PROTEINS 109 



peptid d-alanyl-d-alanine is split by pancreatic juice, but not 

 d-alanyl-1-alanine. In general, those peptids made up of the 

 optical form of the amino acids which is found in nature will 

 be split by enzymes. Peptids made from the optical isomers 

 which do not occur in nature are not split, or are split less 

 readily by enzymes. 



From a study of the polypeptids it has been learned that the 

 ease with which a given polyp eptid may be precipitated from 

 solution depends not only on the size of the molecule, but also 

 upon the particular amino acids present in it. Thus a poly- 

 peptid containing tyrosine, tryptophane or cystin will pre- 

 cipitate at a much lower concentration of a given precipitating 

 reagent than a second polypeptid of equal molecular weight, 

 containing none of these acids. Many polypeptids, both those 

 prepared synthetically and those obtained from protein hydro- 

 lysis will give characteristic protein reactions, though no poly- 

 peptid has been prepared which closely approaches the pro- 

 teins in the complexity and size of its molecule. 



On hydrolizing a protein, that is, breaking it down into sim- 

 pler products, the process evidently is not a symmetrical divi- 

 sion and redivision of the molecule. In tryptic digestion tyro- 

 sine, tryptophane and cystine are split off much quicker than 

 alanine, leucine and valine, whereas glycocoll, proline and 

 phenylalanine are not split off even after prolonged digestion. 

 We must assume, therefore, that certain groups or "building 

 stones," are split off from the great protein molecule, which is 

 thus reduced through various stages to proteoses, peptones and 

 in complete hydrolysis to the amino acids. 



Putrefaction of Proteins. When proteins are broken down 

 by the action of bacteria in the intestines, products are formed 

 some of which are very injurious to the organism. The amino 

 acids produced by the hydrolysis of the proteins are attacked 

 and partially broken down in a variety of ways. From trypto- 

 phane by the removal of its side chain, skatol and indol are 

 produced. 



