l6o PHYSIOLOGY OF NUTRITION 



6. Glyoxylic acid and concentrated sulphuric acid produce a beautiful 

 bluish-violet color (Adamkiewicz and Hopkin's tryptophan reaction). 



The method of Stutzer 1 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 

 extract is then treated with copper hydroxide. The precipitate is filtered off 

 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 nitrogen 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 differ- 

 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 combination 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 157. 6 



The simple proteins may be grouped as follows. 



c 



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

 nung der ProteinstofFe von anderen in Pflanzen vorkommenden Stickoff-Verbindungen. Jour. exp. Landw. 

 28: 103-123. 1881. Idem, Untersuchungen liber die Verdaulichkeit und die quantitative Bestimmung 

 der Eiweissstoffe. Ibid. 29: 473-492. 1881. 



6 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 1, p. 158.] — Ed. 



c 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 1, p. 158.] Haas and 

 Hill, 1 92 1. [See note 3, p. 6.] — Ed. 



