Jm. Murphy et al., PROLINE IS SYNTHESIZED FROM GLUTAMATE DURING INTRAGASTRIC INFUSION BUT NOT DURING INTRAVENOUS-INFUSION IN NEONATAL PIGLETS, The Journal of nutrition, 126(4), 1996, pp. 878-886
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.