142 PHYSIOLOGY OF NUTRITION 



6. Glyoxylic acid and concentrated sulphuric acid produce a beautiful 

 bluish-violet color (Adamkiewicz and Hopkin's tr3^tophan reaction). 



The method of Stutzer^ may be used for the quantitative determination of 

 proteins. This depends upon the fact that with copper hydroxide these sub- 

 stances form a compound that is insoluble in water. The determination is carried 

 out as follows. The triturated plant tissue is boiled with water and the ex- 

 tract is then treated with copper hydroxide. The precipitate is filtered ofif 

 with hot water and is then washed with alcohol and dried. This precipitate 

 contains all the protein material. The other nitrogenous substances of plants 

 form water-soluble compounds with copper hydroxide and are thus removed 

 in the filtrate. For the determination of nitrogeii in the precipitate the well- 

 known method of Kjeldahl may be used. The nitrogen of most organic sub- 

 stances is converted into ammonia by boiling with fuming sulphuric acid 

 and thus remains in the flask as ammonium sulphate, which may then be deter- 

 mined by any of the usual methods. From the result is calculated the amount 

 of protein nitrogen originally present. 



If the entire nitrogen content is determined for one portion of the material 

 and the content in protein nitrogen is determined for another portion, the diflfer- 

 ence between these two numbers gives the amount of the non-protein nitrogen. 



The following table may serve to show the relative amounts of protein nitro- 

 gen and of non-protein nitrogen contained in different plants. The quantities 

 are given as percentages of total nitrogen present. A considerable amount of 

 nitrogen is seen to be present in simple compounds. 



Protein Non-protein 



Nitrogen Nitrogen 



Vetch 67.2 32.8 



Young alfalfa 73.1 26.9 



Potato tubers (July 7) . . 58.7 41 . 3 



From a physiological viewpoint one must distinguish between two groups of 

 proteins; the simple proteins or albuminous bodies, and the conjugated proteins 

 or combinations of simple proteins with other substances. The simple proteins 

 are reserve foods (as, for example, the albumin of aleurone grains), and the 

 compound proteins are essential in the life of the cell. The latter form the 

 principal non-aqueous component of protoplasm, as is evident from the 

 analysis given on page 139.' 



The simple proteins may be grouped as follows." 



1 Stutzer, A., Untersuchungen iiber die quantitative Bestimmung des Protein Stickstoffs und die Tren- 

 nung der Proteinstoffe von anderen in Pflanzen vorkommenden Stickstoff-Verbindungen. Jour. exp. 

 Landw. :28 103-123. 1881. Idem, Untersuchungen uber die Verdaulichkeit und die quantitative Bestim- 

 mung der Eiwissstoffe, Ibid.2g: 473-492. 1881. 



>> Since our knowledge of plant proteins rests almost wholly upon the investigation of seeds, 

 a general classification based on physiological properties is not yet possible. For a discussion 

 of this point see: Osborne, 1909. [See note i, p. 140.] — Ed. 



' This discussion is mainly based on our knowledge of animal proteins; plant proteins appear 

 to differ from these in many respects. See Osborne, 1909. [See note i, p. 140]. Haas and 

 Hill, 1913, p. 312. [See note 3, p. 6.] — Ed. 



