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Glutamic Acid Metabolism in the Lactating Dairy Cow¹

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

A single dose of L-glutamic acid uniformly labeled with carbon-14 was injected into a lactating dairy cow via the portal vein or the jugular vein in four experiments, and the time course of the isotope was followed in respired CO₂, plasma glutamate, plasma glucose and milk constituents. In each experiment the specific activity of plasma glutamate decreased with a half-time of approximately 5 minutes during the first 30 minutes after injection. Expired CO₂ and plasma glucose became labeled rapidly with maximal specific activities occurring at about 12 minutes in CO₂ and 20 minutes after the injection in plasma glucose. Between 34 and 41% of the injected dose appeared in respired CO₂ in the first 3 hours. Milk constituents contained 3 to 8% of the injected dose after 3 hours and 12 to 15% after 48 hours. Lactose accounted for 46 to 54% of all carbon-14 recovered in milk during 48 hours; casein, 27 to 37%; and albumin, 4 to 5%. The specific activity of citrate was very high, whereas that of milk fat was negligible. Aspartate was the most highly labeled amino acid in casein except for glutamate, indicating extensive interconversion of these two amino acids. Proline and arginine had very low specific activities, which means that their synthesis from glutamate was nominal. It is concluded that the amount of carbon from the plasma glutamate pool utilized for gluconeogenesis exceeds that incorporated into milk protein.

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.

Manuscript received 25 June 1968.