 



* A more detailed reference to the action of ferments is given later. 



