330 PROTEINS 



These researches would therefore lead to the conclusion 

 that the proteins, throuy;h the breaking up of various amino 

 acids derived from them, are ultimately responsible for the 

 production of a variety of nitrogen-free alcohols, aldehydes 

 and acids as bye-products, which go to produce the different 

 essential oils, etc. 



The metabolism of proteins in the animal world is, as is 

 well known, a very important process and results in their very 

 complete decomposition with the formation of urea, carbon 

 dioxide and water. Although little is known concerning the 

 metabolism of proteins by plants, there is good reason for be- 

 lieving that the destruction of the protein molecule is far less 

 complete ; the occurrence of urea has in fact only rarely been 

 recorded in plants, namely, in Lycoperdon Bovista * and traces 

 have been isolated from Cichoriuvi endivi, Cucurbita maxima, 

 Cucumis melo, Brassica olereacea and B. napus, Solanum tuber- 

 osum and Spinacia olereacea.^ It has been suggested that 

 many of the simpler nitrogenous compounds, as, for example, 

 caffeine, theobromine, the alkaloids, skatol and allied sub- 

 stances such as indigo, indoxyl, etc., may be products of pro- 

 tein metabolism. 



ESTIMATION OF NITROGEN. 

 The Kjeldahl process for the estimation of nitrogen de- 

 pends on the fact that organic nitrogenous compounds when 

 boiled with concentrated sulphuric acid are decomposed with 

 the formation of ammonium sulphate, | while the carbon and 

 hydrogen are oxidized to carbon dioxide and water respectively 

 by the sulphuric acid. In order to ensure complete destruction 

 of the organic matter, the heating must be continued until the 

 mixture is colourless, or, at most, light straw-coloured. This 



* Bamberger and Landsiedl : " Monatshefte,'' 1903, 24, 21S. 



f Fosse: " Compt. rend.," 1912, 155, 851 ; 1913, 156, 567. 



:{: Organic compounds containing the nitro- or nitroso- groups and also inor- 

 ganic nitrates and nitrites, if heated with concentrated sulphuric acid, would 

 give off oxides of nitrogen which would escape without conversion into am- 

 monium sulphate; this diftkulty may, however, be overcome by adding to the 

 substance from 1-2 grams of zinc dust, and then rapidly pouring over the mix- 

 ture, so as to cover it at once, a solution of 2 grams of salicylic acid in 20 to 30 

 c.c. of concentrated sulphuric acid, carefully heating until frothing ceases, and 

 then proceeding with the addition of potassium sulphate, etc. The nascent 

 hydrogen produced by the zinc and the acid probably reduces the nitro group to 

 the amino group, which is then readily converted into ammonium sulphate. 



