Arginine, ornithine, and proline interconversion is dependent on small intestinal metabolism in neonatal pigs

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-C-14] arginine and [U-C-14] ornithine + [2,3-H-3] 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 coproduct 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 mu mol.kg(-1).h(-1)) by the gut, with a corresponding degradation of ornithine (70.8 mu mol.kg(-1).h(-1)) and no change in proline balance. Gut synthesis of arginine from proline ( 48.1 mu mol.kg(-1).h(-1)) was 50% of its requirement, whereas proline synthesis from arginine (33.0 mu mol.kg(-1).h(-1)) 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.