THE COMPONENTS OF PLANTS AND ANIMALS 31 



peptids is also commonly used as a general term for the more 

 complex substances of this group. The latter show many of 

 the reactions of the proteins or of their less modified deriva- 

 tives. For example, many of them give the biuret reaction 

 characteristic of the proteins, are precipitated by phospho- 

 tungstic acid and undergo cleavage by appropriate proteolytic 

 ferments. Moreover, some of the artificial polypeptids of known 

 composition have also been isolated from the mixture of products 

 resulting from the action of ferments upon the proteins. 



49. Conclusions. Since, therefore, the same comparatively 

 simple crystalline products are obtained as the final result of 

 the complete hydrolysis of all the simple proteins, viz., the 

 various amino acids enumerated in a previous paragraph (47), 

 and since, on the other hand, these cleavage products may be 

 synthesized to form substances closely resembling the proteins, 

 it is believed that the protein molecule is built up of these 

 amino acids, united in substantially the same way as in the 

 artificially prepared polypeptids. In other words, it is be- 

 lieved that the latter are the first steps toward the synthesis 

 of proteins, or indeed that they may, from a systematic point 

 of view, be regarded as the simplest of the proteins. 



It should be noted, however, that while the foregoing method of 

 combination of the amino acids appears to be characteristic of the 

 protein molecule, it is not the only form of combination in which 

 nitrogen enters into it. For example, arginin, apparently a constit- 

 uent of all proteins, contains an atom of imid nitrogen, HN. The 

 proteins also contain amid nitrogen (i.e., NH 2 substituted for the OH 

 of the carboxyl group) which yields ammonia on hydrolysis. Further- 

 more, the proteins are capable of acting as polyacid bases and there- 

 fore the molecule must contain numerous NH 2 end-groups such as 

 that of the amids just mentioned or those of the diamino-acids like 

 lysin and arginin. 



50. Proportions of cleavage products in different proteins. 



While all the simple proteins yield, with a few exceptions, 

 qualitatively the same cleavage products, the relative pro- 

 portions of these " building stones " vary widely in proteins 

 from different sources. This is strikingly illustrated by the 

 following tabulation of the percentages of the various amino 

 acids yielded by a number of proteins according to the researches 

 of Osborne and his associates. 



