Pattern of N¹⁵-excretion in man following administration of N¹⁵-labeled L-phenylalanine

¹ Department of Biochemistry, Medical College of Alabama, Birmingham, Alabama, and the Division of Chemistry, Naval Medical Research Institute, Bethesda, Maryland

On the basis of the same dose (in mEq) per unit body weight of N¹⁵ administered, the patterns of N¹⁵-excretion following oral administration of labeled L-phenylalanine and of ammonium citrate have been compared with those of labeled glycine and aspartic acid in previous studies. In the first few hours, the patterns for ammonia, urea and hippuric acid are markedly different for different amino acids. After 6–8 hours, with the exception of hippuric acid, the patterns are substantially the same, regardless of the particular amino acid administered. The data obtained in the first few hours reflect predominantly the rate of metabolism of the particular amino acid administered. The ratios of the isotope concentration of glycine (calculated from that of hippuric acid) to that of ammonia, following the oral administration of l-phenylalanine, and of ammonium citrate, deviate from theory not only in the early stages of excretion, but in the attainment of a final value of about 0.6; this indicates the possibility that the turnover rate of glycine is higher than the average for ammonia-forming amino acids. Following oral administration of L-phenylalanine and also after ammonium citrate, the isotope concentrations of ammonia and of urea are equal at the maximum point of the latter. From the ammonia-urea relationship, the assumption of a single pool of amino acids in the body seems justified in oral experiments.

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(With the Assistance of James George and Daisy Yen Wu)