The Nitrogen Moieties of Dietary Nonessential Amino Acids Are Distinctively Metabolized in the Gut and Distributed to the Circulation in Rats.

Although previous growth studies in rodents have indicated the importance of dietary nonessential amino acids (NEAAs) as nitrogen sources, individual NEAAs have different growth-promoting activities. This phenomenon might be attributable to differences in the nitrogen metabolism of individual NEAAs. The aim of this study was to compare nitrogen metabolism across dietary NEAAs with the use of their N isotopologues. Male Fischer rats (8 wk old) were given 1.0 g amino acid-defined diets containing either N-labeled glutamate, glutamine (amino or amide), aspartate, alanine, proline, glycine, or serine hourly for 5-6 h. Then, steady-state amino acid concentrations and their N enrichments in the gut and in portal and arterial plasma were measured by an amino acid analyzer and LC tandem mass spectrometry, respectively. The intestinal N distribution and portal-arterial balance of N metabolites indicated that most dietary glutamate nitrogen (>90% of dietary input) was incorporated into various amino acids, including alanine, proline, and citrulline, in the gut. Dietary aspartate nitrogen, alanine nitrogen, and amino nitrogen of glutamine were distributed similarly to other amino acids both in the gut and in the circulation. In contrast, incorporation of the nitrogen moieties of dietary proline, serine, and glycine into other amino acids was less than that of other NEAAs, although interconversion between serine and glycine was very active. Cluster analysis of N enrichment data also indicated that dietary glutamate nitrogen, aspartate nitrogen, alanine nitrogen, and the amino nitrogen of glutamine were distributed similarly to intestinal and circulating amino acids. Further, the analysis revealed close relations between intestinal and arterial N enrichment for each amino acid. The steady-state N enrichment of arterial amino acids indicated that substantial amounts of circulating amino acid nitrogen are derived from dietary NEAAs. The present results revealed similarities and differences among NEAAs in terms of their intestinal nitrogen metabolism in rats and indicated substantial entry of dietary NEAA nitrogen into circulating amino acid nitrogen, presumably primarily through metabolism in the gut.