72 THE GENERAL CHARACTERS OF THE PROTEINS 



strong precipitate with human blood, but not with that of a goat or 

 a dog. Such a serum will, however, precipitate the blood of a 

 species nearly allied to that from which the precipitinogen is ob- 

 tained, and the precipitin produced by the injection of human blood 

 will precipitate that of an anthropoid ape, although the amount of 

 precipitate formed will differ quantitatively in different cases. The 

 specificity of the precipitin reaction is, therefore, not absolute. 



In order to produce precipitins the substance injected must be 

 foreign to the animal employed. With a protein derived from the same 

 species precipitins (the so-called " iso-precipitins ") are obtained only 

 in exceptional cases. To produce the most favourable results the 

 animals employed for producing the precipitins should not be too 

 closely allied to the animal from which the precipitinogen has been 

 obtained, although Uhlenhuth has recently succeeded in obtaining a 

 precipitin for hares' blood, but not for that of a rabbit, by in- 

 troducing into the latter the blood of the former animal. 



In addition to the experiments with native proteins, experiments 

 for production of precipitins from chemically changed proteins have 

 also been carried out. Precipitins have been produced from crys- 

 tallised egg- and serum-albumins, although, according to Obermayer 

 and Pick, the precipitin-producing property of these substances is lost 

 after repeated recrystallisation. The precipitinogenic property is not 

 lost, however, by heating, and precipitins can be produced by the in- 

 jection of coagulated proteins. The property does not appear to 

 be lost even by boiling with per cent, hydrochloric acid or sodium 

 hydroxide solutions; neither does it appear as if the property is 

 readily lost by the tryptic digestion of the precipitinogens, although 

 it is readily lost by the peptic digestion. For this reason it is not, 

 as a rule, possible to produce precipitins by the administration of 

 precipitinogens per os, although the formation may, in certain cases, 

 take place when a particular protein is ingested in such large quanti- 

 ties that it escapes the action of the peptic juice. 



Proteins, therefore, which have undergone considerable changes, 

 either by oxidation or hydrolysis, still possess the property of pro- 

 ducing precipitins, which are specific for the species of animal from 

 which they have been obtained. There is, however, according to 

 Obermayer and Pick, another class of changed proteins in which 

 this kind of specificity has been lost. lodo-, nitro- and diazo-pro- 

 teins, for example, will also yield precipitins, but these are specific, 

 not for a particular animal species, but for other substances of the 

 same class ; thus a precipitin which has been produced by the in- 

 jection of an iodo-protein from ox-serum is not specific for ox-serum, 

 or even the iodo derivatives from ox-serum, but will precipitate iodo- 

 proteins from other sera, and even an iodo-protein derived from a 

 plant. Furthermore, although an animal cannot, as a rule, produce 

 a precipitin for one of its own proteins, it can produce one for a 

 changed protein. In this way a xantho-protein precipitin has been 

 produced by the injection of xantho-protein derived from rabbits' 

 serum into a rabbit. Obermayer and Pick think that the animal 

 species specificity is due to the aromatic groups, and that this 

 particular kind of specificity is lost by the treatment of the protein 

 by reagents, such as halogens, nitric acid, etc., which have a 

 tendency to destroy these groups. 



i 



