PROTEINS AND THEIR CLASSIFICATION. 909 



/. Precipitants. For one or another protein the following reagents cause 

 precipitation : 



1. The addition of an excess of alcohol. 



2. Boiling (heat coagulation). 



3. The addition of mineral acids, e. g., nitric acid. 



4. The salts of the heavy metals, e. g., acetate of lead, copper sul- 



phate, etc. 



5. Addition of neutral salts of the alkalies to a greater or less degree 



of concentration, e. g., sodium chlorid, ammonium sulphate. 



6. Ferrocyanid of potassium after previous acidification by acetic acid. 



7. Tannic acid after previous acidification by acetic acid. 



8. Phosphotungstic or phosphomolybdic acid in the presence of free 



mineral acids. 



9 lodin in solution in v potassium iodid, after previous acidification 

 with a mineral acid. 



10. Picric acid in solutions acidified by organic acids. 



11. Trichloracetic acid. 



This list might be extended still further, but it comprises the precipi- 

 tating reagents that are ordinarily used. Some of them, particularly Nos. 

 7, 8, and 9, give reactions in solutions containing excessively minute traces 

 of protein. 



12. Precipitins. In this connection a brief reference may be made to 

 the interesting group of bodies known as precipitins. As stated 

 on p. 399, the animal organism has the power, when foreign cells 

 are injected into it, of forming anti-bodies by a specific biological 

 reaction. It has been discovered that anti-bodies, or as they 

 are called in this case, precipitins, may be produced in the 

 same way if protein solutions or solutions of animal tissue are in- 

 jected into the circulation. Thus, if cows' milk be injected under 

 the skin of a rabbit there will be produced within the rabbit's 

 blood a precipitin which is capable of precipitating the casein of 

 cows' milk, although it may have no action on the milk of other 

 animals. In the same way any given foreign protein, when injected 

 under the skin of an animal, may cause the production- of a pre- 

 cipitin capable of precipitating that particular protein from its 

 solutions. The precipitin is not absolutely specific for the protein 

 used to produce it, but nearly so. If a rabbit is immunized with 

 human blood a precipitin is produced in the animal's blood 

 which causes a precipitate when mixed with human blood or 

 with that of some of the higher monkeys, but gives no reaction 

 with the blood of other mammals. The reaction may be used, 

 therefore, in a measure to test the blood-relationship of different 

 animals.* It has been suggested that the reaction may also be 

 of practical importance in medicolegal cases, in determining whether 

 a given blood-stain is or is not human blood. For such a pur- 

 pose a human antiserum is first produced by injecting human 

 serum into a rabbit. The serum of the rabbit is then mixed with 

 an extract of the suspected blood-stain made with salt solution; 

 if a precipitate forms it proves that the blood stain is human blood 

 provided the possibility of its being monkey's blood is excluded. 

 Concerning the nature of the precipitins, little is known. They 

 combine quantitatively with the protein precipitated and they 

 are inactivated (hematosera) by a temperature of 70 C. Their 

 reactions are not sufficiently specific to be used as a means of de- 

 tecting or distinguishing closely related proteins. 



II. The Color Reactions of Proteins. 



1. The biuret reaction. The protein solution is made strongly alkaline 

 with caustic soda or potash and a few drops of a dilute solution of 



* For many interesting experiments and the literature see Nuttall, "Blood 

 Immunity and Relationship." Cambridge, 1904. 



