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Whole-Body Nitrogen and Splanchnic Amino Acid Metabolism Differ in Rats Fed Mixed Diets Containing Casein or Its Corresponding Amino Acid Mixture¹

Deutsches Institut für Ernährungsforschung (DIfE), D-14558 Bergholz-Rehbrücke, Germany and ^* Rowett Research Institute, Bucksburn, Aberdeen, AB295SB, Scotland, UK

²To whom correspondence should be addressed. E-mail: metges{at}www.dife.de

Whole-body and splanchnic metabolism of dietary amino acids derived from casein (CAS) or the corresponding crystalline L-amino acid mixture (AA) were compared. Male adult rats were adapted for 9 d to two isoenergetic, isonitrogenous diets (15 g/100 g protein, 5 g/100 g fat) containing either CAS or AA. On d 10, the rats were fed a single mixed meal (3 g dry mass) containing either intrinsically ¹³C-labeled goat casein or the amino acid mixture containing [U-¹³C₆] leucine and [-¹⁵N] lysine. Rats were killed before and 1, 3, 5 and 7 h after meal ingestion and samples of plasma, stomach wall and contents, small intestine and liver were collected. ¹³C and ¹⁵N enrichments of free and protein-bound amino acids in plasma and tissues were analyzed by gas chromatography-combustion isotope ratio mass spectrometry. Urinary nitrogen excretion was higher (P < 0.05) and weight gain lower (P < 0.05) in rats given the AA diet, indicating a lower whole-body net protein synthesis. Free ¹³C-leucine from the AA diet appeared in the intestinal mucosa free pool more rapidly (P < 0.05) than the CAS-¹³C-leucine, probably due to the faster transit through the stomach of the AA group. However, the incorporation of dietary leucine into plasma and liver proteins was higher in the CAS group 7 h after the meal (P < 0.05), whereas lysine incorporation into liver protein was higher in the AA group (P < 0.05). We conclude that whole-body protein homeostasis is better supported by dietary casein-bound than crystalline free amino acids, and that protein-bound leucine, but not lysine, is used more efficiently for liver protein synthesis than dietary free leucine.

KEY WORDS: • ¹³C leucine • ¹⁵N lysine • splanchnic tissue • stable isotopes • mass spectrometry

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