128 PHYSIOLOGICAL CHEMISTRY 



of larger molecular weight in which these amino acids are miss- 

 ing. A study of the proteoses and peptones has developed the 

 important fact that the breaking down of the protein molecule 

 is by no means a symmetrical process. In fact, it probably con- 

 sists in breaking off fragments of the greatest diversity, includ- 

 ing some of the amino acids themselves even in the early stages 

 of the process. There appear to be certain combinations or 

 groups, "nuclei" we may call them, in the protein molecule 

 which are quite resistent to hydrolyzing agents, and even after 

 very vigorous hydrolysis such groups may remain intact. 

 Although the proteoses and peptones are simpler in structure 

 than the proteins, they still are quite complex substances and 

 contain many amino acid molecules, many more than the poly- 

 pep tid of 18 amino acids built up by Fischer. 



By boiling with strong acids or alkalies, or by the action of 

 certain enzymes, proteoses and peptones are split into amino 

 acids as might be expected. 



Neither proteoses nor peptones coagulate on boiling. Both 

 groups give the biuret test. The color is redder than that given 

 by proteins, however. They usually give the xanthoproteic test. 

 Their response to the Millon's and Hopkins-Cole reactions de- 

 pends on the presence of tyrosine and tryptophane in the par- 

 ticular proteose or peptone under investigation. Proteoses usu- 

 ally give these tests. Peptones seldom do. Concentrated nitric 

 acid precipitates some proteoses. These are called primary pro- 

 teoses. It does not precipitate "secondary proteoses" or pep- 

 tones. Peptones will diffuse fairly well through an animal mem- 

 brane. 



Peptids are substances consisting of only a few amino acids 

 united by the "peptid linking." These compounds have been 

 obtained by hydrolyzing protein, and also have been built up 

 artificially in the laboratory, as already has been indicated at 

 an earlier point in the discussion of proteins. 



The final products of hydrolysis, the amino acids are the ul- 

 timate building stones of which the proteins and their simpler 

 derivatives are built up. We have seen that proteins are neces- 



