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1 Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2P5; and 2 The Research Institute, The Hospital for Sick Children, and the Departments of 3 Paediatrics and 4 Nutritional Sciences, University of Toronto, Toronto, Ontario, Canada M5G 1X8
We have previously shown that
arginine deficiency is exacerbated by the removal of dietary proline in
orally, but not parenterally, fed piglets. Therefore, we hypothesized
that the net interconversions of proline, ornithine, and arginine
primarily occur in the small intestine of neonatal piglets. Ten
intragastrically fed piglets received either intraportal (IP) or
intragastric (IG) primed, constant infusions of
[guanido-14C]arginine and
[U-14C]ornithine + [2,3-3H]proline. By
infusing amino acid isotopes via the stomach compared with the portal
vein, we isolated small intestinal first-pass metabolism in vivo.
During IP infusion, fractional net conversions (%) from proline to
ornithine (0), ornithine to arginine (11 ± 6), and
ornithine to proline (5 ± 1) were lower (P < 0.05) than during IG infusion (39 ± 8, 18 ± 6, and 42 ± 12, respectively); we speculate that these data are due to the
localization of ornithine aminotransferase to the gut. The balance of
these conversions indicated a large synthesis of arginine (70.0 µmol · kg
1 · h1)
by the gut, with a corresponding degradation of ornithine (70.8 µmol · kg1 · h1)
and no change in proline balance. Gut synthesis of arginine from
proline (48.1 µmol · kg1 · h1)
was 50% of its requirement, whereas proline synthesis from arginine (33.0 µmol · kg1 · h1)
amounted to 10% of its requirement. Overall, arginine synthesis is
more dependent on the gut than proline synthesis. In situations in
which gut metabolism is compromised, such as during parenteral nutrition or gastrointestinal disease, arginine and proline are individually indispensable because their biosyntheses are negligible.
biosynthesis; intraportal infusion; ornithine aminotransferase; amino acid kinetics; first-pass metabolism
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