50 THE VEGETABLE PROTEINS 



C. Ultimate Composition of Vegetable Proteins. 



Analyses of seed proteins have shown wider variations in composi- 

 tion than have those of animal proteins. Thus the proportion of car- 

 bon in most of them falls between 50 and 55 per cent., nitrogen between 

 15 and 19 per cent, and sulphur between 0*1 per cent, and 2 

 per cent. Phosphorus has not yet been proved to be a constituent of 

 any of the seed proteins thus far studied. It is true that many pre- 

 parations of these substances contain small amounts of phosphorus, but 

 it has been found that this phosphorus disappears from most of them 

 when they are purified by repeated precipitation. We have seen 

 that crude preparations of edestin, when neutralised, yield a small 

 quantity of phosphate in the mixture of salts which results on 

 neutralising and that by reprecipitation this phosphorus disappears. 

 There is little doubt that the phosphorus found in most crude pre- 

 parations of seed proteins is present in them in the form of combined 

 acid and that it forms no part of their molecule. This question will 

 be further considered in connection with the presence of nuclein and 

 nucleoproteins in seeds. 



D. Fractional Precipitation with Ammonium Sulphate. 



Saturation with ammonium sulphate is a means frequently employed 

 in separating proteins from solution, and partial saturation at definite 

 concentrations of this salt has been extensively used in separating them 

 from one another. It appears to be generally believed that the differ- 

 ent proteins are precipitated by ammonium sulphate at definite con- 

 centrations and within narrow limits, and that this salt can be used for 

 characterising the different protein individuals as well as for effecting 

 a sharp separation of mixtures of them. Osborne and Harris (356) 

 found that several of the seed proteins do not show such definite re- 

 lations to solutions of ammonium sulphate as the statements generally 

 current in protein literature would lead one to expect. Several of the 

 globulins which had been separated from seed extracts by fractional 

 precipitation with ammonium sulphate and purified, as far as possible, 

 by repeated precipitation between narrow limits of concentration of 

 this salt, were found to have limits of precipitation lower and wider 

 apart after being reprecipitated by dialysis. The dialysis precipitates 

 thus obtained gave every evidence that they consisted of unchanged 

 protein, and it would appear that fractional precipitation with ammonium 

 sulphate is not such a definite characteristic of these proteins as is 

 generally assumed. This subject, however, requires further investi- 



