Arginine synthesis does not occur during first-pass hepatic metabolism in the neonatal piglet

Abstract

We have shown that first-pass intestinal metabolism is necessary for ϳ50% of whole body arginine synthesis from its major precursor proline in neonatal piglets. Furthermore, the intestine is not the site of increased arginine synthesis observed during dietary arginine deficiency. Primed constant intravenous (iv) and intraportal (ip) infusions of L-[U-14 C]proline, and iv infusion of either L-[guanido-14 C]arginine or L-[4,5-3 H]arginine were used to measure first-pass hepatic arginine synthesis in piglets enterally fed either deficient (0.20 g ⅐ kg Ϫ1 ⅐ day Ϫ1 ) or generous (1.80 g ⅐ kg Ϫ1 ⅐ day Ϫ1 ) quantities of arginine for 5 days. Conversion of arginine to other urea cycle intermediates and arginine recycling were also calculated for both dietary treatments. Arginine synthesis (g ⅐ kg Ϫ1 ⅐ day Ϫ1 ) from proline was greater in piglets (P Ͻ 0.05) fed the deficient arginine diet in both the presence (generous: 0.07; deficient: 0.17; pooled SE ϭ 0.01) and absence (generous: 0.06; deficient: 0.20; pooled SE ϭ 0.01) of first-pass hepatic metabolism. There was no net arginine synthesis from proline during first-pass hepatic metabolism regardless of arginine intake. Arginine conversion to urea, citrulline, and ornithine was significantly greater (P Ͻ 0.05) in piglets fed the generous arginine diet. Calculated arginine fluxes were significantly lower (P ϭ 0.01) for [4,5-3 H]arginine than for [guanido-14 C]arginine, and the discrepancy between the values was greater in piglets fed the deficient arginine diet (35% vs. 20%). Collectively, these findings show that first-pass hepatic metabolism is not a site of net arginine synthesis and that piglets conserve dietary arginine in times of deficiency by decreasing hydrolysis and increasing recycling.proline; arginine biosynthesis; first-pass metabolism IN WEEK-OLD PIGLETS, IT IS PROLINE and not glutamine/glutamate that is the major precursor for the carbon backbone required for arginine synthesis (7,25,31). Addition of proline to an arginine-free diet prevented severe hyperammonemia when neonatal piglets were enterally, but not parenterally, fed (7). These findings suggested that proline can act as an arginine precursor only when first-pass splanchnic metabolism is intact (7). Indeed, Bertolo et al. (4) showed that first-pass intestinal, and not hepatic or peripheral, metabolism was responsible for the conversion of proline to ornithine. These observations were presumably due to the primary localization of ornithine aminotransferase (OAT, EC 2.6.1.13) to the small intestine (18). However, a subsequent study found that although whole body arginine synthesis was twofold greater in enterally fed piglets receiving a deficient arginine diet relative to piglets fed a generous arginine diet, first-pass intestinal metabolism was necessary for 42-63% of whole body arginine synthesis and was not affected by arginine intake (31). Therefore, there must be sites other than first-pass intestinal metabolism involved in arginine synthesis in the neonatal piglet, and it is the synthesis a...

A multitracer stable isotope quantification of the effects of arginine intake on whole body arginine metabolism in neonatal piglets

“…Arginine was extensively converted to ornithine in enterally fed piglets, particularly in those receiving a generous amount of arginine (28), and some of this ornithine may have been recycled back to arginine. A study using rat liver perfusions and isolated hepatocytes revealed that extramitochondrial arginine was converted extensively to ornithine, presumably via mitochondrial arginase II, and subsequently citrulline, in a series of reactions that appeared to be fairly tightly channeled (23).…”

“…The femoral vein catheter was advanced to the vena cava to allow for the sampling of mixed venous blood. Preoperative procedures, surgical procedures, postsurgical care, and piglet housing were as previously described (28,30).…”

“…The complete elemental diet was designed to meet all nutrient requirements of piglets (22) and was continuously infused enterally via the gastric catheter by use of pressure-sensitive infusion pumps. Diet composition (28,29), preparation (39), and piglet adaptation to the enteral diet (28,29) were all as previously described.…”

“…These findings indicated that it was citrulline formation that was limiting to endogenous arginine synthesis; however, rates of conversion were not measured in the previous study (29). The fractional net conversions of proline to metabolites such as citrulline and ornithine have been measured (3,28,31); however, because neither citrulline nor ornithine fluxes were measured, the molar rates of conversions between metabolites could not be calculated. If the molar rates of conversion between the major intermediates in the proline-to-arginine synthetic pathway are known, then it will be possible to identify the contribution of each to whole body arginine synthesis.…”

“…Therefore, the conversion of citrulline to arginine is the best measure of total endogenous arginine synthesis, and this method has been successfully used to measure whole body arginine synthesis in adult humans (9). In previous studies, we used the proline-to-arginine conversion as a measure of whole body arginine synthesis in neonatal piglets (27)(28)(29)31); however, how much of the total endogenous arginine synthesis in piglets that arises from proline is currently unknown. By comparing the rate of conversion of proline to arginine to the rate of the citrulline-to-arginine conversion, the relative importance of proline as an arginine precursor can be assessed.…”