PROTEINS 163 



H H H O H 



I . I 1 ii I 



HOOC— C— N— H+HO OC— C— N— H=HOOC— C— N— C— C— N— H+H 2 0. 



I I ' ' II I I I I 



HH HH HHHH 



These can be linked in any order so that from three different 

 amino acids — a, b, and c — three different combinations could 

 be made, — abc, acb, and bac. Such very simple combinations 

 are not complex enough to be called proteins but instead are 

 polypeptids. Real proteins probably contain hundreds of such 

 acids, which are not only linked together end to end, but may also 

 contain amino acids attached on the side, as illustrated in the 

 accompanying diagram, where amino acids are attached at the 

 places marked R. When one thinks of the combinations possible 



NHCH.CO— NH.CH.CO.NH.CH.CO.NH.CH.COOH 



I I I I 



R R R R 



with such large numbers of units, which may be linked together 

 not only in straight chains but also in rings, it is not surprising that 

 each species of plant and animal has its own specific type of protein 

 and that there may even be individual differences so that every 

 individual organism might have a protein of its own. This has 

 been suggested, and there is evidence in its favor; if it is not true, 

 it is not because of the lack of protein possibilities. 



An important evidence of this specificity of proteins is the 

 precipitin reaction. It has been found that if serum from a horse 

 is injected into a rabbit in increasingly larger doses, the rabbit will 

 form in its blood a substance known as precipitin, a protein related 

 to the globulins. If the precipitin from the rabbit is then added 

 to the blood of the horse in vitro & precipitate is formed, which is a 

 compound of the precipitin with the albumin of the horse serum. 

 This precipitin will react only with horse serum or with the serum 

 of an animal closely related such as the mule and the ass. In this 

 way it is possible to determine relationships, and much work of 

 this sort has been done. The precipitin produced by human blood 

 will react with that from the anthropoid apes thus establishing 

 another link in the evidence for human evolution. Korinek (1924), 

 Raeder (1924), Saltzmann (1924), and Mez (1926) have all been 

 doing work of this kind with plants and find that the same type of 

 reactions can be obtained with plant proteins. Thus serum from a 

 rabbit which has been injected with yeast extract will precipitate 



