66 PHYSIOLOGICAL CHEMISTRY 



of starch there is formed a series of dextrins of gradually decreasing 

 complexity and coincidently with the formation of each dextrin a small 

 amount of sugar is split off and finally nothing but sugar remains. In 

 the case of protein hydrolysis there is a series of proteins of gradually 

 decreasing complexity produced and coincidently with the formation of 

 each new protein substance amino acids are split off and finally the sole 

 products remaining are amino acids. 



Inasmuch as diversity in the method of decomposing a given protein 

 does not result in an equally diversified line of decomposition products, 

 but, on the other hand, yields products which are quite comparable in 

 character, it may be argued that there are probably well-defined lines of 

 cleavage in the individual protein molecule and that no matter what the 

 force brought to bear to tear such a molecule apart, the disintegration, 

 when it comes, will yield in every case certain definite fragments. 

 These fragments may be called the " building stones" of the protein 

 molecule, a term used by some of the German investigators. Take, for 

 example, the decomposition of protein which may be brought about 

 through the action of the enzyme try p sin of the pancreatic juice. 

 When this enzyme is allowed to act upon a given protein, the latter is 

 disintegrated in a series of definite cleavages, resulting in the formation 

 of proteases, peptones, and peptides in regular order, the peptides being 

 the last of the decomposition products which possess protein character- 

 istics. They are all built up from amino acids and are therefore closely 

 related to these acids on the one side and to peptones on the other. 

 We have di-, tri-, tetra-, penta-, deca-, and poly-peptides which are 

 named according to the number of amino acids included in the peptide 

 molecule. Following the peptides there are a diverse assortment of 

 monamino and diamino acids which constitute the final products of 

 the protein decomposition. These acids are devoid of any protein 

 characteristics and are therefore decidedly different from the original 

 substance from which they were derived. From a protein of huge 

 molecular weight, a typical colloid, perhaps but slightly soluble, and 

 entirely non-diffusible, we have passed by way of proteoses, peptones, 

 and peptides to a class of simpler crystalline substances which are, for 

 the most part, readily soluble and diffusible. 



These amino acids after their production in the process of digestion, 

 as just indicated, are synthesized within the cells of the organism to 

 form protein material which goes to build up the tissues of the body. 

 It is thus seen that the amino acids are of prime importance in the 

 animal economy. It was formerly believed that these essential factors 

 in metabolism and nutrition could not be produced within the animal 

 organism from their elements, but were only yielded upon the hydrol- 



