CHEMICAL COMPOSITION OF PROTEIN MOLECULE 79 

 Calculation of Histidine Value. 



Total nitrogen of bases minus total amino nitrogen gives the total 

 non-amino nitrogen, D. This is contained in the arginine and histi- 

 dine. 



Since three-fourths of the arginine nitrogen and two-thirds of the 

 histidine nitrogen are in this form, the total non-amino nitrogen minus 

 three-fourths of the arginine nitrogen (found previously) represents two- 

 thirds of the histidine nitrogen. Hence 



Histidine N - f (D - f Arg.) 



= 1-667 D - 1*125 Arg. 



Calculation of Lysine V^alue. 



Knowing the other values, that of lysine is found by difference : 

 Lysine N = Total N - (Arg. N + Cyst. N + Hist. N). 



11. Determination of the Total Nitrogen of the Monoamino Acids. 

 To the combined filtrate and washings from the phosphotungstate pre- 

 cipitate 50 per cent, caustic soda is carefully added, until the solution 

 becomes turbid by precipitation of lime and is just alkaline. Excess 

 must be carefully avoided as the precipitate may then not completely 

 dissolve in acetic acid, which is added to clear the solution when just 

 alkaline. The acid solution is concentrated in vacuo until salt com- 

 mences to crystallise out. The solution is then washed into a 150 c.c. 

 measuring flask and diluted up to the mark. 



Total nitrogen is estimated in 25 c.c. portions by Kjeldahl's 

 method, using 35 c.c. of sulphuric acid, 15 grams potassium sulphate 

 and 0-25 grams of copper sulphate. The acid must be added care- 

 fully on account of the evolution of hydrochloric acid. The digestion 

 must be continued for three hours after the solution has become 

 clear. 



1 2. Determination of the Amino Nitrogen of the Monoamino Acids. 

 10 c.c. portions of the solution are used for this purpose and the time 

 for the reaction with nitrous acid is six to ten minutes. 



The volume of nitrogen evolved by a given amount of amino nitro- 

 gen is 2*5 times the volume of decinormal acid neutralised, if the same 

 amount is determined by the Kjeldahl method. Therefore, the por- 

 tions of 25 c.c. and 10 c.c. used in these last estimations give results 

 of similar accuracy. 



