Chemical Constituents of the Animal Body. 247 



(4) The Estimation of Some Individual Hydrolysis 

 Products. Tyrosine and tryptophan are readily split off 

 from certain proteins under the influence of digestive 

 ferments, such as trypsin. Methods have been proposed 

 for the estimation of the amounts of these substances set 

 free in this manner, the amount of tyrosine being deter- 

 mined by titration with bromine, with which it forms a 

 definite compound, whilst the amount of tryptophan can 

 be estimated by colorimetric methods. 



(5) The Biological Differentiation of Proteins. If a 

 protein is repeatedly injected into an animal, such as a rabbit 

 or a dog, the serum of that animal acquires the property of 

 forming a precipitate with the injected protein. The latter 

 is called the " antigen," whilst the precipitate is designated 

 a " precipitin," and the serum of the treated animal an 

 " anti-serum." The anti-serum produced by injecting an 

 antigen from one species will produce, as a rule, a precipi- 

 tate with all proteins from that species, but not with 

 proteins from other species, unless they are very closely 

 allied. Thus, by injecting a protein from the human body 

 into a rabbit an anti-serum will be produced which will 

 form a precipitin with human but not with ox-serum. It 

 will, however, produce a slight precipitate with the serum 

 of an anthropoid ape. Other analogous biological reactions 

 are known, but no satisfactory explanation of their chemical 

 or physical mechanism has as yet been offered. 



THE BIOLOGICAL IMPORTANCE OF THE CHEMICAL 

 INVESTIGATION OF PROTEINS. 



In view of the fact that the proteins form the main 

 constituent of animal tissues, and are essential constituents 



"Commercial Organic Analysis, "fourth edition, vol. vii., p. 263, and vol. viii., 

 p. 487. The apparatus has been much improved in various details (such as 

 mechanical methods for shaking), and is supplied in the newest forms by various 

 manufacturers of apparatus. 



