Simultaneous determination of natural-abundance δ15N values and quantities of individual amino acids in proteins from milk of lactating women and from infant hair using gas chromatography/isotope ratio mass spectrometry

Compound-Specific 14N/15N Analysis of Amino Acid Trimethylsilylated Derivatives from Plant Seed Proteins

Isotopic analyses of plant samples are now of considerable importance for food certification and plant physiology. In fact, the natural nitrogen isotope composition (δ¹⁵N) is extremely useful to examine metabolic pathways of N nutrition involving isotope fractionations. However, δ¹⁵N analysis of amino acids is not straightforward and involves specific derivatization procedures to yield volatile derivatives that can be analysed by gas chromatography coupled to isotope ratio mass spectrometry (GC-C-IRMS). Derivatizations other than trimethylsilylation are commonly used since they are believed to be more reliable and accurate. Their major drawback is that they are not associated with metabolite databases allowing identification of derivatives and by-products. Here, we revisit the potential of trimethylsilylated derivatives via concurrent analysis of δ¹⁵N and exact mass GC-MS of plant seed protein samples, allowing facile identification of derivatives using a database used for metabolomics. When multiple silylated derivatives of several amino acids are accounted for, there is a good agreement between theoretical and observed N mole fractions, and δ¹⁵N values are satisfactory, with little fractionation during derivatization. Overall, this technique may be suitable for compound-specific δ¹⁵N analysis, with pros and cons.

Determination of N-acetyl-DL-leucine in the saliva of healthy volunteers and diabetic patients using ultra-performance liquid chromatography with fluorescence detection

BACKGROUND

Recent studies have indicated that N-acetyl-leucine (N-Ac-Leu) is a potential biomarker of diabetes. This study aimed to measure the levels of enantiomers of the chiral molecule N-Ac-DL-Leu in the saliva of patients with type 2 diabetes and further determine the potential association between them.

METHOD

A novel validated method was established using ultra-performance liquid chromatography (UPLC) with fluorescence (FL) detection, in which precolumn derivatization of (R)-(-)-4-(N, N-dimethylaminosulfonyl)-7-(3-aminopyrrolidin-l-yl)-2,1,3-benzoxadiazole [(R)-(-)-DBD-APy] was used for the simultaneous determination and chiral separation of N-Ac-DL-Leu in human saliva.

RESULTS

The labeled N-Ac-DL-Leu diastereomers were completely separated, with a resolution value of 1.93. Additionally, excellent linearity for N-Ac-DL-Leu was observed, with high coefficients of correlation (r² ≥ 0.9999) in the range of 10-300 μM; the limit of quantitation (signal-to-noise ratio=10) was 40-120 pmol/mL, and the mean recoveries of N-Ac-L-Leu and N-Ac-D-Leu were 102.48% and 104.68%, respectively. The levels of N-Ac-Leu in the saliva of diabetic patients and healthy volunteers were determined, and it was found that the levels of N-Ac-DL-Leu in the saliva of diabetic patients were significantly lower than those in healthy volunteers. (p < 0.01).

CONCLUSIONS

The proposed method was successfully applied for the measurement of N-Ac-DL-Leu enantiomers in the saliva of diabetic patients and healthy volunteers.

Plasma amino acid pools in the umbilical cord artery show lower 15N natural isotope abundance relative to the maternal venous pools

¹⁵N natural isotope abundance (NIA) is systematically higher in infants' hair than in that of their mothers at birth. This study aimed to investigate this difference in plasma pools. We compared ¹⁵N NIA values for plasma amino acid (AA) pools (free + protein-bound) in the umbilical cord artery (UCA) and vein (UCV) and in the maternal vein (MV) at birth. This preliminary study included 7 mother-infant dyads. Whole plasma was treated (HCl) to hydrolyze protein. Following derivatization, AAs were separated using gas chromatography and compound-specific ¹⁵N NIA values were measured on-line using an isotope ratio monitoring mass spectrometer. ¹⁵N NIA plasma AA pools in the UCA and UCV were highly correlated to the MV, r ² > 0.89 and r ² > 0.88 (both P < 10^-4) respectively. The full model found a significant effect of sampling compartment (P = 0.02) and AA type (P < 0.0001) on ¹⁵N NIA plasma AA values. ¹⁵N NIA plasma AA was 0.74 ‰ higher (P = 0.01) in the MV than in the UCA. This study indicates that a decrease in ¹⁵N NIA for plasma AA pools occurs in the fetal-placental unit. Trial registration: ClinicalTrials.gov identifier: NCT00607061.

Maternal obesity does not influence human milk protein 15N natural isotope abundance

Obesity increases protein metabolism with a potential effect on nitrogen isotope fractionation. The aim of this study was to test the influence of obesity on human milk extracted protein ¹⁵N natural isotope abundance (NIA) at one month post-partum and to compare human milk extracted protein ¹⁵N NIA and bulk infant hair ¹⁵N NIA. This cross-sectional observational study involved 16 obese mothers (body mass index (BMI) ≥ 30 kg m^-2 before pregnancy) matched with 16 normal-weight mothers (18.5 kg m^-2 ≤ BMI < 25 kg m^-2) for age and pregnancy characteristics. Human milk extracted protein and bulk infant hair ¹⁵N NIA were determined by isotope ratio monitoring by mass spectrometry interfaced to an elemental analyser (IRM-EA/MS). No significant difference was found in human milk protein ¹⁵N NIA values between obese and normal-weight mothers (8.93 ± 0.48 ‰ vs. 8.95 ± 0.27 ‰). However, human milk protein ¹⁵N NIA was significantly lower than bulk infant hair ¹⁵N NIA: 8.94 ± 0.38 ‰ vs. 9.66 ± 0.69 ‰, respectively. On the basis of these results, it is concluded that human milk protein ¹⁵N NIA measured at one month post-partum is not influenced by maternal obesity. These findings suggest that ¹⁵N NIA may be exploited to study metabolism without considering maternal obesity as a confounder.

Exploration of Biological Markers of Feed Efficiency in Young Bulls

The efficiency with which ruminants convert feed to desirable products is difficult to measure under normal commercial settings. We explored the use of potential biological markers from easily obtainable samples, that is, blood, hair, and feces, to characterize potential causes of divergent efficiency when considered as residual feed intake (RFI) or feed conversion efficiency (FCE). A total of 54 Charolais bulls, 20 in period 1 and 34 in period 2, were examined for individual dry matter intake (DMI) and growth. Bulls were offered a diet of 70:30 wrapped grass silage to concentrate for 99 d. At the conclusion of the test period, blood samples were collected for the determination of vitamins B₂ and B₆, and plasma used for the determination of metabolites, natural isotopic ¹⁵N abundance (¹⁵N NIA, expressed as δ¹⁵N ‰) and fractionation (Δ¹⁵N_{plasma proteins-diet} and Δ¹³C_{plasma proteins-diet}) and near-infrared spectroscopy (NIRS). Feces were analyzed by NIRS. Bulls were slaughtered at 15-17 months of age and carcass characteristics determined. Bulls were ranked according to RFI with extremes (SD ± 0.5; n = 31) classified as either efficient (Neg-RFI) or inefficient (Pos-RFI). Extreme bulls were then classified for FCE (high vs low FCE), changing the groups. Pos-RFI bulls consumed 14% more feed than Neg-RFI bulls for the same level of weight gain. Low FCE bulls tended to eat more, but had lower weight gains than high FCE bulls. No differences were detected in carcass conformation, fat scores, hot carcass weight, or dressing percentage. Yet, heart and bladder weights were heavier in Pos-RFI, and rumen weight tended to be heavier in Pos-RFI bulls. RFI did not affect bulk ¹⁵N or ¹³C fractionation. A negative correlation was observed between FCE and Δ¹⁵N_{plasma proteins-diet}. Inefficient bulls (Pos-RFI) had higher δ¹⁵N in glycine compared to Neg-RFI bulls. Similarly, metabolomic analysis showed a tendency for concentrations of glycine and sarcosine to be elevated in Pos-RFI bulls, whereas aspartic acid and carnosine tended to be elevated, and serine tended to be lower in High FCE. Among vitamins, only flavin adenine dinucleotide concentration was higher in the blood of bulls with High FCE. These results suggest that the two feed efficiency metrics differ in the underlying mechanisms of metabolism, where RFI is driven by differences in the energetic requirements of visceral organs and the extent of AA catabolism.

Natural Isotope Abundance in Metabolites: Techniques and Kinetic Isotope Effect Measurement in Plant, Animal, and Human Tissues

The natural isotope abundance in bulk organic matter or tissues is not a sufficient base to investigate physiological properties, biosynthetic mechanisms, and nutrition sources of biological systems. In fact, isotope effects in metabolism lead to a heterogeneous distribution of ²H, ¹⁸O, ¹³C, and ¹⁵N isotopes in metabolites. Therefore, compound-specific isotopic analysis (CSIA) is crucial to biological and medical applications of stable isotopes. Here, we review methods to implement CSIA for ¹⁵N and ¹³C from plant, animal, and human samples and discuss technical solutions that have been used for the conversion to CO₂ and N₂ for IRMS analysis, derivatization and isotope effect measurements. It appears that despite the flexibility of instruments used for CSIA, there is no universal method simply because the chemical nature of metabolites of interest varies considerably. Also, CSIA methods are often limited by isotope effects in sample preparation or the addition of atoms from the derivatizing reagents, and this implies that corrections must be made to calculate a proper δ-value. Therefore, CSIA has an enormous potential for biomedical applications, but its utilization requires precautions for its successful application.

Kinetic commitment in the catalysis of glutamine synthesis by GS1 from Arabidopsis using 14N/15N and solvent isotope effects

Glutamine synthetase (GS, EC 6.3.1.2) catalyzes the production of glutamine from glutamate, ammonium and ATP. Although being essential in plants for N assimilation and recycling, kinetic commitments and transition states of the reaction have not been clearly established yet. Here, we examined ¹²C/¹³C, ¹⁴N/¹⁵N and H₂O/D₂O isotope effects in Arabidopsis GS1 catalysis and compared to the prokaryotic (Escherichia coli) enzyme. A¹⁴N/¹⁵N isotope effect (¹⁵V/K ≈ 1.015, with respect to substrate NH₄⁺) was observed in the prokaryotic enzyme, indicating that ammonium utilization (deprotonation and/or amidation) was partially rate-limiting. In the plant enzyme, the isotope effect was inverse (¹⁵V/K = 0.965), suggesting that the reaction intermediate is involved in an amidation-deamidation equilibrium favoring ¹⁵N. There was no ¹²C/¹³C kinetic isotope effect (¹³V/K = 1.000), suggesting that the amidation step of the catalytic cycle involves a transition state with minimal alteration of overall force constants at the C-5 carbon. Surprisingly, the solvent isotope effect was found to be inverse, that is, with a higher turn-over rate in heavy water (^DV ≈ 0.5), showing that restructuration of the active site due to displacement of H₂O by D₂O facilitates the processing of intermediates.

13C and 15N natural isotope abundance reflects breast cancer cell metabolism

Breast cancer is the most common cancer in women worldwide. Despite the information provided by anatomopathological assessment and molecular markers (such as receptor expression ER, PR, HER2), breast cancer therapies and prognostics depend on the metabolic properties of tumor cells. However, metabolomics have not provided a robust and congruent biomarker yet, likely because individual metabolite contents are insufficient to encapsulate all of the alterations in metabolic fluxes. Here, we took advantage of natural ¹³C and ¹⁵N isotope abundance to show there are isotopic differences between healthy and cancer biopsy tissues or between healthy and malignant cultured cell lines. Isotope mass balance further suggests that these differences are mostly related to lipid metabolism, anaplerosis and urea cycle, three pathways known to be impacted in malignant cells. Our results demonstrate that the isotope signature is a good descriptor of metabolism since it integrates modifications in C partitioning and N excretion altogether. Our present study is thus a starting point to possible clinical applications such as patient screening and biopsy characterization in every cancer that is associated with metabolic changes.

Natural (15)N Abundance in Key Amino Acids from Lamb Muscle: Exploring a New Horizon in Diet Authentication and Assessment of Feed Efficiency in Ruminants

Natural (15)N abundance (δ(15)N) varies between individual amino acids (AAs). We hypothesized that δ(15)N of nontransaminating and essential AAs ("source" AAs, such as phenylalanine) present in animal tissues could be used as a marker of dietary origin, whereas δ(15)N of transaminating AAs ("trophic" AAs, such as glutamic acid) could give more detailed insights into animal feed efficiency. Two diets based on dehydrated Lucerne pellets were tested in growing lambs, which promoted different feed efficiencies. No dietary effects were noted on δ(15)N of any AAs analyzed in lamb muscle. In addition, δ(15)N of phenylalanine was unexpectedly similar to that of glutamic acid, suggesting that δ(15)N of AAs is significantly derived from the metabolism of the rumen microbiota and, thus, are not suited for diet authentication in ruminants. In contrast, the δ(15)N of transaminating AAs facilitates an improved prediction of animal feed efficiency compared to the classical isotopic bulk N analysis.

A Marine Protein-based Dietary Supplement for Subclinical Hair Thinning/Loss: Results of a Multisite, Double-blind, Placebo-controlled Clinical Trial

Introduction:

Since skin and hair quality are potent vitality signals, and hair growth deficiency can cause significant psychological morbidity. In addition to clearly-defined hair loss disorders, milder forms of hair thinning or hair loss appear to be increasingly common, with a suggestion that sub-optimal diets and stressful lifestyles may be involved.

Methods:

Here we assess the value of a dietary marine-extract based dietary supplement in premenopausal women with subclinical hair thinning or hair loss conditions. This multi-site, randomized double-blind, placebo-controlled clinical trial was conducted with impact on hair shedding rate and hair fiber diameter (assessed by phototrichogram) as primary end points upon consumption of the oral supplement compared to a placebo. A total of 96 eligible female subjects were enrolled aged 21–55 years of age from Asian, Caucasian, and Hispanic ethnic backgrounds.

Results:

This study showed that hair shedding was significantly reduced in the first 3–6 months of daily consumption of the oral supplement. Moreover, phototrichogram image analysis revealed a statistically significant increase in the mean vellus-like hair diameter after 6 months of supplement consumption, when compared to the mean vellus-like hair diameters measured at baseline.

Discussion:

These results support the view that a nutritional supplement approach may be useful for women in this age group to deal with subclinical hair thinning or hair loss conditions, and those components of this marine extract-based oral supplement may be a useful adjunct.