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Metabolism of Glutamic Acid, Valine and Arginine by the Lactating Goat¹

Department of Physiological Sciences, School of Veterinary Medicine, University of California, Davis, California 95616

The metabolic fate of uniformly ¹⁴C-labeled L-glutamic acid, L-valine and L-arginine, injected intravenously as a pulse dose into lactating goats, was studied in a series of six experiments. Glutamic acid was the most rapidly and extensively oxidized of the amino acids. In the first 3 hours after injection, respired CO₂ contained about 40% of the carbon-14 from glutamate, and the peak specific activity in CO₂ was reached within 10 to 12 minutes. The slower oxidation of valine and arginine was apparent from carbon-14 recoveries in CO₂ during 3 hours which amounted to 15% and less than 10%, respectively. The maximum specific activity occurred in CO₂ at 20 and 45 minutes following the injection of valine and arginine, respectively. Glutamate was the most glucogenic of the three amino acids. Carbon-14 from glutamate appeared earliest and in largest amounts in plasma glucose, while smaller amounts of ¹⁴C from valine and arginine appeared at a slower rate. More of the carbon-14 from glutamate appeared in lactose than in milk protein, whereas the largest part of the carbon-14 from valine and arginine was recovered in milk protein. Citrate had high specific activity, especially in the glutamic acid experiments, whereas milk fat had very low specific activity after the injection of all three amino acids. It is concluded that, in the goat, processes involving gluconeogenesis acquire a greater part of the glutamate flux than processes involved in milk protein synthesis. The reverse relationship is true for valine and arginine. The relatively slow metabolism of valine and arginine has the effect of conserving these amino acids for protein synthesis; it also results in a prolonged availability of the carbon from these amino acids for gluconeogenesis.

² Present address: Department of Agronomy, Waite Agricultural Research Institute, University of Adelaide, Private Bag No. 1, Glen Osmond, South Australia. Supported by an overseas scholarship from the Australian Dairy Produce Board during the tenure of this study.

³ Present address: Department of Biochemistry, Queens University, Kingston, Ontario, Canada. Postdoctoral Fellow of the National Institute of Arthritis and Metabolic Diseases (F-F2-AM-24, 702-02).

Manuscript received 13 November 1969.