Search

RATIONALE: As mercury (Hg) accumulation in marine animals generally increases with increased trophic level (δ15N values) through the food web, predators accumulate higher levels of Hg. The main source of human Hg intake is the consumption of fish and other marine animals, and Hg concentration in scalp hair is the preferred marker for evaluating consumption of marine animals. Difference in δ15N values between trophic and source amino acids of human consumers could enable us to estimate the trophic level of the consumer without knowing the bulk δ15N value of their prey. METHODS: We measured the δ15N values of 15 amino acids in scalp hair from heavy fish eaters and whale meat eaters using isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/MS), and investigated the correlations between Hg concentrations in the hair and the δ15N values of the individual constituent amino acids. RESULTS: The δ15N values for all trophic amino acids (Ala, Val, Leu, Ile, Pro, Asx and Glx) increased with increases in Hg concentration (p < 0.01), with the highest correlation being with Glx (R2 = 0.725). In contrast, the δ15N value for Thr decreased with increases in Hg concentration (R2 = 0.663, p < 0.01). The difference in δ15N values between Glx and Thr was positively correlated with Hg concentration, showing the highest correlation coefficient (R2 = 0.773, p < 0.01) among the various combinations for amino acids. CONCLUSIONS: The difference in δ15N values between Glx and Thr appears to be the best proxy for the estimation of Hg concentration in scalp hair. [ABSTRACT FROM AUTHOR]

Rationale Nitrogen stable isotope ratios (δ15N 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 δ15N 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 δ15N 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 δ15NAA 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 δ15NAA values of the HP-E and HP groups were not found to be significantly different. Conclusions The 15N-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 15N in the biomolecules of living organisms and contributes to the variability in the nitrogen trophic level effect. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]