Significance of N¹⁵excess in Nitrogenous Compounds of Biological Origin

The purpose of this study was to determine whether the small increase in abundance of N 15 normally found in nitrogenous compounds of biological origin is primarily due to mass discrimination in nitrogen metabolism, or to reproducible analytical errors.This problem was approached by repeatedly determining N 15 in the same series of nine amino acids, either purchased in chromatographically pure form or isolated from proteins of rat liver, dog serum, and six plant sources. Proteins and amino acids included in the study were selected on the premise that concentration or redistribution of N 15 might occur in such processes in urea formation, nitrogen transfer, or nitrogen fixation. Data for N 15 excess in a series of alkaloids were also secured.Standard deviations obtained in series of analyses were too small, and values for N 15 excess in the same amino acids isolated from different sources too variable, to permit interpreting the observed N 15 excess as a reproducible error. Distribution of N 15 in amino acids of the animal proteins studied resembled that observed when N 15 -labelled amino acids or ammonium compounds are given. Differences between results for amino acids of animal and plant origin also supported the idea that the small excess of N 15 normally found is metabolically as well as statistically significant. Results for N 15 in amide nitrogen likewise supported this view. The smallest excess of N 15 occurred in amino acids from proteins of legumes, which fix nitrogen. In synthetic amino acids, the concentration of N 15 was more often below than above normal abundance.