Whole-body nitrogen and splanchnic amino acid metabolism differ in rats fed mixed diets containing casein or its corresponding amino acid mixture.
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 (13)C-labeled goat casein or the amino acid mixture containing [U-(13)C(6)] leucine and [alpha-(15)N] 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. (13)C and (15)N 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 (13)C-leucine from the AA diet appeared in the intestinal mucosa free pool more rapidly (P < 0.05) than the CAS-(13)C-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.
Daenzer
M
M
Deutsches Institut für Ernährungsforschung (DIfE), D-14558 Bergholz-Rehbrücke, Germany.
Petzke
K J
KJ
Bequette
B J
BJ
Metges
C C
CC
D000596:Amino Acids / Q000008:administration & dosage / Q000378:metabolism* / Q000493:pharmacokinetics; D000818:Animals; D002247:Carbon Isotopes; D002364:Caseins / Q000008:administration & dosage* / Q000737:chemistry / Q000493:pharmacokinetics; D004734:Energy Metabolism; D008401:Gas Chromatography-Mass Spectrometry; D005753:Gastric Mucosa / Q000378:metabolism; D005772:Gastrointestinal Transit; D006706:Homeostasis; D007413:Intestinal Mucosa / Q000378:metabolism*; D007700:Kinetics; D007930:Leucine / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008239:Lysine / Q000378:metabolism; D008297:Male; D009584:Nitrogen / Q000378:metabolism* / Q000652:urine; D051381:Rats; D017208:Rats, Wistar; D013152:Splanchnic Circulation; D015430:Weight Gain
D000596:Amino Acids; D002247:Carbon Isotopes; D002364:Caseins; D007930:Leucine; D008239:Lysine; D009584:Nitrogen