THE END-PRODUCTS OF PROTEIN HYDROLYSIS 131 



IV. THE COAGULATED PROTEINS. 



The coagulated proteins may be subdivided into two classes, namely, 

 those in which the coagulation-process has gone so far as to be irrevers- 

 ible, so that the coagulum cannot be brought back into solution again 

 without preliminary decomposition into simpler substances, and those 

 in which the coagulum remains soluble after removal of the coagulating- 

 agent and in which the coagulation-process has therefore remained 

 reversible. The majority of the heat-coagulated proteins belong to the 

 first class, although the incipient stages of heat-coagulation are some- 

 times reversible. On the other hand the coagula produced by alcohol 

 or by ammonium sulphate belong to the second class, although in some 

 instances after more or less prolonged contact with alcohol the coagula 

 produced by alcohol become irreversible. 



The polypeptides or chains of amino-acids out of which proteins 

 are built up, form anhydrides with exceptional ease, either by internal 

 neutralization of carboxyl- and amino-groups, or by the condensation 

 of several molecules, and this tendency increases with increasing 

 length of the amino-acid chain. We can hardly suppose, therefore, 

 that this property has been lost in the much more bulky amino-acid 

 complexes which constitute the proteins. On the other hand the 

 agencies which bring about coagulation are all of such a character 

 (heat, alcohol and concentrated salts) as to suggest that the with- 

 drawal of water from the protein is the chemical basis of the coagulation- 

 process. It appears very probable, therefore, that coagulation is due 

 to the formation of protein anhydrides, and that the irreversible 

 coagula are those in which the anhydride-formation has proceeded 

 furthest. 



THE END-PRODUCTS OF PROTEIN HYDROLYSIS: 

 THE AMINO-ACIDS. 



Decomposition of proteins into simple crystal! izable substances 

 may very readily be brought about by a variety of agencies, but the 

 only methods of decomposition which yield easily interpretable results 

 are those which bring about Hydrolysis, or decomposition of the mole- 

 cule by successive splittings with the addition of the elements of 

 water. Hydrolysis of the proteins (autohydrolysis) will take place 

 spontaneously in neutral protein solutions or even if precipitated 

 proteins be left in long-continued contact with neutral and sterile 

 water. The process is, however, greatly accelerated by the application 

 of heat, especially by temperatures considerably exceeding the tem- 

 perature of boiling water, or by catalyzers, of which the most efficient 

 are acids, alkalies and the protein-digesting (proteolytic) enzymes. 

 Whatever the means of hydrolysis employed, however, the end-result, 

 provided the hydrolysis has been complete, is the same, namely, the 

 production of a mixture of amino-acids. 



