Proline is synthesized from glutamate during intragastric infusion but not during intravenous infusion in neonatal piglets

Glutamate is considered the primary precursor amino acid for proline synthesis in mammals. Evidence exists, however, suggesting that proline may be a dietary indispensable amino acid for 2.5-kg piglets due to inadequate synthesis. This hypothesis was tested by intravenous and intragastric infusion of radiolabeled amino acids in vivo. Piglets (3 to 4 d old) were surgically implanted with catheters in the femoral (infusion) and jugular (sampling) veins and in the stomach (feeding and infusion). Piglets were fed hourly, via the stomach catheter, a semi-purified diet containing 10% dried skim milk, 15% corn oil, amino acids, vitamins and minerals. Experiment 1 was a 2 x 2 factorial design, with 24 piglets adapted to either low or supplemental proline diets (1.3 and 16.4 g proline . kg(-1) respectively) for 7 d, then intravenously infused with either [U-C-14]glutamate or [U-C-14]proline (185 kBq . kg(-1) prime; 370 kBq . kg(-1). h(-1) constant) for 4 h. Experiment 2 followed similar protocols, with eight piglets adapted to the low proline diet for 7 d and [U-C-14]glutamate or [U-C-14]proline infused into the stomach catheter. Piglets infused intravenously with [U-C-14]glutamate did not convert glutamate to proline. Radioactive label was recovered in proline in all of the piglets receiving intragastric infusion of [U-C-14]glutamate. The fractional synthesis rate of proline from intragastric glutamate was 125 mu mol . kg(-1). h(-1), accounting for approximately 40% of the proline accumulated. These data provide conclusive evidence that intravenously infused glutamate is not used as a precursor for proline synthesis and that, although conversion of glutamate to proline occurs in the gastrointestinal tract, the rate is not sufficient to provide the proline accumulated.