Equilibration of ¹⁵N labeled amino compounds in man

Following an overnight fast, either ¹⁵N labeled glycine, or ammonium chloride, or L aspartate was orally administered to human volunteers in equal hourly aliquots for 8-12 h. In a parallel series of experiments, [¹⁵N]glycine and [¹⁵N]ammonium chloride were given intravenously at a constant rate for 10-12 h. The amount of ¹⁵N given ranged between 2.7 and 27.3 mg/h. Five hundred milligrams of sodium benzoate were given hourly. Venous blood and urine samples were collected serially. The ¹⁵N enrichment of the plasma amino nitrogen, urinary ammonia, hippuric acid, and urea were determined. Irrespective of the ¹⁵N carrier used or the route of administration, the plasma amino nitrogen, urinary ammonia, and hippuric acid ¹⁵N enrichments approached a plateau within 6-10 h. The authors concluded that human [¹⁵N]glycine metabolism was not atypical of amino acids and that mixing, rather than nitrogen interchange reactions, determines the rate in attaining an approximate plateau.