118 THE KOLK OF NITROGEN IN PLANT-METABOLISM, ill., 



tannic acid used, and that the presence of sodium chloride makes 

 separation more complete. 



Mey *, 1906, found, in the tannin filtrate from peptic digests, 

 numei'ous peptone-like substances giving the biuret reaction. 



Jiigelow and Cook f, 1906, by numerous experiments, deter- 

 mined the conditions of maximum precipitation for Witte's 

 peptone to be concentration of tannic acid 5%, sodium chloride 

 15%, in the final solution in which precipitation is made. This 

 gave the maximum pi'ecipitation of proteins, and separated 94% 

 of the nitrogen of Witte's peptone. This reagent precipitated 

 proteoses and peptones at 12°C. 



►Suzuki I, 1907, employed a tannin-salt solution in his experi- 

 ments on germinating seeds, to separate total proteins, including 

 peptones. 



Bialosuknia §, 1908, and numerous other workers in plant- 

 proteins, measure the activity of proteolytic enzymes by the 

 increase of nitrogen in the tannin filtrates. 



These references suffice to show the uncertainty which 

 existed with regard to the completeness of the precipitation 

 by tannic acid. There is no doubt concerning the true pro- 

 teins, as Sebelien showed, their precipitability being complete 

 when the correct conditions, as to the amount of reagent and 

 concentration of protein, are found for each case, condi- 

 tions which vary with the nature of the protein. But, in 

 dealing with protein-derivatives, there is" now sufficient 

 evidence to show, that many of the polypeptides are redis- 

 solved by excess of the reagent, so that the filtrates may give 

 a positive biuret reaction. Since the polypeptides may exist 

 in decreasing molecular magnitude, from the very complex 

 to the simple dipeptides, it would appear useless to fix any 

 limits as to which are, and which are not precipitated. But 

 it is quite certain that the smaller members are soluble. 



* MRY—ihid. 48. 1906, 81. 



t BiGELOW k Cook — Journ. Amer. ("hem. Soc. 28, 1906, 1485. 



X SuzDKi— Joiun. Biol. Chein. iii., 1907, 268. 



§ Bialosuknia— Zeit. physiol. Cheni. 58, 1908, 487. 



