TRANSFERENCE OF FOOD MATERIALS 131 



Barfoed's Reagent (copper acetate in dilute acetic acid) may 

 be used with proper precautions.* 



The proteins have been hydrolysed to the stage of peptone 

 with some polypeptides and amino acids. In the intestine 

 there is an enzyme, erepsin, which converts peptone into 

 polypeptides and amino acids. f This enzyme will not act 

 upon natural proteins (except fibrin, vitellin and caseinogen) 

 but it acts readily upon the products of partial digestion. We 

 can show the presence of erepsin by incubating it with some 

 peptone solution. As the peptone is hydrolysed to poly- 

 peptides and amino acids, tests with copper sulphate and alkali 

 show that the pink colour given by peptones with these 

 reagents gives place to the blue colour given by polypeptides 

 and amino acids with the same reagents. We can instead 

 precipitate the peptones with some reagent that precipitates 

 peptones and estimate the amount of nitrogen in the solution 

 by Kjeldahl's method. As the peptone is hydrolysed the 

 amount of nitrogen not precipitated by the peptone precipitant 

 increases. 



The fats have been hydrolysed to glycerine and fatty acids, 

 so like the other two great classes of food substances they have 

 been reduced to their simplest form. 



GENERAL REFERENCE 



J. P. PAVLOV: The Work of the Digestive Glands, 1910, translated by 

 W.H.Thompson. Charles Griffin. 



* H. E. Roaf, Biochem. Journ., 1908, vol. 3, p. 182, and Journ. 

 Physiol., 1921, vol. 54, proc. p. Ix. 

 f O. Cohnheim, Zeit. Physiol. Chem., 1901, vol. 33, p. 451. 



