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The Metabolism of Tyrosine, Aspartic Acid and Asparagine, with Special Reference to Respiratory Exchange and Heat Production¹

Institute of Animal Nutrition, The Pennsylvania State College, State College

Tyrosine, aspartic acid and asparagine fed to rats as supplements to a mixed maintenance ration, in quantities supplying 7.5, 5.8 and 6.0 kilogram-calories per day, respectively, appeared to be absorbed to the extent of 97%, 95% and 97%. The balances of nitrogen indicated that of the total nitrogen ingested as aspartic acid and as asparagine 25.1% and 13.7%, respectively, were retained in the body, and that none of the tyrosine was retained. These balances were presumably conditioned by the composition of the basal ration.

The C:N ratios of the urinary constituents from the different supplements were as follows: from tyrosine 2.77:1; from aspartic acid 0.50:1; and from asparagine 0.45:1. The energy:N ratios of the urine derived from the supplements were as follows: from tyrosine 25.18:1; from aspartic acid 6.30:1; and from asparagine 5.80:1. On the basis of these ratios the calculated fractions of the urinary nitrogen representing metabolized amino acids were approximately, for tyrosine 65%, for aspartic acid 97% and for asparagine 98%. The metabolizable energy values of tyrosine, aspartic acid and asparagine (corrected to a basis of nitrogen equilibrium) were found to be 59.2%, 71.8% and 60.6%, respectively, of the gross energy of the amino acids.

Factors were determined for computing the respiratory exchange and the heat production in the metabolism of each of the amino acids tested. The values obtained for O₂, CO₂ and calories per gram of urinary nitrogen are approximately twice as great for aspartic acid as the corresponding values for asparagine; and the values for tyrosine are approximately four times as great as those for asparagine, thus showing, in confirmation of a previous report, a close correlation between these factors and the ratios of carbon to nitrogen in the materials.

A very close agreement was obtained between the determined respiration and energy factors, expressed per gram of nitrogen of amino acid metabolized, and the values computed on a theoretical basis.

Manuscript received 10 October 1939.