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 ca sein (CAS) or the corresponding crystalline L-amino acid mixture (AA) were compared. Male adult rats were adapted for 9 d to two isoenergetic, isonitr ogenous 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) containin g either intrinsically C-13-labeled goat casein or the amino acid mixture c ontaining [U-C-13(6)] leucine and [alpha-N-15] lysine. Rats were killed bef ore and 1, 3, 5 and 7 h after meal ingestion and samples of plasma, stomach wall and contents, small intestine and liver were collected. C-13 and N-15 enrichments of free and protein-bound amino acids in plasma and tissues we re analyzed by gas chromatography-combustion isotope ratio mass spectrometr y. 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 prot ein synthesis. Free C-13-leucine from the AA diet appeared in the intestina l mucosa free pool more rapidly (P < 0.05) than the CAS-C-13-leucine, proba bly due to the faster transit through the stomach of the AA group. However, the incorporation of dietary leucine into plasma and liver proteins was hi gher in the CAS group 7 h after the meal (P < 0.05), whereas lysine incorpo ration into liver protein was higher in the PLA group (P < 0.05). We conclu de that whole-body protein homeostasis is better supported by dietary casei n-bound than crystalline free amino acids, and that protein-bound leucine, but not lysine, is used more efficiently for liver protein synthesis than d ietary free leucine.