The Nature of the Dietary Protein Impacts the Tissue-to-Diet 15N Discrimination Factors in Laboratory Rats

Abstract

Due to the existence of isotope effects on some metabolic pathways of amino acid and protein metabolism, animal tissues are 15N-enriched relative to their dietary nitrogen sources and this 15N enrichment varies among different tissues and metabolic pools. The magnitude of the tissue-to-diet discrimination (Δ15N) has also been shown to depend on dietary factors. Since dietary protein sources affect amino acid and protein metabolism, we hypothesized that they would impact this discrimination factor, with selective effects at the tissue level. To test this hypothesis, we investigated in rats the influence of a milk or soy protein-based diet on Δ15N in various nitrogen fractions (urea, protein and non-protein fractions) of blood and tissues, focusing on visceral tissues. Regardless of the diet, the different protein fractions of blood and tissues were generally 15N-enriched relative to their non-protein fraction and to the diet (Δ15N>0), with large variations in the Δ15N between tissue proteins. Δ15N values were markedly lower in tissue proteins of rats fed milk proteins compared to those fed soy proteins, in all sampled tissues except in the intestine, and the amplitude of Δ15N differences between diets differed between tissues. Both between-tissue and between-diet Δ15N differences are probably related to modulations of the relative orientation of dietary and endogenous amino acids in the different metabolic pathways. More specifically, the smaller Δ15N values observed in tissue proteins with milk than soy dietary protein may be due to a slightly more direct channeling of dietary amino acids for tissue protein renewal and to a lower recycling of amino acids through fractionating pathways. In conclusion, the present data indicate that natural Δ15N of tissue are sensitive markers of the specific subtle regional modifications of the protein and amino acid metabolism induced by the protein dietary source.

Isotopic and modeling investigation of long-term protein turnover in rat tissues

“…On day 0 of the diet shift, we observed higher ␦ 15 N values in the P fraction than in the AA fraction of all tissues, with absolute differences ranging from ϳ1‰ to 6‰. This is consistent with our previous report that, at natural abundance levels, ␦ 15 N values are higher in the P fraction than in the AA fraction in most rat tissues (38). This difference in ␦ 15 N between the P and AA fractions is linked to the existence of isotopic effects throughout one or several of the metabolic pathways within tissues, with a preferential utilization of AA containing the lighter ( 14 N), rather than the heavier ( 15 N), nitrogen isotope during metabolic interconversions of AA (30), P synthesis (45), and/or P breakdown (46).…”

Discrimination Factors for Stable Isotopes of Carbon and Nitrogen in Blood and Feathers from Chicks and Juveniles of the California Condor

“…These include taxon, type of tissue analyzed, differences in digestive physiology, nutritional or reproductive status, form of waste, macromolecular composition of the diet (i.e., proportions of protein, carbohydrate, and lipid), as well as the extent of routing of dietary macromolecules to the same macromolecules in body tissues (i.e., dietary protein to body protein or dietary fat to body fat) (bearhop et al 2002, kurle 2002, McCutchan et al 2003, Pearson et al 2003, Vande rklift and Ponsard 2003, Robbins et al 2005, Martínez del Rio et al 2009, lecomte et al 2011, Poupin et al 2011. given the propensity for variation, the use of assumed discrimination factors in stable-isotope mixing models can result in significant errors in the estimation of wild animals' foraging ecology (bond andDiamond 2011, Phillips 2012;kurle, unpubl.…”

Linking Population Declines with Diet Quality in Vaux's Swifts

“…An increasing number of studies published over the last two decades have turned to the use of stable isotope analysis to identify trophic relationships, particularly through the analysis of nitrogen stable isotope signatures (δ 15 N) (Fry 2008). As 15 N is a heavy isotope, it becomes preferentially incorporated into the tissues of consumers, and biomagnifies through the food web (Post 2002, Mazerolle and Hobson 2005, Poupin et al 2011. By analyzing stable isotope signatures in certain tissues (e.g., blood, feathers, claws, bones, or feces), one can infer both diet quality and diet changes in an individual based on the turnover rate of that particular tissue (Post 2002).…”