Interpretation of urinary N¹⁵-excretion data following administration of an N¹⁵-labeled amino acid

¹ Department of Biochemistry, Medical College of Alabama, Birmingham, Alabama, Division of Chemistry, Naval Medical Research Institute, Bethesda, Maryland and the Chronic Disease Research Institute, University of Buffalo, Buffalo, New York

A discussion is given of the problems encountered in the interpretation of urinary excretion data when the isotope concentration versus time curve is fitted to an exponential equation. The possibility of evaluating the size of the initial pool of the specific amino acid which is administered in labeled form, and its rate of turnover, is illustrated with ammonia excretion data from intravenous glycine experiments. The calculation of the size of other pools is shown with ammonia, hippuric acid and urea excretion data from oral glycine and phenylalanine experiments. From the cumulative total-N¹⁵ and urea-N¹⁵ excretion data in the later ‘post-equilibrium’ period, the rate of (apparently irreversible) protein synthesis has been calculated. The values (about 1 mEq/hr/kg body weight) obtained in intravenous and oral experiments on man with phenylalanine, glycine, aspartic acid and ammonium citrate are substantially the same. As the labeled-NH₂ is incorporated into more and more compounds and as each of these labeled compounds diffuses into an increasing number of compartments of the body, the apparent size of the mixed amino-N pool, as reflected in the isotope concentration of a urinary compound such as ammonia, seems to expand with time until eventually it equals the total exchangeable-NH₂ in the body.