74 PHYSIOLOGICAL CHEMISTRY. 



COMPONENT GROUPS IN THE PROTEIN COMPLEX. 



In the study of the protein molecule as a whole, a limit is 

 soon reached in any attempt to fix its composition, but much 

 may be learned by observing the various products formed in 

 reactions by which the molecule is broken down under the 

 influence of different agents. Some of these reactions are ap- 

 parently largely hydrolytic in character, and in a degree may be 

 compared to the decomposition of a fat by superheated steam. 

 In this very simple case glycerol and fatty acids are obtained 

 and we conclude that they were not actually formed in the 

 process, but that they were present in combination in the 

 original fat. In treating protein bodies in a similar manner 

 or in subjecting them to the decomposing influences of acids 

 or alkalies, a number of products are formed. These must 

 be either results of peculiar disintegration and subsequent 

 synthesis, or they must represent groups in some way existent 

 in the original complex. The latter view is strengthened by 

 the fact long observed that certain products result, whatever 

 the method of decomposition. Leucine, for example, is found 

 abundantly among the products liberated by subjecting protein 

 to the action of superheated steam, hot hydrochloric, nitric or 

 dilute sulphuric acid, concentrated alkali solutions, bromine 

 water under pressure, or to prolonged pancreatic digestion. 

 The almost necessary conclusion must be that in these varied 

 reactions the leucine could not have been formed from smaller 

 disintegration groups, but must have been set free from some- 

 thing holding it in the protein complex. 



The decomposition reactions are therefore considered very 

 important as suggesting probably the component groups in 

 the large molecule. In the following pages a few of the most 

 important of these reactions and their products will be 

 described. 



Decomposition by Steam under Pressure. By pro- 

 longed heating of protein substances with water certain 

 changes take place, even below a temperature of 100 C. 



