Application of stable isotopes and mass isotopomer distribution analysis to the study of intermediary metabolism of nutrients1

Feeding medium-chain fatty acid-rich formula causes liver steatosis and alters hepatic metabolism in neonatal pigs

Medium-chain fatty acids (MCFA) and long-chain fatty acids (LCFAs) are often added to enhance the caloric value of infant formulas. Evidence suggests that MCFAs promote growth and are preferred over LCFAs due to greater digestibility and ease of absorption. Our hypothesis was that MCFA supplementation would enhance neonatal pig growth to a greater extent than LCFAs. Neonatal pigs (n = 4) were fed a low-energy control (CONT) or two isocaloric high-energy formulas containing fat either from LCFAs, or MCFAs for 20 days. Pigs fed the LCFAs had greater body weight compared with CONT- and MCFA-fed pigs (P < 0.05). In addition, pigs fed the LCFAs and MCFAs had more body fat than those in the CONT group. Liver and kidney weights as a percentage of body weight were greater (P ≤ 0.05) for pigs fed the MCFAs than those fed the CONT formula, and in those fed LCFAs, liver and kidney weights as a percentage of body weight were intermediate (P ≤ 0.05). Pigs in the CONT and LCFA groups had less liver fat (12%) compared with those in the MCFA (26%) group (P ≤ 0.05). Isolated hepatocytes from these pigs were incubated in media containing [13C]tracers of alanine, glucose, glutamate, and propionate. Our data suggest alanine contribution to pyruvate is less in hepatocytes from LCFA and MCFA pigs than those in the CONT group (P < 0.05). These data suggest that a formula rich in MCFAs caused steatosis compared with an isocaloric LCFA formula. In addition, MCFA feeding can alter hepatocyte metabolism and increase total body fat without increasing lean deposition.NEW & NOTEWORTHY Our data suggest that feeding high-energy MCFA formula resulted in hepatic steatosis compared with isoenergetic LCFA or low-energy formulas. Steatosis coincided with greater laurate, myristate, and palmitate accumulation, suggesting elongation of dietary laurate. Data also suggest that hepatocytes metabolized alanine and glucose to pyruvate, but neither entered the tricarboxylic acid (TCA) cycle. In addition, the contribution of alanine and glucose was greater for the low-energy formulas compared with the high-energy formulas.

Acute and chronic inflammation alter immunometabolism in a cutaneous delayed-type hypersensitivity reaction (DTHR) mouse model

T-cell-driven immune responses are responsible for several autoimmune disorders, such as psoriasis vulgaris and rheumatoid arthritis. Identification of metabolic signatures in inflamed tissues is needed to facilitate novel and individualised therapeutic developments. Here we show the temporal metabolic dynamics of T-cell-driven inflammation characterised by nuclear magnetic resonance spectroscopy-based metabolomics, histopathology and immunohistochemistry in acute and chronic cutaneous delayed-type hypersensitivity reaction (DTHR). During acute DTHR, an increase in glutathione and glutathione disulfide is consistent with the ear swelling response and degree of neutrophilic infiltration, while taurine and ascorbate dominate the chronic phase, suggesting a switch in redox metabolism. Lowered amino acids, an increase in cell membrane repair-related metabolites and infiltration of T cells and macrophages further characterise chronic DTHR. Acute and chronic cutaneous DTHR can be distinguished by characteristic metabolic patterns associated with individual inflammatory pathways providing knowledge that will aid target discovery of specialised therapeutics.

Nuclear magnetic resonance spectroscopy-based tissue metabolomics is used to define detailed temporal signatures of acute and chronic inflammation in cutaneous delayed-type hypersensitivity reaction.

Essential amino acid profile of supplemental metabolizable protein affects mammary gland metabolism and whole-body glucose kinetics in dairy cattle

This study investigated mammary gland metabolism and whole-body (WB) rate of appearance (Ra) of glucose in dairy cattle in response to a constant supplemental level of metabolizable protein (MP) composed of different essential AA (EAA) profiles. Five multiparous rumen-fistulated Holstein-Friesian dairy cows (2.8 ± 0.4 lactations; 81 ± 11 d in milk; mean ± standard deviation) were abomasally infused according to a 5 × 5 Latin square design with saline (SAL) or 562 g/d of EAA delivered in different profiles where individual AA content corresponded to their relative content in casein. The profiles consisted of (1) a complete EAA mixture (EAAC), (2) Ile, Leu, and Val (ILV), (3) His, Ile, Leu, Met, Phe, Trp, Val (GR1+ILV), and (4) Arg, His, Lys, Met, Phe, Thr, Trp (GR1+ALT). A total mixed ration (58% corn silage, 16% alfalfa hay, and 26% concentrate on a dry matter basis) was formulated to meet 100 and 83% of net energy and MP requirements, respectively, and was fed at 90% of ad libitum intake on an individual cow basis. Each experimental period consisted of 5 d of continuous abomasal infusion followed by 2 d of no infusion. Arterial and venous blood samples were collected on d 4 of each period for determination of mammary gland AA and glucose metabolism. On d 5 of each period, D-[U-¹³C]glucose (13 mmol priming dose; continuous 3.5 mmol/h for 520 min) was infused into a jugular vein and arterial blood samples were collected before and during infusion to determine WB Ra of glucose. Milk protein yield did not differ between EAAC, GR1+ILV, and GR1+ALT, or between SAL and ILV, and increased over SAL and ILV with EAAC and GR1+ILV. Mammary plasma flow increased with ILV infusion compared with EAAC and GR1+ILV. Infusion of EAAC tended to increase mammary gland net uptake of total EAA and decreased the mammary uptake to milk protein output ratio (U:O) of non-EAA compared with SAL. Infusion of ILV increased mammary net uptake and U:O of Ile, Leu, and Val markedly over all treatments. The U:O of total Ile, Leu, and Val increased numerically (25%) with GR1+ILV infusion compared with EAAC, and the U:O of total Arg, Lys, and Thr tended to decrease, primarily from decreased U:O of Lys. During GR1+ALT infusion, U:O of total Arg, Lys, and Thr was greater than that during EAAC infusion, whereas U:O of Ile, Leu, and Val did not differ from EAAC. Glucose WB Ra increased 16% with GR1+ALT over SAL, and increased numerically 8 and 12% over SAL with EAAC and GR1+ILV, respectively. The average proportion of lactose yield relative to glucose WB Ra did not differ across treatments and averaged 0.53. On average, 28% of milk galactose arose from nonglucose precursors, regardless of treatment. In conclusion, intramammary catabolism of group 2 AA increased to support milk component synthesis when the EAA profile of MP was incomplete with respect to casein. Further, WB and mammary gland glucose metabolism was flexible in support of milk component synthesis, regardless of absorptive EAA profile.

Assessment of the potential vaping-related exposure to carbonyls and epoxides using stable isotope-labeled precursors in the e-liquid

The formation of carbonyls and epoxides in e-cigarette (EC) aerosol is possible due to heating of the liquid constituents. However, high background levels of these compounds have inhibited a clear assessment of exposure during use of ECs. An EC containing an e-liquid replaced with 10% of ¹³C-labeled propylene glycol and glycerol was used in a controlled use clinical study with 20 EC users. In addition, five smokers smoked cigarettes spiked with the described e-liquid. Seven carbonyls (formaldehyde, acetaldehyde, acrolein, acetone, crotonaldehyde, methacrolein, propionaldehyde) were measured in the aerosol and the mainstream smoke. Corresponding biomarkers of exposure were determined in the user’s urine samples. ¹³C-labeled formaldehyde, acetaldehyde and acrolein were found in EC aerosol, while all seven labeled carbonyls were detected in smoke. The labeled biomarkers of exposure to formaldehyde (¹³C-thiazolidine carboxylic acid and ¹³C-N-(1,3-thiazolidine-4-carbonyl)glycine), acrolein (¹³C₃-3-hydroxypropylmercapturic acid) and glycidol (¹³C₃-dihydroxypropylmercapturic acid) were present in the urine of vapers indicating an EC use-specific exposure to these toxicants. However, other sources than vaping contribute to a much higher extent by several orders of magnitude to the overall exposure of these toxicants. Comparing data for the native (unlabeled) and the labeled (exposure-specific) biomarkers revealed vaping as a minor source of user’s exposure to these toxicants while other carbonyls and epoxides were not detectable in the EC aerosol.

Comparing Stable Isotope Enrichment by Gas Chromatography with Time-of-Flight, Quadrupole Time-of-Flight, and Quadrupole Mass Spectrometry

Stable isotope tracers are applied for in vivo and in vitro studies to reveal the activity of enzymes and intracellular metabolic pathways. Most often, such tracers are used with gas chromatography coupled to mass spectrometry (GC-MS) owing to its ease of operation and reproducible mass spectral databases. Differences in isotope tracer performance of the classic GC-quadrupole MS instrument and newer time-of-flight instruments are not well studied. Here, we used three commercially available instruments for the analysis of identical samples from a stable isotope labeling study that used [U-13C6] d-glucose to investigate the metabolism of the bacterium Rothia mucilaginosa with respect to 29 amino acids and hydroxyl acids involved in primary metabolism. The prokaryote R. mucilaginosa belongs to the family of Micrococcaceae and is present and metabolically active in the airways and sputum of cystic fibrosis patients. Overall, all three GC-MS instruments (low-resolution GC-SQ MS, low-resolution GC-TOF MS, and high-resolution GC-QTOF MS) can be used to perform stable isotope tracing studies for glycolytic intermediates, tricarboxylic acid (TCA) metabolites, and amino acids, yielding similar biological results, with high-resolution GC-QTOF MS offering additional capabilities to identify the chemical structures of unknown compounds that might show significant isotope enrichments in biological studies.

A novel quantification method for sulfur-containing biomarkers of formaldehyde and acetaldehyde exposure in human urine and plasma samples

A novel method for the quantification of the sulfur-containing metabolites of formaldehyde (thiazolidine carboxylic acid (TCA) and thiazolidine carbonyl glycine (TCG)) and acetaldehyde (methyl thiazolidine carboxylic acid (MTCA) and methyl thiazolidine carbonyl glycine (MTCG)) was developed and validated for human urine and plasma samples. Targeting the sulfur-containing metabolites of formaldehyde and acetaldehyde in contrast to the commonly used biomarkers formate and acetate overcomes the high intra- and inter-individual variance. Due to their involvement in various endogenous processes, formate and acetate lack the required specificity for assessing the exposure to formaldehyde and acetaldehyde, respectively. Validation was successfully performed according to FDA's Guideline for Bioanalytical Method Validation (2018), showing excellent performance with regard to accuracy, precision, and limits of quantification (LLOQ). TCA, TCG, and MTCG proved to be stable under all investigated conditions, whereas MTCA showed a depletion after 21 months. The method was applied to a set of pilot samples derived from smokers who consumed unfiltered cigarettes spiked with ¹³C-labeled propylene glycol and ¹³C-labeled glycerol. These compounds were used as potential precursors for the formation of ¹³C-formaldehyde and ¹³C-acetaldehyde during combustion. Plasma concentrations were significantly lower as compared to urine, suggesting urine as suitable matrix for a biomonitoring. A smoking-related increase of unlabeled biomarker concentrations could not be shown due to the ubiquitous distribution in the environment. While the metabolites of ¹³C-acetaldehyde were not detected, the described method allowed for the quantification of ¹³C-formaldehyde uptake from cigarette smoking by targeting the biomarkers ¹³C-TCA and ¹³C-TCG in urine.Graphical abstract.

Analysis of metabolic flux in felid spermatozoa using metabolomics and 13C-based fluxomics†

Spermatozoa from three feline species-the domestic cat (Felis catus), the cheetah (Acinonyx jubatus), and the clouded leopard (Neofelis nebulosa)-were analyzed using metabolomic profiling and 13C-based fluxomics to address questions raised regarding their energy metabolism. Metabolic profiles and utilization of 13C-labeled energy substrates were detected and quantified using gas chromatography-mass spectrometry (GC-MS). Spermatozoa were collected by electroejaculation and incubated in media supplemented with 1.0 mM [U13C]-glucose, [U13C]-fructose, or [U13C]-pyruvate. Evaluation of intracellular metabolites following GC-MS analysis revealed the uptake and utilization of labeled glucose and fructose in sperm, as indicated by the presence of heavy ions in glycolytic products lactate and pyruvate. Despite evidence of substrate utilization, neither glucose nor fructose had an effect on the sperm motility index of ejaculated spermatozoa from any of the three felid species, and limited entry of pyruvate derived from these hexose substrates into mitochondria and the tricarboxylic acid cycle was detected. However, pathway utilization was species-specific for the limited number of individuals (four to seven males per species) assessed in these studies. An inhibitor of fatty acid beta-oxidation (FAO), etomoxir, altered metabolic profiles of all three felid species but decreased motility only in the cheetah. While fluxomic analysis provided direct evidence that glucose and fructose undergo catabolic metabolism, other endogenous substrates such as endogenous lipids may provide energy to fuel motility.

Impact of post-ruminally infused macronutrients on bovine mammary gland expression of genes involved in fatty acid synthesis, energy metabolism, and protein synthesis measured in RNA isolated from milk fat

Background

Characterising the regulation of milk component synthesis in response to macronutrient supply is critical for understanding the implications of nutritional interventions on milk production. Gene expression in mammary gland secretory cells was measured using RNA isolated from milk fat globules from 6 Holstein-Friesian cows receiving 5-d abomasal infusions of saline, essential amino acids (AA), or glucose (GG) or palm olein (LG) without (LAA) or with (HAA) essential AA, according to a 6 × 6 Latin square design. RNA was isolated from milk fat samples collected on d 5 of infusion and subjected to real-time quantitative PCR. We hypothesised that mRNA expression of genes involved in de novo milk fatty acid (FA) synthesis would be differently affected by GG and LG, and that expression of genes regulating transfer of tricarboxylic acid cycle intermediates would increase at the HAA level. We also hypothesised that the HAA level would affect genes regulating endoplasmic reticulum (ER) homeostasis but would not affect genes related to the mechanistic target of rapamycin complex 1 (mTORC1) or the integrated stress response (ISR) network.

Results

Infusion of GG did not affect de novo milk FA yield but decreased expression of FA synthase (FASN). Infusion of LG decreased de novo FA yield and tended to decrease expression of acetyl-CoA carboxylase 1 (ACC1). The HAA level increased both de novo FA yield and expression of ACC1, and tended to decrease expression of mitochondrial phosphoenolpyruvate carboxykinase (PCK2). mRNA expression of mTORC1 signaling participants was not affected by GG, LG, or AA level. Expression of the ε subunit of the ISR constituent eukaryotic translation initiation factor 2B (EIF2B5) tended to increase at the HAA level, but only in the presence of LG. X-box binding protein 1 (XBP1) mRNA was activated in response to LG and the HAA level.

Conclusions

Results show that expression of genes involved in de novo FA synthesis responded to glucogenic, lipogenic, and aminogenic substrates, whereas genes regulating intermediate flux through the tricarboxylic acid cycle were not majorly affected. Results also suggest that after 5 d of AA supplementation, milk protein synthesis is supported by enhanced ER biogenesis instead of signaling through the mTORC1 or ISR networks.

Postruminal infusions of amino acids or glucose affect metabolisms of splanchnic, mammary, and other peripheral tissues and drive amino acid use in dairy cows

Effects of AA and glucose infusions on efficiency of use of essential AA (EAA) were studied according to a 2 × 2 factorial using 5 multicatheterized cows in a 4 × 4 Latin square plus one cow, with 2-wk periods. The diet provided 87% of energy and 70% of metabolizable protein requirements, and the 4 treatments were abomasal infusions of (1) water, (2) an AA mixture with a casein profile (695 g/d), (3) glucose (1,454 g/d), or (4) a combination of AA and glucose infusions. Milk samples were collected on the last 6 milkings. On d 14, 6 blood samples were collected from arterial, and portal, hepatic, and mammary venous vessels. Splanchnic plasma flow was calculated by dilution of p-aminohippurate and mammary flow by the Fick principle using Phe + Tyr. The net flux of AA across tissues [splanchnic, i.e., portal-drained viscera (PDV) + liver, and mammary gland] was calculated as the efflux minus the influx across that tissue. The efficiency of EAA was calculated as the sum of exported true proteins [milk protein yield (MPY), scurf, and metabolic fecal protein] multiplied by their respective AA profile and divided by the predicted AA supply minus AA endogenous urinary loss. In addition, catabolism was estimated for each tissue: AA supply - (portal net flux + metabolic fecal protein) for the PDV; -hepatic net flux for the liver; splanchnic net flux - (-mammary net flux + scurf) for the other peripheral tissues; and -mammary net flux - milk for the mammary gland. The MIXED procedure (SAS Institute Inc., Cary, NC) was used with cow as a random effect. No AA × glucose interaction existed for most of the measured parameters. With infusions of AA and glucose, MPY increased by 17 and 14%, respectively. The decreased efficiency of EAA-N with AA infusion resulted from increased EAA-N in MPY smaller than the increased EAA-N supply and was accompanied by increased liver catabolism of His + Met + Phe (representing group 1 AA) and increased mammary and PDV catabolisms of group 2 AA-N (Ile, Leu, Lys, and Val). In contrast, the increased efficiency of EAA-N with glucose infusion, resulting from increased EAA-N in MPY with no change in EAA-N supply, was accompanied by decreased mammary catabolism of group 2 AA-N and hepatic catabolism of His + Met + Phe. No mammary catabolism of His, Met, and Phe existed in all treatments, as indicated by the mammary uptake to milk output ratio close to one for these EAA. Therefore, the mammary gland contributes significantly to variations of efficiency of group 2 AA-N through variations of AA catabolism, in response to both AA and glucose supplies, whereas additional PDV catabolism was observed with increased AA supply. Partition of AA use between tissues allows to delineate their anabolic or catabolic fate across tissues and better understand changes of efficiency of EAA in response to protein and energy supplies.

Mammary gland metabolite utilization in response to exogenous glucose or long-chain fatty acids at low and high metabolizable protein levels

We investigated mammary gland metabolism in lactating dairy cattle in response to energy from glucogenic (glucose; GG) or lipogenic (palm olein; LG) substrates at low (LMP) and high (HMP) metabolizable protein levels. According to a 6 × 6 Latin square design, 6 rumen-fistulated second-lactation Holstein-Friesian dairy cows (97 ± 13 d in milk) were abomasally infused with saline (LMP-C); isoenergetic infusions (digestible energy basis) of 1,319 g/d glucose (LMP-GG), 676 g/d palm olein (LMP-LG), or 844 g/d essential AA (EAA; HMP-C); or isoenergetic infusions of 1,319 g/d glucose + 844 g/d EAA (HMP-GG) or 676 g/d palm olein + 844 g/d EAA (HMP-LG). Each experimental period consisted of 5 d of continuous infusion followed by 2 d of rest. A total mixed ration (42% corn silage, 31% grass silage, and 27% concentrate on a dry matter basis) formulated to meet 100 and 83% of net energy and metabolizable protein requirements, respectively, was fed at 90% of ad libitum intake by individual cow. Arterial and venous blood samples were collected on d 5 of each period. Infusing GG or LG at the HMP level did not affect milk yield or composition differently than at the LMP level. Neither GG nor LG infusion stimulated milk protein or lactose yield, but fat yield tended to decrease with GG and tended to increase with LG. Infusion of GG increased arterial plasma concentrations of glucose and insulin and decreased concentrations of β-hydroxybutyrate (BHB), nonesterified fatty acids, long-chain fatty acids (LCFA), total AA, EAA, and group 2 AA. Infusion of LG increased arterial triacylglycerides (TAG) and LCFA but did not affect EAA concentrations. Compared with the LMP level, the HMP level increased arterial concentrations of BHB, urea, and all EAA groups and decreased the concentration of total non-EAA. Mammary plasma flow increased with GG and was not affected by LG or protein level. Uptake and clearance of total EAA and group 2 AA were affected or tended to be affected by GG × AA interactions, with their uptakes being lower and their clearances higher with GG, but only at the LMP level. Infusion of LG did not affect uptake or clearance of any AA group. The HMP level increased uptake and decreased clearance of all EAA groups and decreased non-EAA uptake. Infusion of GG tended to increase mammary glucose uptake, and tended to decrease BHB uptake only at the LMP level. Infusion of LG increased mammary uptake of TAG and LCFA and increased or tended to increase clearance of TAG and LCFA. We suspect GG increased mammary plasma flow to maintain intramammary energy and AA balance and stimulated lipogenesis in adipose, accounting for depressed arterial BHB and group 2 AA concentrations. Mammary glucose uptake did not cover estimated requirements for lactose and fat synthesis at the HMP level, except during HMP-GG infusion. Results of this study illustrate flexibility in mammary metabolite utilization when absorptive supply of glucogenic, lipogenic, and aminogenic substrate is increased.

Amino Acid Metabolism in Dairy Cows and their Regulation in Milk Synthesis

Background:

Reducing dietary Crude Protein (CP) and supplementing with certain Amino Acids (AAs) has been known as a potential solution to improve Nitrogen (N) efficiency in dairy production. Thus understanding how AAs are utilized in various sites along the gut is critical.

Objective:

AA flow from the intestine to Portal-drained Viscera (PDV) and liver then to the mammary gland was elaborated in this article. Recoveries in individual AA in PDV and liver seem to share similar AA pattern with input: output ratio in mammary gland, which subdivides essential AA (EAA) into two groups, Lysine (Lys) and Branchedchain AA (BCAA) in group 1, input: output ratio > 1; Methionine (Met), Histidine (His), Phenylalanine (Phe) etc. in group 2, input: output ratio close to 1. AAs in the mammary gland are either utilized for milk protein synthesis or retained as body tissue, or catabolized. The fractional removal of AAs and the number and activity of AA transporters together contribute to the ability of AAs going through mammary cells. Mammalian Target of Rapamycin (mTOR) pathway is closely related to milk protein synthesis and provides alternatives for AA regulation of milk protein synthesis, which connects AA with lactose synthesis via α-lactalbumin (gene: LALBA) and links with milk fat synthesis via Sterol Regulatory Element-binding Transcription Protein 1 (SREBP1) and Peroxisome Proliferatoractivated Receptor (PPAR).

Conclusion:

Overall, AA flow across various tissues reveals AA metabolism and utilization in dairy cows on one hand. While the function of AA in the biosynthesis of milk protein, fat and lactose at both transcriptional and posttranscriptional level from another angle provides the possibility for us to regulate them for higher efficiency.

The consequences of porcine IVM medium supplementation with follicular fluid become reflected in embryo quality, yield and gene expression patterns

Oocyte and embryo developmental competence are shaped by multiple extrinsic and intrinsic factors. One of the most extensive research areas in the last decade is the regulation of lipid metabolism in oocytes and embryos of different species. We hypothesized that differences in developmental competence of oocytes and embryos between prepubertal and cyclic gilts may arise due to distinct fatty acid profiles in follicular fluid. We found that supplementation of oocyte maturation media with follicular fluid from prepubertal pigs affected quality and development of embryos from prepubertal pigs while embryos of cyclic pigs were not affected. PLIN2, SCD and ACACA transcripts involved in lipid metabolism were upregulated in embryos originating from oocytes of prepubertal pigs matured with autologous follicular fluid. The surface occupied by lipid droplets tend to increase in oocytes matured with follicular fluid from prepubertal pigs regardless oocyte origin. The change into follicular fluid of cyclic pigs increased the efficiency of embryo culture and improved quality, while gene expression was similar to embryos obtained from cyclic gilts. We assume that the follicular fluids of prepubertal and cyclic pigs influenced the quality of oocytes and embryos obtained from prepubertal pigs which are more susceptible to suboptimal in vitro culture conditions.

Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease

The ability to efficiently adapt metabolism by substrate sensing, trafficking, storage, and utilization, dependent on availability and requirement, is known as metabolic flexibility. In this review, we discuss the breadth and depth of metabolic flexibility and its impact on health and disease. Metabolic flexibility is essential to maintain energy homeostasis in times of either caloric excess or caloric restriction, and in times of either low or high energy demand, such as during exercise. The liver, adipose tissue, and muscle govern systemic metabolic flexibility and manage nutrient sensing, uptake, transport, storage, and expenditure by communication via endocrine cues. At a molecular level, metabolic flexibility relies on the configuration of metabolic pathways, which are regulated by key metabolic enzymes and transcription factors, many of which interact closely with the mitochondria. Disrupted metabolic flexibility, or metabolic inflexibility, however, is associated with many pathological conditions including metabolic syndrome, type 2 diabetes mellitus, and cancer. Multiple factors such as dietary composition and feeding frequency, exercise training, and use of pharmacological compounds, influence metabolic flexibility and will be discussed here. Last, we outline important advances in metabolic flexibility research and discuss medical horizons and translational aspects.

Roux-en-Y gastric bypass compared with equivalent diet restriction: Mechanistic insights into diabetes remission

AIMS

To investigate the physiological mechanisms leading to rapid improvement in diabetes after Roux-en-Y gastric bypass (RYGB) and specifically the contribution of the concurrent peri-operative dietary restrictions, which may also alter glucose metabolism.

MATERIALS AND METHODS

In order to assess the differential contributions of diet and surgery to the mechanisms leading to the rapid improvement in diabetes after RYGB we enrolled 10 patients with type 2 diabetes scheduled to undergo RYGB. All patients underwent a 10-day inpatient supervised dietary intervention equivalent to the peri-operative diet (diet-only period), followed by, after a re-equilibration (washout) period, an identical period of pair-matched diet in conjunction with RYGB (diet and RYGB period). We conducted extensive metabolic assessments during a 6-hour mixed-meal challenge test, with stable isotope glucose tracer infusion performed before and after each intervention.

RESULTS

Similar improvements in glucose levels, β-cell function, insulin sensitivity and post-meal hepatic insulin resistance were observed with both interventions. Both interventions led to significant reductions in fasting and postprandial acyl ghrelin. The diet-only intervention induced greater improvements in basal hepatic glucose output and post-meal gastric inhibitory polypeptide (GIP) secretion. The diet and RYGB intervention induced significantly greater increases in post-meal glucagon-like peptide-1 (GLP-1), peptide YY (PYY) and glucagon levels.

CONCLUSIONS

Strict peri-operative dietary restriction is a main contributor to the rapid improvement in glucose metabolism after RYGB. The RYGB-induced changes in the incretin hormones GLP-1 and PYY probably play a major role in long-term compliance with such major dietary restrictions through central and peripheral mechanisms.

Novel quantitative insights into carbon sources for synthesis of poly hydroxybutyrate in Synechocystis PCC 6803

Many freshwater cyanobacteria accumulate polyhydroxybutyrate (PHB) under nitrogen or phosphorus deprivation. While prior literature has shed lights on transcriptomic and metabolomic changes in the model cyanobacterium Synechocystis PCC 6803 cells, the quantitative contributions of the newly fixed carbon following nitrogen deprivation or the externally added acetate to PHB synthesis are not clear. Similarly, it is not clear how photomixotrophy affects precursor contributions. In this study, we show that (i) the pre-growth mode (photoautotrophic or photomixotrophic), while significantly impacting glycogen levels, does not have any significant effect on PHB levels, (ii) the carbon fixed following nitrogen deprivation contributes 26% of C for PHB synthesis in photoautotrophically pre-grown cells and its contribution to the PHB synthesis goes down with the addition of acetate at the resuspension phase or with photomixotrophic pre-growth, (iii) the acetate added at the start of nitrogen deprivation, doubles the intracellular PHB levels and contributes 44-48% to PHB synthesis and this value is not greatly affected by how the cells were pre-grown. Indirectly, the labeling studies also show that the intracellular C recycling is the most important source of precursors for PHB synthesis, contributing about 74-87% of the C for PHB synthesis in the absence of acetate. The addition of acetate significantly reduces its contribution. In photoautotrophic pre-growth followed by acetate addition under nitrogen starvation, the contribution of intracellular C reduces to about 34%. Thus, our study provides several novel quantitative insights on how prior nutritional status affects the precursor contributions for PHB synthesis.

Metabolic tracing analysis reveals substrate-specific metabolic deficits in platelet storage lesion

BACKGROUND

Storage of platelets (PLTs) results in a progressive defect termed PLT storage lesion (PSL). The PSL is characterized by poor PLT quality on a variety of assays. Metabolic defects are thought to underlie the PSL; thus this study was designed to quantitatively probe specific metabolic pathways over PLT storage.

STUDY DESIGN AND METHODS

Relative incorporation of stable isotope-labeled substrates was quantified by isotopologue analysis of key acyl-coenzyme A (CoA) thioester products for fresh, viable (after collection, Days 2-5), and expired PLTs (after Day 5). We examined the incorporation of acetate, glucose, and palmitate into acetyl- and succinyl-CoA via liquid chromatography-tandem mass spectrometry.

RESULTS

Storage-related defects in the incorporation of acetyl-CoA derived from acetate and palmitate were observed. Carbon derived from palmitate and acetate in succinyl-CoA was reduced over storage time. Glucose incorporation into succinyl-CoA increased in viable PLTs and then decreased in expired PLTs. Carbon derived from octanoate and pyruvate remained partially able to incorporate into acetyl- and succinyl-CoA in expired PLTs, with high variability in pyruvate incorporation.

CONCLUSION

Isotopologue analysis is useful in probing substrate specific defects in the PSL.

Using stable isotope techniques in nutrition assessments and tracking of global targets post-2015

Stable isotopes are non-radioactive, safe and are applied for various purposes in human health assessment in trace amounts that minimally disturb normal physiology. The International Atomic Energy Agency supports the use of stable isotope techniques to design and evaluate interventions addressing malnutrition in all its forms with focus on infant and young child feeding; maternal and adolescent nutrition; diet quality; prevention and control of non-communicable diseases; healthy ageing and gut function. These techniques can be used to objectively measure: (1) amount of human milk consumed and whether an infant is exclusively breastfed; (2) body composition in the context of re-feeding programmes for moderate and severe acute malnutrition and as an indicator of the risk for obesity; (3) bioavailability and bioconversion of pro-vitamin A and vitamin A body stores following vitamin A intervention programmes; (4) absorption and retention of iron, zinc and protein; (5) total energy expenditure for validation of physical activity measurement and dietary assessment tools and (6) diagnosis of Helicobacter pylori. Stable isotope techniques will be invaluable in the tracking of global targets on exclusive breast-feeding childhood obesity and anaemia among women. Efforts are underway to make nuclear techniques more affordable, field-friendly and less invasive, and to develop less sophisticated but precise equipment. Advocacy for the wide adoption of the techniques is needed.

Stable isotopes and LC-MS for monitoring metabolic disturbances in Friedreich's ataxia platelets

BACKGROUND

Friedreich's ataxia (FRDA) is an autosomal recessive disease with metabolic abnormalities that have been proposed to play an important role in the resulting neurodegeneration and cardiomyopathy. The inability to access the highly affected neuronal and cardiac tissues has hampered metabolic evaluation and biomarker development.

METHODS

Employment of a LC-MS-based method to determine whether platelets isolated from patients with FRDA exhibit differentiable metabolism compared with healthy controls.

RESULTS

Isotopologue analysis showed a marked decrease in glucose incorporation with a concomitant increase in palmitate-derived acyl-CoA thioesters in FRDA platelets compared with controls.

CONCLUSION

Our findings demonstrate that platelets can be used as a surrogate tissue for in vivo biomarker studies to monitor new therapeutic approaches for the treatment of FRDA.

Biosynthesis of milk fat, protein, and lactose: roles of transcriptional and posttranscriptional regulation

The demand for high-quality milk is increasing worldwide. The efficiency of milk synthesis can be improved by taking advantage of the accumulated knowledge of the transcriptional and posttranscriptional regulation of genes coding for proteins involved in the synthesis of fat, protein, and lactose in the mammary gland. Research in this area is relatively new, but data accumulated in the last 10 years provide a relatively clear picture. Milk fat synthesis appears to be regulated, at least in bovines, by an interactive network between SREBP1, PPARγ, and LXRα, with a potential role for other transcription factors, such as Spot14, ChREBP, and Sp1. Milk protein synthesis is highly regulated by insulin, amino acids, and amino acid transporters via transcriptional and posttranscriptional routes, with the insulin-mTOR pathway playing a central role. The transcriptional regulation of lactose synthesis is still poorly understood, but it is clear that glucose transporters play an important role. They can also cooperatively interact with amino acid transporters and the mTOR pathway. Recent data indicate the possibility of nutrigenomic interventions to increase milk fat synthesis by feeding long-chain fatty acids and milk protein synthesis by feeding amino acids. We propose a transcriptional network model to account for all available findings. This model encompasses a complex network of proteins that control milk synthesis with a cross talk between milk fat, protein, and lactose regulation, with mTOR functioning as a central hub.

Nutritional regulation of the anabolic fate of amino acids within the liver in mammals: concepts arising from in vivo studies

At the crossroad between nutrient supply and requirements, the liver plays a central role in partitioning nitrogenous nutrients among tissues. The present review examines the utilisation of amino acids (AA) within the liver in various physiopathological states in mammals and how the fates of AA are regulated. AA uptake by the liver is generally driven by the net portal appearance of AA. This coordination is lost when demands by peripheral tissues is important (rapid growth or lactation), or when certain metabolic pathways within the liver become a priority (synthesis of acute-phase proteins). Data obtained in various species have shown that oxidation of AA and export protein synthesis usually responds to nutrient supply. Gluconeogenesis from AA is less dependent on hepatic delivery and the nature of nutrients supplied, and hormones like insulin are involved in the regulatory processes. Gluconeogenesis is regulated by nutritional factors very differently between mammals (glucose absorbed from the diet is important in single-stomached animals, while in carnivores, glucose from endogenous origin is key). The underlying mechanisms explaining how the liver adapts its AA utilisation to the body requirements are complex. The highly adaptable hepatic metabolism must be capable to deal with the various nutritional/physiological challenges that mammals have to face to maintain homeostasis. Whereas the liver responds generally to nutritional parameters in various physiological states occurring throughout life, other complex signalling pathways at systemic and tissue level (hormones, cytokines, nutrients, etc.) are involved additionally in specific physiological/nutritional states to prioritise certain metabolic pathways (pathological states or when nutritional requirements are uncovered).

Integrated, Step-Wise, Mass-Isotopomeric Flux Analysis of the TCA Cycle

Mass isotopomer multi-ordinate spectral analysis (MIMOSA) is a step-wise flux analysis platform to measure discrete glycolytic and mitochondrial metabolic rates. Importantly, direct citrate synthesis rates were obtained by deconvolving the mass spectra generated from [U-(13)C6]-D-glucose labeling for position-specific enrichments of mitochondrial acetyl-CoA, oxaloacetate, and citrate. Comprehensive steady-state and dynamic analyses of key metabolic rates (pyruvate dehydrogenase, β-oxidation, pyruvate carboxylase, isocitrate dehydrogenase, and PEP/pyruvate cycling) were calculated from the position-specific transfer of (13)C from sequential precursors to their products. Important limitations of previous techniques were identified. In INS-1 cells, citrate synthase rates correlated with both insulin secretion and oxygen consumption. Pyruvate carboxylase rates were substantially lower than previously reported but showed the highest fold change in response to glucose stimulation. In conclusion, MIMOSA measures key metabolic rates from the precursor/product position-specific transfer of (13)C-label between metabolites and has broad applicability to any glucose-oxidizing cell.

Abomasal amino acid infusion in postpartum dairy cows: Effect on whole-body, splanchnic, and mammary glucose metabolism

Nine Holstein cows fitted with rumen cannulas and indwelling catheters in splanchnic blood vessels were used to study the effects of supplementing AA on milk lactose secretion, whole-body rate of appearance (WB-Ra) of glucose, and tissue metabolism of glucose, lactate, glycerol, and β-OH-butyrate (BHBA) in postpartum dairy cows according to a generalized randomized incomplete block design with repeated measures in time. At calving, cows were blocked according to parity (second and third or greater) and were allocated to 2 treatments: abomasal infusion of water (n=4) or abomasal infusion of free AA with casein profile (AA-CN; n=5) in addition to the same basal diet. The AA-CN infusion started with half the maximal dose at 1 d in milk (DIM) and then steadily decreased from 791 to 226 g/d from DIM 2 to 29 to cover the estimated essential AA deficit. On DIM 5, 15, and 29, D[6,6-(2)H2]-glucose (23.7 mmol/h) was infused into a jugular vein for 5h, and 6 blood samples were taken from arterial, portal, hepatic, and mammary sources at 45-min intervals, starting 1h after the initiation of the D[6,6-(2)H2]glucose infusion. Trans-organ fluxes were calculated as veno-arterial differences times plasma flow (splanchnic: downstream dilution of deacetylated para-aminohippurate; mammary: Fick principle using Phe+Tyr). Energy-corrected milk and lactose yields increased on average with AA-CN by 6.4 kg/d and 353 g/d, respectively, with no DIM × treatment interaction. Despite increased AA supply and increased demand for lactose secretion with AA-CN, net hepatic release of glucose remained unchanged, but WB-Ra of glucose tended to increase with AA-CN. Portal true flux of glucose increased with AA-CN and represented, on average, 17% of WB-Ra. Splanchnic true flux of glucose was unaltered by treatments and was numerically equivalent to WB-Ra, averaging 729 and 741 mmol/h, respectively. Mammary glucose utilization increased with AA-CN infusion, averaging 78% of WB-Ra, and increased gradually as lactation advanced. Net portal, hepatic, splanchnic, and mammary fluxes of lactate, glycerol, and BHBA were not affected by AA infusion. Increasing the supply of AA in postpartum dairy cows elevated the WB-Ra of glucose without affecting the true liver glucose release. The greater WB-Ra of glucose with abomasal AA infusion seemed to originate mainly from greater true portal-drained viscera release of glucose. Glucose utilization by the portal-drained viscera was unaffected by abomasal AA infusion, but the exact mechanism behind the greater true portal glucose release could not be assessed in the current study. The increased mammary glucose uptake was in line with the increased milk lactose yield. In early postpartum lactation, the demand for AA seems to be so high that even with increased AA supply, cows have metabolic priorities for AA other than hepatic gluconeogenesis.

The Role of TCA Cycle Anaplerosis in Ketosis and Fatty Liver in Periparturient Dairy Cows

The transition to lactation period in dairy cattle is characterized by metabolic challenges, negative energy balance, and adipose tissue mobilization. Metabolism of mobilized adipose tissue is part of the adaptive response to negative energy balance in dairy cattle; however, the capacity of the liver to completely oxidize nonesterified fatty acids may be limited and is reflective of oxaloacetate pool, the carbon carrier of the tricarboxylic acid cycle. Alternative metabolic fates of acetyl-CoA from nonesterified fatty acids include esterification to triacylglycerides and ketogenesis, and when excessive, these pathways lead to fatty liver and ketosis. Examination of the anaplerotic and cataplerotic pull of oxaloacetate by the tricarboxylic acid cycle and gluconeogenesis may provide insight into the balance of oxidation and esterification of acetyl-CoA within the liver of periparturient dairy cows.

'Isotopo' a database application for facile analysis and management of mass isotopomer data

The composition of stable-isotope labelled isotopologues/isotopomers in metabolic products can be measured by mass spectrometry and supports the analysis of pathways and fluxes. As a prerequisite, the original mass spectra have to be processed, managed and stored to rapidly calculate, analyse and compare isotopomer enrichments to study, for instance, bacterial metabolism in infection. For such applications, we provide here the database application ‘Isotopo’. This software package includes (i) a database to store and process isotopomer data, (ii) a parser to upload and translate different data formats for such data and (iii) an improved application to process and convert signal intensities from mass spectra of ¹³C-labelled metabolites such as tertbutyldimethylsilyl-derivatives of amino acids. Relative mass intensities and isotopomer distributions are calculated applying a partial least square method with iterative refinement for high precision data. The data output includes formats such as graphs for overall enrichments in amino acids. The package is user-friendly for easy and robust data management of multiple experiments.

Availability: The ‘Isotopo’ software is available at the following web link (section Download): http://spp1316.uni-wuerzburg.de/bioinformatics/isotopo/. The package contains three additional files: software executable setup (installer), one data set file (discussed in this article) and one excel file (which can be used to convert data from excel to ‘.iso’ format). The ‘Isotopo’ software is compatible only with the Microsoft Windows operating system.

Database URL:http://spp1316.uni-wuerzburg.de/bioinformatics/isotopo/.

Short communication: Effect of dietary manipulation of crude protein content and nonfibrous-to-fibrous-carbohydrate ratio on energy balance in early-lactation dairy cows

Disparities between nutrient intake and demand often result in a state of negative energy balance (EB) in the early-lactation dairy cow. Reducing dietary crude protein (CP) content and providing glucogenic nutrients may overcome this issue. This study evaluates whether or not offering a diet lower in CP and higher in nonfiber carbohydrates (LP-NFC) can improve EB and the metabolic status of the early-lactation dairy cow compared with a diet higher in CP and fibrous carbohydrates (HP-FC). Twenty Holstein-Friesian dairy cows were assigned to 1 of 2 dietary treatments in a randomized block design. Diets were isoenergetic (6.57 MJ of net energy for lactation) and formulated to contain 15% CP and 6% starch (HP-FC), or 12% CP and 28% starch (LP-NFC) and were offered for the first 63 d of lactation. Intake and milk yield were determined daily, whereas milk and blood samples, weights, and body condition scores were collected weekly. Intakes (mean ± standard errors of the mean, SEM) of dry matter (17.4 ± 0.6 kg/d) and energy (113.0 ± 4.6 MJ of net energy for lactation) were not different between treatments. However, the HP-FC group had a higher milk yield (31.8 vs. 28.9 ± 1.4 kg/d) and a lower EB compared with the LP-NFC group. Blood urea N concentration (3.5 vs. 1.8 ± 0.2 mmol/L) was higher, whereas bilirubin (6.0 vs. 6.7 ± 0.2 mmol/L) and β-hydroxybutyrate concentrations (0.7 vs. 0.8 ± 0.05 mmol/L) were lower in the HP-FC group compared with the LP-NFC group. These data suggest that EB can be improved during early lactation through the manipulation of milk output by offering a lower CP, higher NFC diet.

Exploring metabolism flexibility in complex organisms through quantitative study of precursor sets for system outputs

Background

When studying metabolism at the organ level, a major challenge is to understand the matter exchanges between the input and output components of the system. For example, in nutrition, biochemical models have been developed to study the metabolism of the mammary gland in relation to the synthesis of milk components. These models were designed to account for the quantitative constraints observed on inputs and outputs of the system. In these models, a compatible flux distribution is first selected. Alternatively, an infinite family of compatible set of flux rates may have to be studied when the constraints raised by observations are insufficient to identify a single flux distribution. The precursors of output nutrients are traced back with analyses similar to the computation of yield rates. However, the computation of the quantitative contributions of precursors may lack precision, mainly because some precursors are involved in the composition of several nutrients and because some metabolites are cycled in loops.

Results

We formally modeled the quantitative allocation of input nutrients among the branches of the metabolic network (AIO). It corresponds to yield information which, if standardized across all the outputs of the system, allows a precise quantitative understanding of their precursors. By solving nonlinear optimization problems, we introduced a method to study the variability of AIO coefficients when parsing the space of flux distributions that are compatible with both model stoichiometry and experimental data. Applied to a model of the metabolism of the mammary gland, our method made it possible to distinguish the effects of different nutritional treatments, although it cannot be proved that the mammary gland optimizes a specific linear combination of flux variables, including those based on energy. Altogether, our study indicated that the mammary gland possesses considerable metabolic flexibility.

Conclusion

Our method enables to study the variability of a metabolic network with respect to efficiency (i.e. yield rates). It allows a quantitative comparison of the respective contributions of precursors to the production of a set of nutrients by a metabolic network, regardless of the choice of the flux distribution within the different branches of the network.

1,25-dihydroxyvitamin D regulation of glucose metabolism in Harvey-ras transformed MCF10A human breast epithelial cells

This study was designed to investigate the impact of 1,25-dihydroxyvitamin D (1,25(OH)2D) on glucose metabolism during early cancer progression. Untransformed and ras-oncogene transfected (ras) MCF10A human breast epithelial cells were employed to model early breast cancer progression. 1,25(OH)2D modified the response of the ras cells to glucose restriction, suggesting 1,25(OH)2D may reduce the ras cell glucose addiction noted in cancer cells. To understand the 1,25(OH)2D regulation of glucose metabolism, following four-day 1,25(OH)2D treatment, metabolite fluxes at the cell membrane were measured by a nanoprobe biosensor, [(13)C6]glucose flux by (13)C-mass isotopomer distribution analysis of media metabolites, intracellular metabolite levels by NMR, and gene expression of related enzymes was assessed. Treatment with 1,25(OH)2D reduced glycolysis as flux of glucose to 3-phosphoglycerate was reduced by 15% (P=0.017) and 32% (P<0.003) in MCF10A and ras cells respectively. In the ras cells, 1,25(OH)2D reduced lactate dehydrogenase activity by 15% (P<0.05) with a concomitant 10% reduction in the flux of glucose to lactate (P=0.006), and reduction in the level of intracellular lactate by 55% (P=0.029). Treatment with 1,25(OH)2D reduced flux of glucose to acetyl-coA 24% (P=0.002) and 41% (P<0.001), and flux to oxaloacetate 33% (P=0.003) and 34% (P=0.027) in the MCF10A and ras cells, respectively, suggesting a reduction in tricarboxylic acid (TCA) cycle activity. The results suggest a novel mechanism involving the regulation of glucose metabolism by which 1,25(OH)2D may prevent breast cancer progression.

13C-isotope-based protocol for prenyl lipid metabolic analysis in zebrafish embryos

Metabolism has a decisive role in many fundamental biological processes, including organism development and tissue homeostasis. Here we describe a protocol for fast and reliable (13)C-isotope-based in vivo metabolic profiling. This protocol covers the loading of isotope precursor; extraction, preparation and quantification of the labeled lipid metabolites (e.g., the prenyl lipid CoQ10) by the means of HPLC-MS; and its analysis in zebrafish embryos. This protocol can be applied to different types of experimental settings, including tissue-specific metabolic analyses or dynamic metabolic changes that occur during vertebrate embryogenesis. The protocol takes 5-7 d to complete, requiring minimal equipment and analytical expertise, and it represents a unique alternative to the existing ex vivo (e.g., cell lines) isotope-based metabolic methods. This procedure represents a valuable approach for researchers interested in studying the effect of gene manipulation on lipid metabolism in zebrafish and in understanding the genetic conditions that result in metabolism dysfunction.

Altered glucose metabolism in Harvey-ras transformed MCF10A cells

Metabolic reprogramming that alters the utilization of glucose including the "Warburg effect" is critical in the development of a tumorigenic phenotype. However, the effects of the Harvey-ras (H-ras) oncogene on cellular energy metabolism during mammary carcinogenesis are not known. The purpose of this study was to determine the effect of H-ras transformation on glucose metabolism using the untransformed MCF10A and H-ras oncogene transfected (MCF10A-ras) human breast epithelial cells, a model for early breast cancer progression. We measured the metabolite fluxes at the cell membrane by a selective micro-biosensor, [(13)C6 ]glucose flux by (13)C-mass isotopomer distribution analysis of media metabolites, intracellular metabolite levels by NMR, and gene expression of glucose metabolism enzymes by quantitative PCR. Results from these studies indicated that MCF10A-ras cells exhibited enhanced glycolytic activity and lactate production, decreased glucose flux through the tricarboxylic acid (TCA) cycle, as well as an increase in the utilization of glucose in the pentose phosphate pathway (PPP). These results provide evidence for a role of H-ras oncogene in the metabolic reprogramming of MCF10A cells during early mammary carcinogenesis.

Software LS-MIDA for efficient mass isotopomer distribution analysis in metabolic modelling

BACKGROUND

The knowledge of metabolic pathways and fluxes is important to understand the adaptation of organisms to their biotic and abiotic environment. The specific distribution of stable isotope labelled precursors into metabolic products can be taken as fingerprints of the metabolic events and dynamics through the metabolic networks. An open-source software is required that easily and rapidly calculates from mass spectra of labelled metabolites, derivatives and their fragments global isotope excess and isotopomer distribution.

RESULTS

The open-source software "Least Square Mass Isotopomer Analyzer" (LS-MIDA) is presented that processes experimental mass spectrometry (MS) data on the basis of metabolite information such as the number of atoms in the compound, mass to charge ratio (m/e or m/z) values of the compounds and fragments under study, and the experimental relative MS intensities reflecting the enrichments of isotopomers in 13C- or 15 N-labelled compounds, in comparison to the natural abundances in the unlabelled molecules. The software uses Brauman's least square method of linear regression. As a result, global isotope enrichments of the metabolite or fragment under study and the molar abundances of each isotopomer are obtained and displayed.

CONCLUSIONS

The new software provides an open-source platform that easily and rapidly converts experimental MS patterns of labelled metabolites into isotopomer enrichments that are the basis for subsequent observation-driven analysis of pathways and fluxes, as well as for model-driven metabolic flux calculations.

Isotopomer enrichment assay for very short chain fatty acids and its metabolic applications

The present work illustrated an accurate GC/MS measurement for the low isotopomer enrichment assay of formic acid, acetic acid, propionic aicd, butyric acid, and pentanoic acid. The pentafluorobenzyl bromide derivatives of these very short chain fatty acids have high sensitivity of isotopoic enrichment due to their low natural isotopomer distribution in negative chemical ionization mass spectrometric mode. Pentafluorobenzyl bromide derivatization reaction was optimized in terms of pH, temperature, reaction time, and the amount of pentafluorobenzyl bromide versus sample. The precision, stability, and accuracy of this method for the isotopomer analysis were validated. This method was applied to measure the enrichments of formic acid, acetic acid, and propionic acid in the perfusate from rat liver exposed to Krebs-Ringer bicarbonate buffer only, 0-1mM [3,4-(13)C(2)]-4-hydroxynonanoate, and 0-2mM [5,6,7-(13)C(3)]heptanoate. The enrichments of acetic acid and propionic acid in the perfusate are comparable to the labeling pattern of acetyl-CoA and propionyl-CoA in the rat liver tissues. The enrichment of the acetic acid assay is much more sensitive and precise than the enrichment of acetyl-CoA by LC-MS/MS. The reversibility of propionyl-CoA from succinyl-CoA was confirmed by the low labeling of M1 and M2 of propionic acid from [5,6,7-(13)C(3)]heptanoate perfusates.

Symposium 2: Modern approaches to nutritional research challenges: Targeted and non-targeted approaches for metabolite profiling in nutritional research

The present report discusses targeted and non-targeted approaches to monitor single nutrients and global metabolite profiles in nutritional research. Non-targeted approaches such as metabolomics allow for the global description of metabolites in a biological sample and combine an analytical platform with multivariate data analysis to visualise patterns between sample groups. In nutritional research metabolomics has generated much interest as it has the potential to identify changes to metabolic pathways induced by diet or single nutrients, to explore relationships between diet and disease and to discover biomarkers of diet and disease. Although still in its infancy, a number of studies applying this technology have been performed; for example, the first study in 2003 investigated isoflavone metabolism in females, while the most recent study has demonstrated changes to various metabolic pathways during a glucose tolerance test. As a relatively new technology metabolomics is faced with a number of limitations and challenges including the standardisation of study design and methodology and the need for careful consideration of data analysis, interpretation and identification. Targeted approaches are used to monitor single or multiple nutrient and/or metabolite status to obtain information on concentration, absorption, distribution, metabolism and elimination. Such applications are currently widespread in nutritional research and one example, using stable isotopes to monitor nutrient status, is discussed in more detail. These applications represent innovative approaches in nutritional research to investigate the role of both single nutrients and diet in health and disease.

Probing pyruvate metabolism in normal and mutant fibroblast cell lines using 13C-labeled mass isotopomer analysis and mass spectrometry

Fibroblast cell lines are frequently used to diagnose genetic mitochondrial defects in children. The effect of enzyme deficiency on overall flux rate through metabolic pathways is, however, not generally considered. We have transposed an experimental paradigm that was developed for isolated perfused organs using (13)C-labeled substrates and (13)C-isotopomer analysis to probe pyruvate mitochondrial metabolism in cultured human fibroblast cell lines with normal or genetically mutant pyruvate decarboxylation (PDC) or carboxylation (PC) activity. Cells were incubated with 1mM [U-(13)C]pyruvate, and the (13)C-molar percent enrichment (MPE) of intracellular pyruvate, citrate, malate (as a surrogate of oxaloacetate) and aspartate was assessed by mass spectrometry. We estimated various flux ratios relevant to metabolic pathways involved in energy production, namely pyruvate formation, PDC, PC, and citrate recycling in the citric acid cycle (CAC). In all cell lines, exogenous pyruvate was predominately decarboxylated (PC/PDC ratios 0.01-0.3). PC-deficient cell lines displayed an expected negligible contribution of PC flux to oxaloacetate formation for citrate synthesis (PC/CS), which was associated with a greater contribution of PDC to acetyl-CoA formation (PDC/CS), and greater recycling of (13)C-labeled citrate into the CAC. In PDH-deficient cell lines, metabolic flux alterations were most apparent in cells with more than 50% reduction in enzyme activity. This led to an unexpected lower PC/CS flux ratio, while the PDC/CS flux ratio was unchanged. These data illustrate the usefulness of this approach in identifying unexpected metabolic consequences of genetic defects related to pyruvate metabolism.

Applications of metabolomics and proteomics to the mdx mouse model of Duchenne muscular dystrophy: lessons from downstream of the transcriptome

Functional genomic studies are dominated by transcriptomic approaches, in part reflecting the vast amount of information that can be obtained, the ability to amplify mRNA and the availability of commercially standardized functional genomic DNA microarrays and related techniques. This can be contrasted with proteomics, metabolomics and metabolic flux analysis (fluxomics), which have all been much slower in development, despite these techniques each providing a unique viewpoint of what is happening in the overall biological system. Here, we give an overview of developments in these fields 'downstream' of the transcriptome by considering the characterization of one particular, but widely used, mouse model of human disease. The mdx mouse is a model of Duchenne muscular dystrophy (DMD) and has been widely used to understand the progressive skeletal muscle wasting that accompanies DMD, and more recently the associated cardiomyopathy, as well as to unravel the roles of the other isoforms of dystrophin, such as those found in the brain. Studies using proteomics, metabolomics and fluxomics have characterized perturbations in calcium homeostasis in dystrophic skeletal muscle, provided an understanding of the role of dystrophin in skeletal muscle regeneration, and defined the changes in substrate energy metabolism in the working heart. More importantly, they all point to perturbations in proteins, metabolites and metabolic fluxes reflecting mitochondrial energetic alterations, even in the early stage of the dystrophic pathology. Philosophically, these studies also illustrate an important lesson relevant to both functional genomics and the mouse phenotyping in that the knowledge generated has advanced our understanding of cell biology and physiological organization as much as it has advanced our understanding of the disease.

Application of stable isotope-labeled compounds in metabolism and in metabolism-mediated toxicity studies

Stable isotope-labeled compounds have been synthesized and utilized by scientists from various areas of biomedical research during the last several decades. Compounds labeled with stable isotopes, such as deuterium and carbon-13, have been used effectively by drug metabolism scientists and toxicologists to gain better understanding of drugs' disposition and their potential role in target organ toxicities. The combination of stable isotope-labeling techniques with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy, which allows rapid acquisition and interpretation of data, has promoted greater use of these stable isotope-labeled compounds in absorption, distribution, metabolism, and excretion (ADME) studies. Examples of the use of stable isotope-labeled compounds in elucidating structures of metabolites and delineating complex metabolic pathways are presented in this review. The application of labeled compounds in mechanistic toxicity studies will be discussed by providing an example of how strategic placement of a deuterium atom in a drug molecule mitigated specific-specific renal toxicity. Other examples from the literature demonstrating the application of stable isotope-labeled compounds in understanding metabolism-mediated toxicities are presented. Furthermore, an example of how a stable isotope-labeled compound was utilized to better understand some of the gene changes in toxicogenomic studies is discussed. The interpretation of large sets of data produced from toxicogenomics studies can be a challenge. One approach that could be used to simplify interpretation of the data, especially from studies designed to link gene changes with the formation of reactive metabolites thought to be responsible for toxicities, is through the use of stable isotope-labeled compounds. This is a relatively unexplored territory and needs to be further investigated. The employment of analytical techniques, especially mass spectrometry and NMR, used in conjunction with stable isotope-labeled compounds to establish and understand mechanistic link between reactive metabolite formation, genomic, and proteomic changes and onset of toxicity is proposed. The use of stable isotope-labeled compounds in early human ADME studies as a way of identifying and possibly quantifying all drug-related components present in systemic circulation is suggested.