SIGNIFICANCE OF N15 EXCESS IN NITROGENOUS COMPOUNDS OF BIOLOGICAL ORIGIN

The purpose of this study was to determine whether the small increase in abundance of N15 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 N15 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 N15 might occur in such processes in urea formation, nitrogen transfer, or nitrogen fixation. Data for N15 excess in a series of alkaloids were also secured.Standard deviations obtained in series of analyses were too small, and values for N15 excess in the same amino acids isolated from different sources too variable, to permit interpreting the observed N15 excess as a reproducible error. Distribution of N15 in amino acids of the animal proteins studied resembled that observed when N15-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 N15 normally found is metabolically as well as statistically significant. Results for N15 in amide nitrogen likewise supported this view. The smallest excess of N15 occurred in amino acids from proteins of legumes, which fix nitrogen. In synthetic amino acids, the concentration of N15 was more often below than above normal abundance.