434 PROTEINS 



coagulated, and many are not coagulated even by boiling 

 water. 



Heat coagulation is best effected as follows. The solution 

 is first boiled, and from 1-3 drops of dilute acetic acid are 

 added for each 10 c.c. of liquid, the liquid being boiled before 

 the addition of each drop. 



{b) The addition of absolute alcohol to a neutral or faintly 

 acid solution of a native protein * will precipitate it from 

 solution unchanged. If, however, it be left in contact with 

 the alcohol for some time, the protein is rendered insoluble and 

 is coagulated. 



4. Reversible precipitation. 



Certain salts, such as sodium chloride and the sulphates 

 of sodium, magnesium, and ammonium, etc., have the pro- 

 perty of throwing proteins, except peptones, out of solution. 

 This is, however, purely a physical phenomenon, and must 

 be distinguished from the chemical precipitation described 

 below, inasmuch as the proteins are precipitated unchanged, and 

 retain all their original properties and solubilities. Absolute 

 alcohol, also, as mentioned above, precipitates the proteins 

 unchanged, though the precipitate must not be left in contact 

 with the alcohol, or else it will become coagulated. 



With regard to the precipitating power of these various 

 salts, it should be mentioned that saturated ammonium sul- 

 phate will precipitate all proteins except peptones, and con- 

 sequently a solution which on saturation with ammonium 

 sulphate remains clear, can be regarded as free from protein. 



Furthermore, zinc sulphate is approximately 



equivalent to ammonium sulphate, 



saturated sodium chloride is 



approximately equivalent to saturated magnesium 



sulphate, or 

 1/2 saturated ammon- 

 ium sulphate. 



Solubilities of Proteins and their Physiological Significance. 



In view of the number of proteins in the plant and their 

 different characteristic solubilities, it is easy to see the impor- 



* The term native protein is applied to proteins which have been isolated 

 from the tissues by some simple process which does not involve any material 

 alteration in their original properties. 



