PROTEINS AND THEIR CLASSIFICATION. 1027 



are all so alike in their general properties in spite of the great differences in 

 the complexity of their molecular structure. (3) The fact that they show 

 both basic and acid characters. (4) The fact that they all give the biuret 

 reaction* (see below). 



In addition to the amino-acids some proteins -egg-albumin, for example 

 yield a carbohydrate body upon decomposition. The carbohydrate ob- 

 tained is an amino-sugar compound, usually glucosamin, C 6 H 13 NO 5 . It is 

 detected by its reducing action and by the formation of an osazone. It seems 

 probable, therefore, that some of the proteins at least contain such a group- 

 ing as part of the molecular complex, but at present it is undetermined how 

 many possess this peculiarity of structure. 



General Reactions of the Proteins. It is evident from what has been 

 said in the preceding paragraph that proteins may give different reactions 

 according to the kinds of groupings contained in the molecule. The reac- 

 tions common to all proteins are few in number, the most certain perhaps 

 being the biuret reaction, the hydrolysis by proteolytic enzymes or putre- 

 factive organisms, and the nature of the split products formed by these latter 

 hydrolyses or by the action of boiling dilute acids. A very large number 

 of reactions, however, have been described which hold for some or all of 

 the proteins usually found in the tissues and liquids of the body. These 

 reactions may be described under two heads: (1) Precipitation of the protein 

 when in solution; (2) color reactions. 



/. 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 chlprid, 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 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. 425, 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 



* For further details, see Cohnheim, "Chemie der Eiweisskorper," second 

 edition, 1904; or Abderhalden, "Lehrbuch d. physiol. Chemie," 1914; and 

 Rosenheim, in "Science Progress," April and July, 1908. 



