The nature of the free amino groups in the native proteins

Abstract

The fact that at least some proteins contain a small but definite proportion of their nitrogen in the form of free amino groups capable of reacting with nitrous acid has been proven by one of us. Levites and Skraup failed to obtain lysine from proteins which had been treated with nitrous acid before hydrolysis. This indicates that the ω-NH₂ group of lysine, NH ₂·(CH₂)₄·CH₂(NH₂)-COOH, may be free in the protein molecule. As lysine is peculiar among the amino acids in possessing an ω-NH₂ group, it appeared possible that this might be responsible for all or most of the amino nitrogen determined. We have, therefore, determined the free amino nitrogen in ten proteins in which the lysine has already been quantitatively determined in either our laboratory or in Osborne's.

Without exception the free amino N of the native proteins equals within a fraction of a per cent. that of the ω-group of the lysine. It appears that the ω-group of lysine constitutes practically all of the free amino nitrogen of the native proteins determinable with nitrous acid. The albumoses show appreciably more free amino nitrogen, which is to be expected from the fact that the protein molecule is partially broken down in their preparation.