Transamination Governs Nitrogen Isotope Heterogeneity of Amino Acids in Rats

The nitrogen isotope composition (delta N-15) of different amino acids carries different dietary information. We hypothesized that transamination and de novo synthesis create three groups that largely explain their dietary information. Rats were fed with N-15-labeled amino acids. The redistribution of the dietary N-15 labels among the muscular amino acids was analyzed. Subsequently, the labeling was changed and the nitrogen isotope turnover was analyzed. The amino acids had a common nitrogen half-life of similar to 20 d, but differed in delta N-15. Nontransaminating and essential amino acids largely conserved the delta N-15 of the source and, hence, trace the origin in heterogeneous diets. Nonessential and nontransaminating amino acids showed a nitrogen isotope composition between their dietary composition and that of their de novo synthesis pool, likely indicating their fraction of de novo synthesis. The bulk of amino acids, which are transaminating, derived their N from a common N pool and hence their delta N-15 was similar.