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Quantitative Aspects of Glycine and Alanine Nitrogen Metabolism in Postabsorptive Young Men: Effects of Level of Nitrogen and Dispensable Amino Acid Intake¹

Y. M. Yu^2,*, R. D. Yang^*, D. E. Matthews, Zhi M. Wen^*, J. F. Burke, D. M. Bier and V. R. Young^*

^* Laboratory of Human Nutrition, Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, MA 02139 Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110 Department of Surgery, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114

The nutritionally indispensable amino acids (IAA) alone do not maintain body nitrogen (N) balance; a source of "nonspecific" nitrogen from dispensable amino acids (DAA), such as from glycine and alanine or other N compounds, is required. However, the in vivo regulation of the metabolism of these amino acids in humans with varying nutritional states has received little study. Hence, the effects of N intake and the IAA:DAA ratio on kinetic aspects of whole-body alanine and glycine metabolism were examined in eight healthy young adult male subjects. They received an L-amino acid diet supplying N equivalent to about 1.5 g and 0.6 g protein (N x 6.25) per kilogram body weight per day. All were studies at each N level with the IAA:DAA ratio (wt/wt) of 1:1 and 1:0, each for a 7-d diet period. Constant primed, intravenous infusions of L-[l-¹³C]leucine together with either L-[¹⁵N]alanine (four subjects) or [¹⁵N]glycine (four subjects) were given to each subject at the end of the diet period, after an overnight fast, to determine rates of de novo whole-body alanine and glycine N synthesis. The rate of alanine synthesis was similar (P > 0.05) for all four diets. Glycine de novo N synthesis declined (P < 0.01) with removal of dietary DAA, especially at the lower intake, where the mean rates [micromoles/(kilogram·hour)] were 59 and 20 for 1:1 and 1:0 ratios, respectively. The possible significance of reduced rates of glycine N synthesis for maintenance of protein nutritional status in the healthy adult is discussed.

KEY WORDS: • alanine • glycine • metabolism • dispensable amino acids • indispensable amino acids • synthesis • [¹⁵N]glycine • [⁵N]alanine • [l-¹³C]leucine

¹ This study was supported, in part, by grants from the National Institutes of Health: from the National Institute of Arthritis, Diabetes, Digestive and Kidney Diseases, grants AM 15856 and AM 25994, from the National Institute of Child Health and Human Development, grant HD 10667, and from the Division of Research Resources, grant RR 954 and RR 88. Additional funding from the U.S. Department of Agriculture was through grant 81-CRCR-0070.

² On study leave from the Department of Surgery, Capital Hospital, Chinese Academy of Medical Sciences, Beijing, People's Republic of China.

Manuscript received 27 August 1984.

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