The dietary protein paradox and threonine ¹⁵ N‐depletion: Pyridoxal‐5'‐phosphate enzyme activity as a mechanism for the δ ¹⁵ N trophic level effect

Rationale Nitrogen stable isotope ratios (δ 15 N values) are used to reconstruct dietary patterns, but the biochemical mechanism(s) responsible for the diet to tissue trophic level effect and its variability are not fully understood. Here δ 15 N amino acid (AA) values and physiological measurements (nitrogen intake, plasma albumin concentrations, liver‐reduced glutathione concentrations and leucine oxidation rates) are used to investigate increased dietary protein consumption and oxidative stress (vitamin E deficiency) in rat total plasma protein. Methods Using gas chromatography/combustion/isotope ratio mass spectrometry, the δ 15 N values from N ‐pivaloyl‐ i ‐propyl esters of 15 AAs are reported for rats ( n = 40) fed casein‐based diets with: adequate protein (AP, 13.8%; n = 10), medium protein (MP, 25.7%; n = 10), high protein (HP, 51.3%; n = 10) or HP without vitamin E (HP‐E; n = 10) for 18 weeks. Results Between the HP and AP groups, the δ 15 N AA values of threonine (–4.0‰), serine (+1.4‰) and glycine (+1.2‰) display the largest differences and show significant correlations with: nitrogen intake, plasma albumin concentrations, liver‐reduced glutathione concentrations and leucine oxidation rates. This indicates increased AA catabolism by the dietary induction of shared common metabolic pathways involving the enzymes threonine ammonia‐lyase (EC 4.3.1.19), serine hydroxymethyltransferase (EC 2.1.2.1) and the glycine cleavage system (EC 2.1.2.10). The δ 15 N AA values of the HP‐E and HP groups were not found to be significantly different. Conclusions The 15 N‐depleted results of threonine are linked to increased activity of threonine ammonia‐lyase, and show potential as a possible biomarker for protein intake and/or gluconeogenesis. We hypothesize that the inverse nitrogen equilibrium isotope effects of Schiff base formation, between AAs and pyridoxal‐5'‐phosphate cofactor enzymes, play a key role in the bioaccumulation and depletion of 15 N in the biomolecules of living organisms and contributes to the variability in the nitrogen trophic level effect.