PROTEINS 111 



are acted on by certain enzymes, and perhaps by other means. 

 They are characterized by their insolubility in the usual mild 

 protein solvents, such as water, salt solutions, etc. Little is 

 known of what takes place in the protein molecule in coagula- 

 tion. Although coagulated proteins are fairly insoluble, still 

 they will dissolve in strong acids or alkalies, or in dilute acids 

 or alkalies on warming slightly for some time. In the latter 

 case they are converted into metaproteins. In the process of 

 cooking food, most of the proteins are coagulated, but they are 

 converted into metaprotein by the acid of the gastric juice, and 

 then digested. Some proteins are digested more easily after 

 coagulation, others less so. Coagulated proteins give all 

 color tests given by the original protein. The protein mole- 

 cule evidently has not been greatly altered or broken down in 

 their formation. 



Secondary Protein Derivatives. Proteoses and Peptones 

 are formed from proteins by the action of strong acids or alka- 

 lies, or by the action of enzymes. The protein molecule is broken 

 up into fragments of varying size and properties. From what 

 is known of the size and complexity of the protein molecule it 

 is easy to understand the great diversity in size, composition and 

 properties of the products formed on splitting it into fragments. 

 Much labor has been spent in the study of these compounds. As 

 yet little is known of the individual substances. For conven- 

 ience, they are divided according to the concentration of am- 

 monium sulphate required to precipitate them. Those which 

 precipitate on saturating with ammonium sulphate are called 

 proteoses. Those which will not precipitate on saturating with 

 ammonium sulphate are called peptones. Such a method of 

 separation is unsatisfactory, but it serves its purpose until more 

 is known of the constitution of the individual substances. It is 

 now known that factors other than the size of the molecule affect 

 the ease with which a substance may be precipitated with am- 

 monium sulphate. Thus substances of the above nature which 

 contain tyrosin, cystin and tryptophane are precipitated at 

 lower concentrations of ammonium sulphate than are those even 



