ANALYSIS OF TISSUES 575 



B. PROTEINS. 



Although several proteins are generally present together in animal and 

 vegetable tissues it is not usual to determine the amount of each protein, in 

 them ; only the total amount of protein is ascertained. 



The total amount of protein is estimated by Kjeldahl's method (p. 35); 

 1-2 gm. of moist material, or -2 --4 gm. of dry material, are taken for the 

 analysis. 



Since most proteins contain about 15 per cent, of nitrogen the result in 

 gm. of nitrogen is multiplied by - or 6*6. The figure 6*25 is generally used ; 

 in the case of vegetable proteins the figure 5*68 should be used (p. 450). 



This value will include the nitrogen contained in the nitrogenous ex- 

 tractives. Their amount is so small in most tissues that it is neglected. 



In order to estimate the amount of the individual proteins the properties 

 -of these proteins must be taken into account so that they may be separated 

 from one another. Separation of proteins cannot in most cases be effected by 

 ammonium sulphate but zinc sulphate may be used. Proteins behave in 

 exactly the same way to zinc sulphate as they do to ammonium sulphate. 

 Either the precipitate may be filtered off and its total nitrogen determined or 

 the loss of nitrogen in the filtrate may be determined. 



The analysis of the proteins in milk may be taken as a typical example. 



Analysis of the Proteins in Milk. 



Total Protein. 



(1) The nitrogen in 5 c.c. is determined by Kjeldahl's method and the 

 figure multiplied by 6-38. 



The non-protein nitrogen in cow's milk varies from 0*022 to 0*034 pel 

 cent., in human milk from 0-014 to 0*026 per cent. Taking these figures 

 into account, the protein nitrogen in cow's milk would therefore be 94 per 

 cent, of the total, in human milk 91 per cent. 



(2) The amount of total protein in milk may be rapidly determined by 

 deration after the addition of formaldehyde. Strontium hydroxide should be 

 used as alkali. 



10 c.c. of milk are titrated with -iN alkali using 0*5 c.c. of i per cent, 

 phenolphthalein solution as indicator until a faintly pink colour is obtained. 

 2 c.c. or more of formalin neutralised with the same alkali to phenol- 

 phthalein are added and the mixture is titrated with *iN alkali until the 

 ;same pink colour is again obtained. 



The number of c.c. used multiplied by *i 7 gives the amount of total proteins. 



