A Comparative Study of the Metabolism of Certain Amino Acids, with Special Reference to Respiratory Exchange and Heat Production

Publisher Website
Google Scholar
Abstract
Glutamic acid fed to rats as a supplement to a mixed maintenance ration, in a quantity supplying 7.5 kilogram-calories of gross energy per day, was absorbed to the extent of 95%, while glycine fed as a supplement to the same basal ration, in quantities supplying 7.4 and 5.6 kilogram-calories, and alanine fed in a quantity supplying 7.5 kilogram-calories per day, were completely absorbed. From the ratios of carbon to nitrogen and of energy to nitrogen in the urine, it was calculated that approximately 3% of glutamic acid, 7% of glycine and 29% of alanine, respectively, were excreted in the urine in the form of the unoxidized amino acids, the remaining portions of the urinary nitrogen, derived from the amino acids, representing the amino acids metabolized. The metabolizable energy of glutamic acid was found to be 80.8% of its gross energy, while the metabolizability of the energy of alanine was only 65.68%, and that of glycine 64.53%, at the higher level, and 67.54% at the lower level of feeding. Factors were determined for computing the respiratory exchange and the heat production in the metabolism of each of the three amino acids. These factors were found to be closely correlated with the ratios of carbon to nitrogen in the different amino acids. A similar correlation was found to exist between the respiration and energy factors for some proteins, namely, casein, gelatin and heart muscle protein, and the ratios of carbon to nitrogen in these substances. A graphic representation of these relationships indicates that the proteins were more completely metabolized than were the single amino acids. A tentative method has been suggested for making practical use of these relationships in indirect calorimetry.