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 (δ15 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 δ15 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 δ15 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 δ15 NAA 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 δ15 NAA 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. Copyright © 2017 John Wiley & Sons, Ltd.