28 THE VEGETABLE PROTEINS 



and indicate that the molecular proportion of base with which the 

 protein reacts is equivalent to the proportion of acid found in the salts 

 of edestin, which have been already described. 



Like the amphoteric amino-acids the proteins have acid properties 

 as well as basic, but the latter are commonly much more manifest. 

 Thus the writer (312, 315) has found that edestin salts, when suspended 

 in water and gradually treated with a decinormal solution of potassium 

 hydrate, begin to dissolve after all of the combined acid has been 

 neutralised, and an excess of alkali is present as indicated by a red 

 colour with phenolphthalein. When a sufficient, but definite, excess 

 of alkali is added, all of the edestin passes into solution. The pro- 

 portion of alkali required to dissolve a given quantity of neutral edestin 

 is nearly the same as that required to neutralise the acidity of the in- 

 soluble edestin chloride, in accord with the view that this solution in- 

 volves a reaction with alkali in molecular proportions. Furthermore, 

 when neutral edestin is present in excess the amount dissolved is propor- 

 tional to the amount of alkali added, showing the edestin to be dissolved 

 through the formation of an alkaline salt, the protein functioning as an 

 acid. The strength of this protein acid, however, is slight, and its salts 

 are so readily hydrolysed by water that they show a strong alkaline 

 reaction towards phenolphthalein. Weak bases, like ammonium 

 hydroxide, form less stable salts with edestin, hence 1 3 c.c. of a deci- 

 normal solution of ammonium hydrate are required to dissolve a 

 quantity of edestin which is completely dissolved by I c.c. of a deci- 

 normal solution of potassium hydroxide. 



It was formerly supposed that many proteins were strongly acid in 

 their nature and formed relatively stable salts with bases, as milk 

 casein is now generally considered to do. We, therefore, find many 

 seed proteins described in the older literature as caseins, and among 

 these the so-called legumin from peas and beans was long regarded as 

 a protein of strongly acid character. More recent studies of legumin, 

 however, have shown that the solubility caused by the addition of small 

 quantities of alkali is due to the basic and not to the acid nature of 

 this protein. 



Legumin can be extracted from various leguminous seeds by neutral 

 sodium chloride solution, and the preparations obtained by dialysis, 

 like those of edestin, are distinctly acid toward phenolphthalein. This 

 acidity is probably not caused by the protein itself but by the combined 

 acid of the legumin salt. Such preparations usually require about 2 

 c.c. of decinormal potassium hydroxide solution per gramme to make 

 them neutral to phenolphthalein, a quantity slightly greater than that 



