A study of the occurrence of peptide nitrogen in the blood.

Abstract

The nitrogen found in tungstic acid blood filtrates, which cannot be attributed definitely to any known nitrogenous substances, has been studied by the following method: To 10 cc. of tungstic acid blood filtrate in a large test-tube graduated to 25 cc. are added 10 cc. of a saturated solution (room temperature) of barium hydroxide. The test-tube is fitted with a reflux condenser, and the contents kept boiling gently for 20 to 24 hours. The solution is then concentrated to 5 cc., a drop of 0.25 percent phenolphthalein is added and sufficient 2 percent sulfuric acid to precipitate the barium. Water is added to the 25 cc. mark, and the contents shaken and filtered. The determination of the free and hydrolyzed amino acids is carried out according to the method of Folin. 1 The difference between the free and total amino-acid nitrogen is considered as peptide nitrogen.

In a recent paper Blau 2 came to the conclusion that the amount of peptide nitrogen is by far too small to account for the undetermined nitrogen. The values found in the table, obtained by the method described, show that the peptide nitrogen makes up a very high percent of the unknown nitrogen. The reason that a more exact check on the total nitrogen is not obtained, is due probably to unavoidable small losses in the various methods and the variable quantities of the more complex amino-acids, which do not give a color proportional to their nitrogen content. It appears that easily hydrolyzed peptides are responsible for a large percent of the undetermined rest nitrogen.