438 PROTEINS 



injury resulted. Pearsall and Ewing,* moreover, have shown 

 that rapid outward diffusion of chlorine ions results when 

 the living plant tissue is brought to hydrogen ion concen- 

 trations equal to or greater than the isoelectric point of the 

 chief proteins. 



As the result of a detailed study of the colloidal properties 

 of the mycelium of Fusarium lycopersici, Scott f comes to the 

 conclusion that the living tissue of the fungus behaves analo- 

 gously to an amphoteric protein colloid with an isoelectric 

 point near P^ == 5-4. 



B. Chemical Properties. 



I. Precipitation reactions. 



The proteins have both acid and basic properties ; thus, 

 casein may be looked upon as typically acid, seeing that it 

 dissolves in alkalis to form sodium and potassium salts, whilst 

 the histones and protamines are powerful bases. All proteins, 

 however, have basic properties, which enable them to form 

 insoluble salts with a great many of the ordinary alkaloidal 

 reagents, such as phosphotungstic, tannic, picric, ferrocyanic, 

 and trichloro-acetic acids. Pittom, J however, finds that many 

 of the simpler polypeptides are not precipitated by phospho- 

 tungstic acid. They are also precipitated by potassium ter- 

 iodide (a solution of iodine in potassium iodide) and by the 

 double iodides of potassium with mercury, bismuth, and cad- 

 mium. The strong mineral acids also precipitate proteins. In 

 consequence of this dual nature of proteins they are classed as 

 amphoteric electrolytes. 



The salts of the heavy metals also produce insoluble pre- 

 cipitates with the proteins, a fact which is made use of in the 

 administration of egg albumin as an antidote in cases of poison 

 with salts of these metals. Moreover, the antiseptic action of 

 mercuric chloride is most probably connected with this forma- 

 tion of insoluble salts. 



Amongst the salts most frequently used as precipitants for 

 proteins are the chloride and acetate of iron, colloidal iron 



* Pearsall and Ewing : " New Phytol.," 1924, 23, 193. 



t Scott : " Missouri Agric. Exp. Sta. Res. Bull.," 1926, 92, i. 



X Pittom : " Biochem. Journ.," 1914, 8, 157. 



