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Measuring synthesis rates of nitrogen-containing polymers by using stable isotope tracers^1,2

M. Z. Fan^*,3, L. I. Chiba, P. D. Matzat, X. Yang^*, Y. L. Yin^*,, Y. Mine^# and H. H. Stein^||

^* Department of Animal and Poultry Science, University of Guelph, ON, Canada N1G 2W1; and Department of Animal Sciences, Auburn University, Auburn, AL 36849-5415; and Elanco Animal Health, Greenfield, IN 46140; and Institute of Subtropical Agricultural Research, The Chinese Academy of Sciences, Changsha, P.O. Box 10, Hunan Province, China 410125; and ^# Department of Food Science, University of Guelph, ON, Canada N1G 2W1; and and ^|| Department of Animal and Range Sciences, South Dakota State University, Brookings 57007

³ Corresponding author: mfan{at}uoguelph.ca

The major N-containing polymer compounds in the body include protein, RNA, and DNA. The endogenous gastrointestinal secretions as well as the portal-drained visceral and peripheral immune responses are basic physiological functions. Elevated endogenous secretions and immune activities, as affected by developmental stages, diets, and management factors, decrease the availability of dietary nutrients for peripheral muscle synthesis and deposition. Measurements of in vivo protein, RNA, and DNA synthesis rates associated with the viscera, peripheral immune cells, and skeletal muscles should, in principle, be the sensitive biochemical and cellular endpoints for studying factors affecting nonruminant nutrition, metabolism, and growth. The selection of stable isotope tracers for precursors, routes of tracer delivery, and mass spectrometric analyses of tracer enrichments are the major methodological considerations. To measure in vivo protein, RNA, and DNA synthesis rates, oral feeding with heavy water (²H₂O), and continuous infusion of [U-¹³C]glucose and [¹⁵N]Gly intravenously for labeling the sugar moieties ribose and deoxyribose and de novo purine base synthesis have been established. Flooding doses of tracer Phe, for example, L-[ring-²H₅]Phe, via the i.p. route are reliable and cost-effective for measuring in vivo protein synthesis rates, especially for the viscera in small nonruminants. Therefore, measurements of the major N-containing polymer synthesis rates in the viscera, the peripheral immune cells, and muscles through oral feeding with ²H₂O and/or i.p. flooding doses of Phe tracers are the emerging tools for studying nonruminant nutrition, metabolism, and growth under research and field test conditions.

Key Words: in vivo synthesis rate • nonruminant • stable isotope tracer

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