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Partitioning of Amino Acids in Lactating Cows: Oxidation to Carbon Dioxide^1,2,

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

The rate and extent of oxidation of 20 different amino acids has been evaluated as part of a study on partitioning of amino acids in normal lactating cows. Only four amino acids—glutamate, aspartate, alanine and glutamine—were extensively oxidized, and when expressed as a percentage of the injected dose oxidized during 3 h after intravenous injection, the value for these four amino acids was not significantly different from those obtained with the volatile fatty acids. The oxidation of all other amino acids was significantly slower, effectively prolonging their availability for protein synthesis. The pattern of oxidation among the amino acids supports the hypothesis that catabolized protein can provide a protracted source of anaplerotic precursors that augment the metabolic role of the tricarboxylic acid cycle and would have survival advantage in both acute and chronic exigencies. Glutamate and aspartate provide the earliest and largest influx to the tricarboxylic acid cycle, with alanine and, especially, glutamine appearing to serve as reservoirs of amino-nitrogen. Several dispensable amino acids are metabolized as slowly as the indispensable ones, indicating that metabolic conservation is not restricted to the latter group.

KEY WORDS: • amino acids • lactating cows • ruminant metabolism • tricarboxylic acid cycle

¹ Presented in part at the 72nd Annual Meeting of the Federation of American Societies for Experimental Biology, May 1988, at Las Vegas, NV [BLACK, A. L., ANAND, R. S., BROWN, C. B. & NAKAGIRI, J. A. (1988) Partitioning of nutrient utilization in lactating cows: oxidation of amino acids. FASEB J. 2: A847 (abs.)].

² This work was supported by grants from the National Institutes of Health, HD02375 and from the U.S. Department of Agriculture, Grant #5901-0410-9-0275-0.

³ Current address: Department of Physiology, California State University, Long Beach, CA.

⁴ Deceased, 1988.

Manuscript received 30 June 1989. Revision accepted 5 February 1990.

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