13C gas chromatography-combustion isotope ratio mass spectrometry analysis of N-pivaloyl amino acid esters of tissue and plasma samples

Lipid extraction alters amino acid composition and bulk, but not amino acid, carbon and nitrogen isotope values

RATIONALE

Concerns exist over observed shifts in value and variance of nitrogen isotopes following physicochemical extraction of lipids from organic matter. The mechanisms behind these apparent changes in bulk tissue δ15N values are not fully understood yet have major implications for analytical costs and integrity of data interpretations.

METHODS

Changes in proximate analysis, amino acid composition, C:N ratios, bulk tissue and amino acid δ13C and δ15N values, and resulting isotope-based food web metrics were compared between lipid-intact and lipid-extracted muscle tissue of fishes spanning <1% to >20% muscle fat content to identify mechanisms of nitrogen isotope fractionation associated with physicochemical lipid extraction.

RESULTS

Bulk δ13C and δ15N values increased and %N, C:N ratios and crude protein content decreased following lipid extraction. Resulting bulk isotope niche spacing and overlap varied significantly between lipid-intact and lipid-extracted tissues. While amino acid composition significantly changed during lipid extraction, particularly for lipid-associated amino acids (e.g., Glu, Lys, Ser), individual amino acid δ13C and δ15N values, and their associated compound-specific isotope analysis of amino acids (CSIA-AA)-based food web metrics, did not.

CONCLUSIONS

Physicochemical lipid extraction caused significant tissue composition changes (e.g., leaching of amino acids and 15N-deplete nitrogenous waste) that affected δ13C and δ15N values and tissue %C and %N beyond simply removing lipids. However, lipid extraction did not alter individual amino acid δ13C or δ15N values or their associated CSIA-AA-based food web metrics.

Reconstructions of individual fish trophic geographies using isotopic analysis of eye-lens amino acids

Fish eye lenses are a proteinaceous structure that grows by accumulating layers in a chronological manner. Each layer becomes metabolically inert, capturing the ratio of heavy/light carbon and nitrogen isotopes at time of formation. Therefore, eye lenses contain chronological isotopic records and can be used to create a temporal isotopic history throughout an individual's lifetime. We analyzed eye lens amino-acid δ15N to address spatio-temporal baseline variability and to reconstruct trophic histories of 10 individual Red Snapper. Proteins from sequential eye lens laminae were derivatized to measure 10 amino acids, from which glutamic acid (trophic) and phenylalanine (source) were used to estimate trophic positions at different points in life. Best-fitting regressions were generated to represent individual (R2 ≥ 0.89) and generalized (R2 = 0.77) trophic trajectory for Red Snapper. The resulting trophic trajectories indicated an increase in trophic position with increasing length. Until recently, there has not been a lifetime isotopic structure with enough organic nitrogen to recreate geographic histories using compound-specific stable isotope analysis of amino acids (CSIA-AA). This study confirms that eye-lens laminae can be used to reconstruct trophogeographic histories via CSIA-AA.

Position-specific carbon isotope analysis of serine by gas chromatography/Orbitrap mass spectrometry, and an application to plant metabolism

RATIONALE

Position-specific ¹³ C/¹² C ratios within amino acids remain largely unexplored in environmental samples due to methodological limitations. We hypothesized that natural-abundance isotope patterns in serine may serve as a proxy for plant metabolic fluxes including photorespiration. Here we describe an Orbitrap method optimized for the position-specific carbon isotope analysis of serine to test our hypothesis and discuss the generalizability of this method to other amino acids.

METHODS

Position-specific carbon isotope ratios of serine were measured using a Thermo Scientific™ Q Exactive™ GC Orbitrap™. Amino acids were hydrolyzed from Arabidopsis biomass, purified from potential matrix interferences, and derivatized alongside standards. Derivatized serine (N,O-bis(trifluoroacetyl)methyl ester) was isolated using gas chromatography, trapped in a reservoir, and purged into the electron ionization source over tens of minutes, producing fragment ions containing different combinations of atoms from the serine-derivative molecule. The ¹³ C/¹² C ratios of fragments with monoisotopic masses of 110.0217, 138.0166, and 165.0037 Da were monitored in the mass analyzer and used to calculate position-specific δ¹³ C values relative to a working standard.

RESULTS

This methodology constrains position-specific δ¹³ C values for nanomole amounts of serine isolated from chemically complex mixtures. The δ¹³ C values of fragment ions of serine were characterized with ≤1‰ precisions, leading to propagated standard errors of 0.7-5‰ for each carbon position. Position-specific δ¹³ C values differed by up to ca 28 ± 5‰ between serine molecules hydrolyzed from plants grown under contrasting pCO₂ , selected to promote different fluxes through photosynthesis and photorespiration. The method was validated using pure serine standards characterized offline.

CONCLUSIONS

This study presents the first Orbitrap-based measurements of natural-abundance, position-specific carbon isotope variation in an amino acid isolated from a biological matrix. We present a method for the precise characterization of isotope ratios in serine and propose applications probing metabolism in plants. We discuss the potential for extending these approaches to other amino acids, paving the way for novel applications.

Influence of fuel oil on Platymonas helgolandica: An acute toxicity evaluation to amino acids

It is highly likely that the toxicity of water accommodated fractions (WAF) will influence marine microalgae, and consequently lead to potential risk for the marine ecological environment. However, it was often neglected whether WAF can influence the transformation of relative compounds in organisms. The metabolism of amino acids (AAs) can be used to track physiological changes in microalgae because amino acids are the basis of proteins and enzymes. In this study, using marine Chlorophyta Platymonas helgolandica as the test organism, the effects of different concentrations of WAF on AA compositions and stable carbon isotope ratios (δ¹³C) of individual AAs of Platymonas helgolandica were investigated. The results showed that the WAF of #180 fuel oil had an obvious suppressing effect on the growth and chlorophyll a content of microalgae. The growth inhibitory rate at 96 h was 80.66% at a WAF concentration of 0.50 mg L^-1 compared with the control. Furthermore, seven among the 16 AAs, including alanine, cysteine, proline, aspartic acid, lysine, histidine and tyrosine, had relatively high abundance. Under the glycolysis pathway, the cysteine abundance was higher than control, meaning that the biosynthesized pathway of alanine through cysteine as a precursor could be damaged. Phosphoenolpyruvate (PEP) was an important synthesis precursor of alanine (leucine) and aromatic AA family (Phenylalanine and tyrosine), and played an important role in δ¹³C_AAs fractionation under the WAF stress. Under the TCA pathway, to protect cell metabolism activities under WAF stress, the δ¹³C value of threonine and proline abundance in microalgae with the increase in WAF stress. Therefore, δ¹³C_AAs fractionation can be used as a novel method for toxicity evaluation of WAF on future.

Determination of Arginine δ15N Values in Plant and Animal Proteins by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry

Nitrogen (N) stable isotope techniques are widely used in ecology, archaeology, and forensic science to explore trophic relationships and provenances of organisms and materials, most widely using bulk δ¹⁵N values of whole organisms, tissues, or other materials. However, compound-specific isotope values can provide more diagnostic isotope "fingerprints" and specific information about metabolic processes. Existing techniques for nitrogen isotope analysis allow the determination of δ¹⁵N values of 14 amino acids (AAs), accounting for ca. 75% of plant protein and collagen N. The majority of remaining N is from arginine, comprising 16 and 14% of collagen and plant protein N, respectively. We therefore aimed to develop a method to detect arginine and determine its δ¹⁵N value (δ¹⁵N_Arg) by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS), to further contribute to the understanding of the metabolic routing of this important AA. We demonstrate that arginine, as its N-acetyl isopropyl ester, is amenable to GC analysis using a 15 m midpolarity DB-35 column, eluting with baseline resolution from other AAs. The recorded δ¹⁵N value by GC-C-IRMS was within the error of that of the underivatized compound determined by elemental analyzer-isotope ratio mass spectrometry (EA-IRMS). The newly developed GC-C-IRMS method was applied to modern plant protein and cattle collagen, enabling their δ¹⁵N_Arg values to be related to AA biosynthesis. Determination of archaeological cattle collagen δ¹⁵N_Arg values confirmed the suitability of this method to provide further insights into past diets and ecosystems. Bulk collagen δ¹⁵N value reconstruction including δ¹⁵N_Arg values better reflect the measured bulk values, as the isotopic ratio of 91% of collagen N can now be determined at the compound-specific level.

Stable carbon isotope diagnostics of mammalian metabolism, a high-resolution isotomics approach using amino acid carboxyl groups

The carbon isotopic compositions of amino acids are increasingly measured to characterize diets and metabolic response to diets. We report a new high-resolution system to measure the stable carbon isotopic composition of carboxyl atoms within amino acids. The automated system used HPLC to separate amino acids followed by addition of ninhydrin for decarboxylation and transfer of the evolved CO2 to a stable isotope ratio mass spectrometer for δ13CCARBOXYL measurement. The ninhydrin reaction was conducted at acidic pH (1.5) and elevated temperature (160 oC) giving yields close to 100% for most common amino acids. Eight mammalian keratin samples from herbivores (kudu and caribou), omnivores (humans) and carnivores (bowhead and humpback zooplanktivorous whales) were analysed with this new system. The data provide an initial calibration of reference materials to be used in studies of this type and is the first report of carboxyl carbon isotope distributions in mammals. Results showed widespread 13C enrichments in both essential and non-essential amino acid carboxyl groups, likely linked to decarboxylation of amino acids during normal metabolism. Analyses of non-essential amino acid isotope profiles showed (1) consistent and general taxon-level metabolic differences between the herbivore, human and whale samples, (2) marked differences among individual humans, ruminants and whales (3) evidence for gluconeogenesis in the wildlife samples, and (4) extensive 13C enrichment likely associated with fasting in the humpback whale sample. Future mammalian research related to the metabolism of growth, reproduction, aging and disease may benefit from using this technique. Values obtained for internationally available samples USGS42 and USGS43 (Tibetan and Indian human hair) provide a first characterization of reference materials for δ13CCARBOXYL profiles.

Effect of oil spill on compound-specific stable carbon isotope composition of amino acid in Nitzschia closterium and Heterosigma akashiwo

Marine microalgae, the most important primary producers in marine ecosystems, are susceptible to toxicants and usually selected as the promising organisms for chemical risk testing. The stable carbon isotope ratios (δ¹³C) of amino acids (AAs) were measured to determine if compound-specific stable isotope analysis could be used to provide insight into the toxic effects of oil spill on the marine microalgae. The water accommodated fractions (WAF) of #180 fuel oil were selected as the toxic targets with different concentrations in acute (96 h) toxic tests. Naphthalene, phenanthrene, anthracene, and fluoranthene were detected as the predominant congeners of polycyclic aromatic hydrocarbons in the WAF, and may be the main toxic components. At the same WAF concentration, the δ¹³C values of leucine in Nitzschia closterium increased with the culture time, whereas decreased in Heterosigma akashiwo. However, with the increasing WAF concentrations, the δ¹³C values of glutamic acid exhibited the increasing trend in both of the two kinds of microalgae at the same culture time. The similar phenomenon was also observed for histidine in H. akashiwo, while opposite in N. closterium, but both enriching in δ¹³C compared to the controlled experiments without the WAF. Under the different culture times and WAF concentrations, the δ¹³C values of alanine and methionine showed unexpected trend. These findings indicated that the synthesis pathways of AA in microalgae cells were affected by the WAF, thus leading to the carbon isotopic fractionation in AAs. The present study provide a novel approach to explore the toxicity mechanism of primary producers under oil spill stress, and monitor and evaluate the marine ecological risk of oil spill pollution.

Development of a Method to Isolate Glutamic Acid from Foodstuffs for a Precise Determination of Their Stable Carbon Isotope Ratio

Recent growing health awareness is leading to increasingly conscious decisions by consumers regarding the production and traceability of food. Stable isotopic compositions provide useful information for tracing the origin of foodstuffs and processes of food production. Plants exhibit different ratios of stable carbon isotopes (δ13C) because they utilized different photosynthetic (carbon fixation) pathways and grow in various environments. The origins of glutamic acid in foodstuffs can be differentiated on the basis of these photosynthetic characteristics. Here, we have developed a method to isolate glutamic acid in foodstuffs for determining the δ13C value by elemental analyzer-isotope-ratio mass spectrometry (EA/IRMS) without unintended isotopic fractionation. Briefly, following acid-hydrolysis, samples were defatted and passed through activated carbon and a cation-exchange column. Then, glutamic acid was isolated using preparative HPLC. This method is applicable to measuring, with a low standard deviation, the δ13C values of glutamic acid from foodstuffs derived from C3 and C4 plants and marine algae.

Correlations between mercury concentration, and stable isotope ratios of carbon and nitrogen of amino acids in scalp hair from whale meat eaters and heavy fish eaters

RATIONALE

As mercury (Hg) accumulation in marine animals generally increases with increased trophic level (δ¹⁵ N 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 δ¹⁵ N 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 δ¹⁵ N value of their prey.

METHODS

We measured the δ¹⁵ N 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 δ¹⁵ N values of the individual constituent amino acids.

RESULTS

The δ¹⁵ N 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 δ¹⁵ N value for Thr decreased with increases in Hg concentration (R²  = 0.663, p < 0.01). The difference in δ¹⁵ N 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 δ¹⁵ N values between Glx and Thr appears to be the best proxy for the estimation of Hg concentration in scalp hair. Copyright © 2017 John Wiley & Sons, Ltd.

δ13C analysis of amino acids in human hair using trimethylsilyl derivatives and gas chromatography/combustion/isotope ratio mass spectrometry

RATIONALE

To provide a simple one-step derivatization procedure for the analysis of a wide variety of amino acids in human hair by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) derivatization is already widely used outside the IRMS community, is applicable to a variety of functional groups, and provides products that are common entries in mass spectral databases, thus simplifying compound identification.

METHODS

Method optimization and validation were performed on a mixture of ten standard amino acids found abundantly in human hair. The method was then applied to the analysis of scalp hair from six human subjects. The hair was washed, hydrolyzed with 6 M HCl, derivatized using BSTFA in acetonitrile and analyzed using gas chromatography (GC) with concurrent quadrupole and isotope ratio mass spectrometry (IRMS) detectors.

RESULTS

The reproducibility for the δ(13)C measurements, including the derivatization procedure and GC/C/IRMS analysis, on a day-to-day comparison was between 0.19‰ and 0.35‰ (SD, N = 12), with an average standard deviation of 0.26‰. Because trimethylsilylation adds 3N carbon atoms (where N = # reactive protons) to each amino acid, the δ(13)C values for amino acid derivatives were corrected using a mass balance correction and the measured kinetic isotope effect (KIE). The KIE values ranged from 0.984 to 1.020.

CONCLUSIONS

The procedure gave consistent δ(13)C values with precision similar to other derivatization methods for the range of sample sizes studied: 50-1000 µg of each amino acid. The method gave δ(13)C values consistent with the known literature values when applied to the analysis of amino acids in human hair.

Multitissue assessment of in vivo postprandial intracellular lipid partitioning in rats using localized 1H-[13C] magnetic resonance spectroscopy

Excess accumulation of lipids in nonadipose tissues such as skeletal muscle and liver has been implicated in the development of obesity-related disorders, but the cause of this ectopic lipid overload remains unknown. The aim of this study was to determine in vivo postprandial lipid partitioning in rat skeletal muscle and liver, using localized 1H-[13C] magnetic resonance spectroscopy in combination with the oral administration of 13C-labeled lipids. Six rats were measured at baseline and 5 and 24 h after administration of 400 mg [U-13C]-labeled algal lipids. Five hours after administration, fractional 13C enrichments of the lipid pools in muscle and liver were increased 3.9-fold and 4.6-fold (P<0.05), respectively, indicating that part of the ingested lipids had been taken up by muscle and liver tissue. At 24 h, fractional 13C enrichments of muscle and liver lipids were decreased 1.6-fold and 2.2-fold (P<0.05), respectively, compared with the 5 h values. This can be interpreted as a depletion of 13C-labeled lipids from the intracellular lipid pools as a consequence of lipid turnover. In conclusion, the novel application of 1H-[13C] magnetic resonance spectroscopy in combination with the oral administration of 13C-labeled lipids is applicable for the longitudinal assessment of in vivo lipid partitioning between multiple tissues.

Measurement of 13C and 15N isotope labeling by gas chromatography/combustion/isotope ratio mass spectrometry to study amino acid fluxes in a plant-microbe symbiotic association

We have developed a method based on a double labeling with stable isotopes and gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses to study amino acid exchange in a symbiotic plant-microbe association. Isotopic precision was studied for 21 standards including 15 amino acid derivatives, three N-protected amino acid methyl esters, three amines and one international standard. High correlations were observed between the δ(13)C and δ(15)N values obtained by GC/C/IRMS and those obtained by an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (R(2) = 0.9868 and 0.9992, respectively). The mean precision measured was 0.04‰ for δ(13)C and 0.28‰ for δ(15)N (n = 15). This method was applied in vivo to the symbiotic relationship between alfalfa (Medicago sativa L.) and N(2)-fixing bacteria. Plants were simultaneously labeled over 10 days with (13)C-depleted CO(2) ((12)CO(2)), which was assimilated through photosynthesis by leaves, and (15)N(2) fixed via nodules. Subsequently, the C and N isotope compositions (i.e. δ(13)C and δ(15)N) of free amino acids were analyzed in leaves and nodules by GC/C/IRMS. The method revealed the pattern of C and N exchange between leaves and nodules, highlighting that γ-aminobutanoic acid and glycine may represent an important form of C transport from leaves to the nodules. The results confirmed the validity, reliability and accuracy of the method for assessing C and N fluxes between plants and symbiotic bacteria and support the use of this technique in a broad range of metabolic and fluxomic studies.

Advances in natural stable isotope ratio analysis of human hair to determine nutritional and metabolic status

PURPOSE OF REVIEW

We review the literature on the use of stable isotope ratios at natural abundance to reveal information about dietary habits and specific nutrient intakes in human hair protein (keratin) and amino acids. In particular, we examine whether hair isotopic compositions can be used as unbiased biomarkers to provide information about nutritional status, metabolism, and diseases.

RECENT FINDINGS

Although the majority of research on the stable isotope ratio analysis of hair has focused on bulk protein, methods have been recently employed to examine amino acid-specific isotope ratios using gas chromatography or liquid chromatography coupled to an isotope ratio mass spectrometer. The isotopic measurement of amino acids has the potential to answer research questions on amino acid nutrition, metabolism, and disease processes and can contribute to a better understanding of the variations in bulk protein isotope ratio values. First results suggest that stable isotope ratios are promising as unbiased nutritional biomarkers in epidemiological research. However, variations in stable isotope ratios of human hair are also influenced by nutrition-dependent nitrogen balance, and more controlled clinical research is needed to examine these effects in human hair.

SUMMARY

Stable isotope ratio analysis at natural abundance in human hair protein offers a noninvasive method to reveal information about long-term nutritional exposure to specific nutrients, nutritional habits, and in the diagnostics of diseases leading to nutritional stress and impaired nitrogen balance.

Hair protein and amino acid 13C and 15N abundances take more than 4 weeks to clearly prove influences of animal protein intake in young women with a habitual daily protein consumption of more than 1 g per kg body weight

A high protein or meat intake might be a risk factor for metabolic disorders. Stable isotopic abundances (SIA) of hair can be used as biomarkers for animal protein intake due to characteristic isotopic patterns of food proteins. We investigated if an additional meat intake (M, 200 g pork fillet/day) or an omission of meat and meat products (NOM) can influence the natural (15)N and (13)C SIA within 4 weeks in hair and plasma of young women. The daily protein intake (means +/- SD) was 1.40 +/- 0.29, 2.25 +/- 0.35, and 1.15 +/- 0.26 g/kg at baseline, during M, and during NOM, respectively. At baseline the animal protein intake correlated with bulk SIA of hair ((15)N: R(2) = 0.416; (13)C: R(2) = 0.664; n = 14). However, isotope ratio mass spectrometry (IRMS) analyses have not shown that hair and plasma SIA were changed significantly after M or NOM. Possible reasons were discussed. Urinary SIA were significantly lower after M than after NOM ((15)N: p = 0.039; (13)C: p = 0.006) and close to those of pork fillet. Characteristic patterns of SIA were measured in individual amino acids (AA) by gas chromatography/combustion isotope ratio mass spectrometry (GC/C/IRMS). The results confirmed considerable differences in SIA between AA (delta(15)N, up to 22 per thousand; delta(13)C, up to 31 per thousand). Plots of (15)N versus (13)C abundances in hair revealed characteristic differences between indispensable and dispensable AA. The intervention-dependent changes of AA-specific SIA were not as clear as expected. Although the AA-specific SIA may reveal more detailed characteristics of physiological conditions, further methodological research is required. We suggest that the SIA of leucine can be potential markers of protein intake. The reliability of SIA as biomarkers of protein intake still have to be tested in longer lasting intervention studies in humans. The results may have implications in the assessment for possible benefits and risks of protein consumption.

Determination of 13C isotopic enrichment of valine and threonine by GC–C–IRMS after formation of the N(O,S)-ethoxycarbonyl ethyl ester derivatives of the amino acids

We describe a new method of assessing, in a single run, (13)C isotopic enrichment of both Val and Thr by gas chromatography-combustion-isotope-ratio mass spectrometry (GC-C-IRMS). This method characterised by a rapid one-step derivatisation procedure performed at room temperature to form the N(O,S)-ethoxycarbonyl ethyl ester derivatives, and a polar column for GC. The suitability of this method for Val and Thr in in-vivo samples (mucosal hydrolysate) was demonstrated by studying protein metabolism with two tracers ((13)C-valine or (13)C-threonine). The intra-day and inter-day repeatability were both assessed either with standards or with in-vivo samples at natural abundance and at low (13)C isotopic enrichment. For inter-day repeatability CVs were between 0.8 and 1.5% at natural abundance and lower than 5.5% at 0.112 and 0.190 atom% enrichment for Val and Thr, respectively. Overall isotopic precision was studied for eleven standard amino acid derivatives (those of Val, Ala, Leu, Iso, Gly, Pro, Asp, Thr, Ser, Met, and Phe) and was assessed at 0.32 per thousand. The (13)C isotopic measurement was then extended to the other amino acids (Ala, Val, Leu, Iso, Gly, Pro, Thr, and Phe) at natural abundance for in-vivo samples. The isotopic precision was better than 0.002 atom% per amino acid (for n = 4 rats). This analytical method was finally applied to an animal study to measure Thr utilization in protein synthesis.

Optimisation of derivatisation procedures for the determination of delta13C values of amino acids by gas chromatography/combustion/isotope ratio mass spectrometry

Compound-specific stable carbon isotope analysis of amino acids by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is a highly selective and sensitive method for probing the biosynthetic/diagenetic pathways, pool size and turnover rates of proteins, previously intractable to bulk isotope analyses. However, amino acids are polyfunctional, non-volatile compounds which require derivatisation prior to GC analysis. While a wide range of derivatives exist for the GC analysis of amino acids only a handful have been utilised for their GC/C/IRMS analysis. Significantly, none of those derivatives currently employed appear completely satisfactory and a thorough assessment of their relative utility is lacking. Seven derivatives (three previously reported and four novel) for obtaining delta(13)C values of amino acids via GC/C/IRMS analysis were compared. More specifically, standard mixtures of 15 protein amino acids were converted into N-acetylmethyl (NACME) esters, N-acetyl n-propyl (NANP) esters, N-acetyl i-propyl (NAIP) esters, N-trifluoroacetyl-i-propyl (TFA-IP) esters, N-pivaloyl methyl (NPME) esters, N-pivaloyl n-propyl (NPNP) esters and N-pivaloyl i-propyl (NPIP) esters. Each derivative was assessed with respect to its applicability to carbon isotope determinations of all the common alpha-amino acids, reaction yield, chromatographic resolution, stability, analyte-to-derivative carbon ratio, kinetic isotope effects and errors associated with their carbon isotope determinations. The NACME derivative was concluded to be the preferred derivative mainly due to the highest analyte-to-derivative carbon ratio being achieved, resulting in the lowest analytical errors for amino acid delta(13)C value determinations, ranging from +/-0.6 per thousand for phenylalanine, leucine and isoleucine to +/-1.1 per thousand for serine and glycine.

Carbon and nitrogen stable isotopic composition of hair protein and amino acids can be used as biomarkers for animal-derived dietary protein intake in humans

The stable nitrogen (15N) and carbon (13C) isotopic composition of tissues reflects the isotopic pattern of food sources. We investigated whether the isotopic composition of human hair can be used as a biomarker to predict the dietary intake of animal-derived food. Hair samples were collected from subjects during a 1987-1988 German nutrition survey (VERA) in which dietary information was collected using a 7-d dietary record. Samples of 50 men and 50 women were randomly selected, in addition to 27 samples of subjects with a reported low meat intake. Isotope ratio MS was used to analyze hair bulk and amino acid-specific isotopic composition. Its relation with and feasibility for predicting animal protein intake were tested using regression analysis and cross-tabulation of observed and predicted dietary data and comparison of the individual values for the binary categories of high and low intake. 15N and 13C abundances strongly predicted relative animal protein and meat intake (R2= 0.31, P < 0.01 and R2= 0.20, P <0.01, respectively). Distinct patterns of individual hair amino acid 15N and 13C abundances were observed. In contrast to bulk values, the isotopic abundances in individual amino acids did not show discriminating ability across sex and isotope-specific categories. We conclude that hair 13C values are as predictive for animal protein consumption as hair 15N values. Bulk isotopic abundance of hair can be used as a biomarker for animal protein intake to validate dietary assessment methods provided that the correlation between isotopic abundances and dietary protein intake is verified in dietary intervention studies.

Choice of dietary protein of vegetarians and omnivores is reflected in their hair protein 13C and 15N abundance

Stable isotopic (15N, 13C) composition of tissues depends on isotopic pattern of food sources. We investigated whether the isotopic compositions of human hair protein and amino acids reflect the habitual dietary protein intake. Hair samples were analyzed from 100 omnivores (selected randomly out of the 1987-1988 German nutrition survey VERA), and from 15 ovo-lacto-vegetarians (OLV), and from 6 vegans recruited separately. Hair bulk and amino acid specific isotopic compositions were analyzed by isotope-ratio mass spectrometry (EA/IRMS and GC/C/IRMS, respectively) and the results were correlated with data of the 7 day dietary records. Hair bulk 15N and 13C abundances clearly reflect the particular eating habits. Vegans can be distinguished from OLV and both are significantly distinct from omnivores in both 15N and 13C abundances. 15N and 13C abundances rose with a higher proportion of animal to total protein intake (PAPI). Individual proportions of animal protein consumption (IPAP) were calculated using isotopic abundances and a linear regression model using animal protein consumption data of vegans (PAPI = 0) and omnivores (mean PAPI = 0.639). IPAP values positively correlated with the intake of protein, meat, meat products, and animal protein. Distinct patterns for hair amino acid specific 15N and 13C abundances were measured but with lower resolution between food preference groups compared with bulk values. In conclusion, hair 13C and 15N values both reflected the extent of animal protein consumption. Bulk isotopic abundance of hair can be tested for future use in the validation of dietary assessment methods.

Isotope ratio monitoring of small molecules and macromolecules by liquid chromatography coupled to isotope ratio mass spectrometry

In the field of isotope ratio mass spectrometry, the introduction of an interface allowing the connection of liquid chromatography (LC) and isotope ratio mass spectrometry (IRMS) has opened a range of new perspectives. The LC interface is based on a chemical oxidation, producing CO2 from organic molecules. While first results were obtained from the analysis of low molecular weight compounds, the application of compound-specific isotope analysis by irm-LC/MS to other molecules, in particular biomolecules, is presented here. The influence of the LC flow rate on the CO2 signal and on the observed delta13C values is demonstrated. The limits of quantification for angiotensin III and for leucine were 100 and 38 pmol, respectively, with a standard deviation of the delta13C values better than 0.4 per thousand. Also, accuracy and precision of delta13C values for elemental analyser-IRMS and flow injection analysis-IRMS (FIA-LC/MS) were compared. For compounds with molecular weights ranging from 131 to 66,390 Da, precision was better than 0.3 per thousand, and accuracy varied from 0.1 to 0.7 per thousand. In a second part of the work, a two-dimensional (2D)-LC method for the separation of 15 underivatised amino acids is demonstrated; the precision of delta13C values for several amino acids by irm-LC/MS was better than 0.3 per thousand at natural abundance. For labelled mixtures, the coefficient of variation was between 1% at 0.07 atom % excess (APE) for threonine and alanine, and around 10% at 0.03 APE for valine and phenylalanine. The application of irm-LC/MS to the determination of the isotopic enrichment of 13C-threonine in an extract of rat colon mucosa demonstrated a precision of 0.5 per thousand, or 0.001 atom %.

Effects of fatty acids on hepatic amino acid catabolism and fibrinogen synthesis in young healthy volunteers

Increased synthesis rate of fibrinogen, an independent risk factor for cardiovascular disease, was recently reported in obese insulin-resistant female adolescents with chronic elevated nonesterified fatty acids (NEFA). It is unknown whether a short-term change of NEFA concentrations controls hepatic fibrinogen synthesis. Therefore, 10 healthy male volunteers (24.5 +/- 3.3 yr, body mass index 23.5 +/- 2.9 kg/m2) were investigated in random order under basal and elevated NEFA for 8 h. Leucine metabolism, the fractional synthesis rates (FSR) of plasma fibrinogen, and endogenous urea production rates were measured during primed, continuous infusion of [1-13C]leucine and [15N2]urea, respectively. Plasma alpha-[13C]ketoisocaproic acid and [15N2]urea enrichment values were measured with GC-MS. Plasma fibrinogen was isolated with the beta-alanine method, and fibrinogen-related [13C]leucine enrichment was analyzed by GC-CIRMS. Lipofundin infusion and subcutaneous heparin tripled NEFA and triglycerides in the tests. Plasma glucose, circulating insulin, human C-peptide, and plasma glucagon were not changed by the study procedure. Fibrinogen FSR were significantly lower in tests with NEFA elevation (18.44 +/- 4.67%) than in control tests (21.48 +/- 4.32%; P < 0.05). Plasma fibrinogen concentrations measured were not significantly different (NEFA test subjects: 1.85 +/- 0.33, controls: 1.97 +/- 0.54 g/l). Parameters of leucine metabolism, such as leucine rate of appearance, leucine oxidation, and nonoxidative leucine disposal, were not influenced by NEFA elevation, and endogenous urea production remained unchanged. NEFA contributes to short-term regulation of fibrinogen FSR in healthy volunteers under unchanged hormonal status, leucine metabolism, and overall amino acid catabolism. Its contribution might be of relevance at least after fat-rich meals, counteracting by reduction of FSR the blood viscosity increase implied by hyperlipidemia.

Effects of hydrolysis on the delta13C values of individual amino acids derived from polypeptides and proteins

This study investigates the effects of hydrolysis on the delta13C values of individual amino acids (IAAs) derived from polypeptide standards, and modern and ancient bone collagen. All IAAs were derivatised to their trifluoroacetyl/isopropyl (TFA/IP) esters for delta13C determination using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Firstly, authentic single poly amino acid standards (SPAAs; n = 5) were hydrolysed for 4, 10, 24 and 48 h. As expected, IAA yields increased as a function of hydrolysis time. Significantly, it was only after 24 h of hydrolysis that IAA delta13C values were statistically identical to bulk SPAA values for all five standards. The accuracy of IAA delta13C values was thus shown to be a function of yield; however, poly phenylalanine demonstrated accurate IAA delta13C values with yields of only 1.4 and 4.3%, after 24 and 48 h of hydrolysis time, respectively. Authentic mixed poly amino acid standards (MPAAs; n = 5) comprising two different amino acids were then hydrolysed for 24 h. Percentage recoveries ranged from 36-95%. Estimates of bulk MPAA delta13C values calculated from measured IAA delta13C values agreed within experimental error with measured bulk MPAA values for three out of the five standards. Finally, the experimental procedure was applied to modern rat (MBCs; n = 20) and ancient ovi-caprine and bovine (ABCs; n = 27) bone collagen samples where the delta13C values of 12 out of its 18 constituent amino acids were determined. Estimated bulk MBC and ABC delta13C values were calculated from constituent amino acid delta13C values using mass balance. With the exclusion of three ABC samples, calculated bulk bone collagen delta13C values (delta13C(BCcal)) were shown to correlate extremely well with measured bone collagen values (delta13C(BCmes)) for both modern and ancient samples, where R2 = 0.91 and 0.84, respectively. Significantly, the variation between calculated and measured bone collagen values (Delta13C(BCcal-BCmes)) exhibited similar ranges for both MBC (from -2.6 to +1.2 per thousand ) and ABC (from -2.7 to +2.2 per thousand ) samples, providing evidence for the preservation of intact collagen in the ancient samples. These results demonstrate that the experimental procedures employed in the acid hydrolytic cleavage of peptides or proteins to their constituent amino acids does not involve significant isotopic fractionation.

Low-abundance plasma and urinary [(15)N]urea enrichments analyzed by gas chromatography/combustion/isotope ratio mass spectrometry

We report a method for determining plasma und urinary [(15)N]urea enrichments in an abundance range between 0.37 and 0.52 (15)N atom% (0-0.15 atom% excess (APE) (15)N) using a dimethylaminomethylene derivative. Compared with conventional off-line preparation and (15)N analysis of urea, this method requires only small sample volumes (0.5 ml of plasma and 25 microl of urine). The (15)N/(14)N ratio of urea derivatives was measured by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Two peaks were separated; one was identified by gas chromatography/mass spectrometry (GC/MS) as the complete derivatized urea. Calibration of the complete urea derivative was performed by linear regression of enrichment values of known standard mixtures. Replicate standard (6-465 per thousand delta(15)N) derivatizations showed a relative standard deviation ranging from 0.1 to 7%. In order to test the feasibility of the method, human subjects and rats ingested a single meal containing either 200 mg of [(15)N]glycine (95 AP (15)N) or 0.4 mg of [(15)N]-alpha-lysine (95 AP (15)N), respectively. Urine and plasma were collected at hourly intervals over 7 h after the meal intake. After (15)N glycine intake, maximum urinary urea (15)N enrichments were 330 and 430 per thousand delta(15)N (0.12 and 0.16 APE (15)N) measured by GC/C/IRMS, whereas plasma [(15)N]glycine enrichments were 2.5 and 3.3 APE (15)N in the two human subjects 2 h after the meal. (15)N enrichments of total urine and urine samples devoid of ammonia were higher enriched than urinary [(15)N]urea measured by GC/C/IRMS, reflecting the presence of other urinary N-containing substances (e.g. creatinine). In rats plasma urea (15)N enrichments were 15-20 times higher than those in urinary urea (10-20 per thousand delta(15)N). The different [(15)N]urea enrichments observed after ingestion of [(15)N]-labeled glycine and lysine confirm known differences in the metabolism of these amino acids.

Determination of (13)C natural abundance of amino acid enantiomers in soil: methodological considerations and first results

The application of a combined gas chromatography-combustion/isotope ratio mass spectrometry (GC-C/IRMS) method for stable carbon isotope analysis of amino acid enantiomers in soil samples is presented. Triplicate delta(13)C analyses of pentafluoropropionyl (PFP) isopropyl ester derivatives of 27 amino acid enantiomers revealed that discrimination of (13)C during derivatization is different for different amino acid enantiomers and different amounts. Injection of increasing amounts of amino acid derivatives showed that the isotopic signal varied up to 10 per thousand for D-aspartic acid. Correction for the delta(13)C signal of underivatized amino acid enantiomers is possible for all investigated amino acid enantiomers using logarithmic functions. Operating the GC-C/IRMS system in the split-mode (split ratio 1:12) is possible but resulted in a higher isotopic discrimination. The detection limit approached 3 ng for some amino acid enantiomers in the splitless mode, while the lower limit of routine determination exceeded 10 ng injection amount. The upper limit at which accurate stable isotope values were obtained was 200 ng injection amount. Compound-specific delta(13)C analysis of alanine, valine, aspartic and glutamic acid showed that the D-forms were enriched in (13)C relative to the L-forms, suggesting that microbes significantly contributed to the formation of the D-enantiomers in soil.

Whole-body nitrogen and splanchnic amino acid metabolism differ in rats fed mixed diets containing casein or its corresponding amino acid mixture

Whole-body and splanchnic metabolism of dietary amino acids derived from casein (CAS) or the corresponding crystalline L-amino acid mixture (AA) were compared. Male adult rats were adapted for 9 d to two isoenergetic, isonitrogenous diets (15 g/100 g protein, 5 g/100 g fat) containing either CAS or AA. On d 10, the rats were fed a single mixed meal (3 g dry mass) containing either intrinsically (13)C-labeled goat casein or the amino acid mixture containing [U-(13)C(6)] leucine and [alpha-(15)N] lysine. Rats were killed before and 1, 3, 5 and 7 h after meal ingestion and samples of plasma, stomach wall and contents, small intestine and liver were collected. (13)C and (15)N enrichments of free and protein-bound amino acids in plasma and tissues were analyzed by gas chromatography-combustion isotope ratio mass spectrometry. Urinary nitrogen excretion was higher (P < 0.05) and weight gain lower (P < 0.05) in rats given the AA diet, indicating a lower whole-body net protein synthesis. Free (13)C-leucine from the AA diet appeared in the intestinal mucosa free pool more rapidly (P < 0.05) than the CAS-(13)C-leucine, probably due to the faster transit through the stomach of the AA group. However, the incorporation of dietary leucine into plasma and liver proteins was higher in the CAS group 7 h after the meal (P < 0.05), whereas lysine incorporation into liver protein was higher in the AA group (P < 0.05). We conclude that whole-body protein homeostasis is better supported by dietary casein-bound than crystalline free amino acids, and that protein-bound leucine, but not lysine, is used more efficiently for liver protein synthesis than dietary free leucine.

Kinetics of L-[1-(13)C]leucine when ingested with free amino acids, unlabeled or intrinsically labeled casein

In two groups of five adults, each adapted to two different dietary regimens for 6 days, the metabolic fate of dietary [1-(13)C]leucine was examined when ingested either together with a mixture of free amino acids simulating casein (extrinsically labeled; condition A), along with the intact casein (extrinsically labeled; condition B), or bound to casein (intrinsically labeled; condition C). Fed state leucine oxidation (Ox), nonoxidative leucine disposal (NOLD), protein breakdown, and splanchnic uptake have been compared using an 8-h oral [1-(13)C]leucine and intravenous [(2)H(3)]leucine tracer protocol while giving eight equal hourly mixed meals. Lower leucine Ox, increased NOLD, and net protein synthesis were found with condition C compared with condition A (19.3 vs. 24.9; 77 vs. 55.8; 18.9 vs. 12.3 micromol. kg(-1). 30 min(-1); P < 0.05). Ox and NOLD did not differ between conditions B and C. Splanchnic leucine uptake calculated from [1-(13)C]- and [(2)H(3)]leucine plasma enrichments was between 24 and 35%. These findings indicate that the form in which leucine is consumed affects its immediate metabolic fate and retention by the body; the implications of these findings for the tracer balance technique and estimation of amino acid requirements are discussed.