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 s ynthesis in mammals. Evidence exists, however, suggesting that proline may be a dietary indispensable amino acid for 2.5-kg piglets due to i nadequate synthesis. This hypothesis was tested by intravenous and int ragastric infusion of radiolabeled amino acids in vivo. Piglets (3 to 4 d old) were surgically implanted with catheters in the femoral (infu sion) and jugular (sampling) veins and in the stomach (feeding and inf usion). Piglets were fed hourly, via the stomach catheter, a semi-puri fied diet containing 10% dried skim milk, 15% corn oil, amino acids, v itamins and minerals. Experiment 1 was a 2 x 2 factorial design, with 24 piglets adapted to either low or supplemental proline diets (1.3 an d 16.4 g proline . kg(-1) respectively) for 7 d, then intravenously in fused 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 foll owed 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]glutamat e did not convert glutamate to proline. Radioactive label was recovere d in proline in all of the piglets receiving intragastric infusion of [U-C-14]glutamate. The fractional synthesis rate of proline from intra gastric glutamate was 125 mu mol . kg(-1). h(-1), accounting for appro ximately 40% of the proline accumulated. These data provide conclusive evidence that intravenously infused glutamate is not used as a precur sor for proline synthesis and that, although conversion of glutamate t o proline occurs in the gastrointestinal tract, the rate is not suffic ient to provide the proline accumulated.