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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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