Quantitative analysis of 15N-labeled positional isomers of glutamine and citrulline via electrospray ionization tandem mass spectrometry of their dansyl derivatives

Effects of improved amino acid balance diet on lysine mammary utilization, whole body protein turnover and muscle protein breakdown on lactating sows

BACKGROUND

The study objective was to test the hypothesis that low crude protein (CP) diet with crystalline amino acids (CAA) supplementation improves Lys utilization efficiency for milk production and reduces protein turnover and muscle protein breakdown. Eighteen lactating multiparous Yorkshire sows were allotted to 1 of 2 isocaloric diets (10.80 MJ/kg net energy): control (CON; 19.24% CP) and reduced CP with "optimal" AA profile (OPT; 14.00% CP). Sow body weight and backfat were recorded on d 1 and 21 of lactation and piglets were weighed on d 1, 14, 18, and 21 of lactation. Between d 14 and 18, a subset of 9 sows (CON = 4, OPT = 5) was infused with a mixed solution of 3-[methyl-2H3]histidine (bolus injection) and [13C]bicarbonate (priming dose) first, then a constant 2-h [13C]bicarbonate infusion followed by a 6-h primed constant [1-13C]lysine infusion. Serial blood and milk sampling were performed to determine plasma and milk Lys enrichment, Lys oxidation rate, whole body protein turnover, and muscle protein breakdown.

RESULTS

Over the 21-d lactation period, compared to CON, sows fed OPT had greater litter growth rate (P < 0.05). Compared to CON, sows fed OPT had greater efficiency of Lys (P < 0.05), Lys mammary flux (P < 0.01) and whole-body protein turnover efficiency (P < 0.05). Compared to CON, sows fed OPT tended to have lower whole body protein breakdown rate (P = 0.069). Muscle protein breakdown rate did not differ between OPT and CON (P = 0.197).

CONCLUSION

Feeding an improved AA balance diet increased efficiency of Lys and reduced whole-body protein turnover and protein breakdown. These results imply that the lower maternal N retention observed in lactating sows fed improved AA balance diets in previous studies may be a result of greater partitioning of AA towards milk rather than greater body protein breakdown.

Distinct Amino Acid Profile Characterizes Youth With or at Risk for Type 2 Diabetes

Branched-chain amino acids (BCAAs) and aromatic AAs (AAAs) are associated with increased risk for type 2 diabetes in adults. Studies in youth show conflicting results. We hypothesized that an AA metabolomic signature can be defined to identify youth at risk for β-cell failure and the development of type 2 diabetes. We performed targeted AA metabolomics analysis on 127 adolescents (65 girls; 15.5 [SD ±1.9] years old, Tanner stage II-V) with normal weight or obesity across the spectrum of glycemia, with assessment of AA concentrations by mass spectrometry, at fasting, and steady state of a hyperinsulinemic-euglycemic clamp, with determination of insulin sensitivity (IS) per fat-free mass (FFM). We measured insulin secretion during a 2-h hyperglycemic clamp and calculated the disposition index per FFM (DIFFM), a measure of β-cell function. Our results showed that concentration of glycine (Gly) and the glutamine (Gln)-to-glutamate (Glu) ratio were lower, whereas BCAA, tyrosine, and lysine (Lys) concentrations were higher in the groups with obesity and dysglycemia compared with those with normal weight. Gly and Gln-to-Glu ratio were positively related to IS and DIFFM, with opposite relationships observed for BCAAs, AAAs, and Lys. We conclude that a metabolic signature of low Gly concentration and low Gln-to-Glu ratio, and elevated BCAAs, AAAs, and Lys concentrations may constitute a biomarker to identify youth at risk for β-cell failure.

ARTICLE HIGHLIGHTS

Arginine kinetics are altered in a pilot sample of adolescents and young adults with Barth syndrome

Barth syndrome (BTHS) is a rare, X-linked cardiomyopathy that is characterized by abnormalities in glucose and lipid metabolism, with less known regarding amino acid metabolism. This pilot study characterized whole-body arginine kinetics and found lower arginine rate of appearance into plasma (0.69 ± 0.09 vs. 0.88 ± 0.06 μmol/kgFFM/min, p < 0.01) and arginine non-oxidative disposal rate (0.64 ± 0.11 vs. 0.80 ± 0.03 μmol/kgFFM/min, p < 0.02) in adolescents and young adults with BTHS compared to Controls. This study provides a foundation for more in-depth studies on how arginine and potentially other amino acid abnormalities contribute to the pathology and clinical manifestations of BTHS.

Modeling age-dependent developmental changes in the expression of genes involved in citrulline synthesis using pig enteroids

BACKGROUND

Age-dependent changes in the intestinal gene expression of enzymes involved in the metabolism of citrulline and arginine are well characterized. Enteroids, a novel ex-vivo model that recreates the three-dimensional structure of the intestinal crypt-villus unit, have shown to replicate molecular and physiological profiles of the intestinal segment from where they originated ("location memory").

OBJECTIVE

The present study tested the hypothesis that enteroids recapitulate the developmental changes observed in vivo regarding citrulline production in pigs ("developmental memory").

METHODS

Preterm (10- and 5-d preterm) and term pigs at birth, together with 7- and 35-d-old pigs were studied. Gene expression was measured in jejunal samples and in enteroids derived from this segment. Whole body citrulline production was measured by isotope dilution and enteroid citrulline production by accumulation in the media.

RESULTS

With the exception of arginase I and inducible nitric oxide synthase, all the genes investigated expressed in jejunum were expressed by enteroids. In the jejunum, established markers of development (lactase and sucrase-isomaltase), as well as genes that code for enzymes involved in the production and utilization of citrulline and arginine, underwent the ontogenic changes described in the literature. However, enteroid expression of these genes, as well as citrulline production, failed to recapitulate the changes observed in vivo.

CONCLUSIONS

Under culture conditions used in our study, enteroids derived from jejunal crypts of pigs at different ages failed to replicate the gene expression observed in whole tissue and whole body citrulline production. Additional extracellular cues may be needed to reproduce the age-dependent phenotype.

Derangement of Arginine and Related Amino Acids in Children Undergoing Surgery for Congenital Heart Disease With Cardiopulmonary Bypass

Supplemental Digital Content is available in the text.

Arginine is a conditionally essential amino acid, the precursor for nitric oxide and a key factor in cell proliferation, protein synthesis, and energy metabolism. When there is increased demand in the setting of inflammation, ischemia-reperfusion injury, and organ dysfunction, endogenous arginine production falls short, and external supplementation may be necessary. The goal of this study was to assess changes in concentrations of plasma arginine, citrulline, ornithine, glutamine, and plasma arginase in infants and children undergoing surgery for congenital heart disease with cardiopulmonary bypass.

Developmental changes in the utilization of citrulline by neonatal pigs

Developmental changes in the renal expression and activity of argininosuccinate synthase (ASS1) and argininosuccinate lyase (ASL), enzymes that use citrulline for the production of arginine, have been reported. Thus, the ability of neonates, and especially premature neonates, to produce arginine may be compromised. To determine the utilization of citrulline in vivo, we measured renal expression of ASS1 and ASL and conducted citrulline compartmental and noncompartmental kinetics using [¹⁵N]citrulline in pigs of five different ages (from 10 days preterm to 5 wk of age). The tracer was given in substrate amounts to also test the ability of neonatal pigs to use exogenous citrulline. Preterm and term pigs at birth had lower ASS1 and ASL expression than older animals, which was reflected in the longer half-life of citrulline in the neonatal groups. The production and utilization of citrulline by 1-wk-old pigs was greater than in pigs of other ages, including 5-wk-old animals. Plasma citrulline concentration was not able to capture these differences in citrulline production and utilization. In conclusion, the developmental changes in renal ASS1 and ASL gene expression are reflected in the ability of the pigs to use citrulline. However, it seems that there is an excess capacity to use citrulline at all ages, including during prematurity, since the bolus dose of tracer did not result in an increase in endogenous citrulline. Our results support the idea that citrulline supplementation in neonatal, including premature, pigs is a viable option to increase arginine availability.

Ileal digestibility of intrinsically labeled hen's egg and meat protein determined with the dual stable isotope tracer method in Indian adults

Background

Protein quality assessment through the Digestible Indispensable Amino Acid Score requires accurate measurements of true ileal protein and amino acid digestibility, for which a dual isotope technique was recently developed. However, the ileal digestibility of indispensable amino acids (IAA) in humans from high-quality proteins is not well known.

Objective

The aim of this study was to intrinsically label hen's egg and meat protein by the use of uniformly 2H-labeled amino acids, and to measure their true ileal indispensable amino acid (IAA) digestibility via the dual isotope method in humans.

Design

2H-labeled lyophilized boiled egg white protein, whole boiled egg, and cooked meat were obtained from layer hens (BV-300) administered a uniformly 2H-labeled amino acid mix orally for 35 d with their daily feed. The ileal IAA digestibility of these proteins was determined with reference to digestibility of previously characterized [U-13C]spirulina in a dual tracer method in healthy Indian subjects whose intestinal health was measured by the plasma kynurenine-to-tryptophan (KT) ratio.

Results

All subjects had normal KT ratios. The mean ± SD true ileal IAA digestibility of 2H-labeled egg white protein, whole boiled egg, and cooked meat was 86.3% ± 4.6%, 89.4% ± 4.5%, and 92.0% ± 2.8%, respectively. Leucine digestibility correlated with the KT ratio (r = -0.772; P = 0.009).

Conclusions

Uniformly 2H-labeled hen's egg and meat protein can be used to measure ileal IAA digestibility by the dual isotope tracer approach in humans. The mean IAA digestibility values for these high-quality proteins in the healthy Indians studied were similar to values obtained in earlier human and animal experiments. Leucine digestibility in these meal matrices correlated with the KT ratio, but this aspect needs further evaluation. This trial was registered at the Clinical Trials Registry of India (http://ctri.nic.in) as CTRI/2018/03/012265.

Citrulline Generation Test: What Does It Measure?

BACKGROUND

The citrulline generation test (CGT) has been proposed as a tool to determine gut function. However, the increase in plasma citrulline concentration that follows a bolus dose of alanyl-glutamine may also result from a reduction in citrulline clearance due to competition with glutamine for transport.

MATERIALS AND METHODS

A swine model was developed, and stable isotope tracers were used to determine the mechanism behind the increase in plasma citrulline that follows a bolus dose of alanyl-glutamine. Plasma concentrations and enrichments were determined, and a non-steady-state model was used to calculate rates of appearance, disappearance, and conversion.

RESULTS

The pig model recapitulated the increase in plasma citrulline observed in humans after a dose of alanyl-glutamine. The dipeptide was rapidly hydrolyzed to its constitutive amino acids. Both citrulline plasma concentration and citrulline rate of appearance increased by ≈45% after the bolus dose of alanyl-glutamine. The conversion of citrulline to arginine and the rate of appearance of arginine also increased. Glutamine contributed up to 25% ± 2% of the rate of appearance of citrulline. No changes in the rate of disappearance of citrulline were observed.

CONCLUSION

Our results indicate that a single bolus dose of alanyl-glutamine increases plasma citrulline concentration by increasing citrulline production without any effect on citrulline disposal. Our findings strongly indicate that the CGT assesses the metabolic response of the gut and that CGT can become a useful tool to evaluate gut mass and function.

Prematurity reduces citrulline-arginine-nitric oxide production and precedes the onset of necrotizing enterocolitis in piglets

Necrotizing enterocolitis (NEC) is associated with low plasma arginine and vascular dysfunction. It is not clear whether low intestinal citrulline production, the precursor for arginine synthesis, occurs before and thus predisposes to NEC or if it results from tissue damage. This study was designed to test the hypothesis that whole body rates of citrulline, arginine, and nitric oxide synthesis are low in premature pigs and that they precede NEC. Piglets delivered by cesarean section at 103 days [preterm (PT)], 110 days [near-term (NT)], or 114 days [full-term (FT)] of gestation were given total parenteral nutrition and after 2 days orogastrically fed infant formula for 42 h to induce NEC. Citrulline and arginine fluxes were determined before and during the feeding protocol. Gross macroscopic and histological NEC scores and plasma fatty acid binding protein (iFABP) concentration were determined as indicators of NEC. Intestinal gene expression for enzymes of the arginine pathway were quantitated. A lower ( P < 0.05) survival rate was observed for PT (8/27) than for NT (9/9) and FT pigs (11/11). PT pigs had higher macroscopic gross ( P < 0.05) and histological NEC ( P < 0.05) scores and iFABP concentration ( P < 0.05) than pigs of more advanced gestational age. PT pigs had lower citrulline production and arginine fluxes ( P < 0.05) throughout and a reduced gene expression in genes of the citrulline-arginine pathway. In summary, intestinal enzyme expression and whole body citrulline and arginine fluxes were reduced in PT pigs compared with animals of more advance gestational age and preceded the development of NEC. NEW & NOTEWORTHY Arginine supplementation prevents necrotizing enterocolitis (NEC), the most common gastrointestinal emergency of prematurity. Citrulline (precursor for arginine) production is reduced during NEC, and this is believed to be a consequence of intestinal damage. In a swine model of NEC, we show that intestinal gene expression of the enzymes for citrulline production and whole body citrulline and arginine fluxes are reduced and precede the onset of NEC in premature pigs. Reduced citrulline production during prematurity may be a predisposition to NEC.

Ex Vivo Enteroids Recapitulate In Vivo Citrulline Production in Mice

Background

The endogenous production of arginine relies on the synthesis of citrulline by enteral ornithine transcarbamylase (OTC). Mutations in the gene coding for this enzyme are the most frequent cause of urea cycle disorders. There is a lack of correlation between in vivo metabolic function and DNA sequence, transcript abundance, or in vitro enzyme activity.

Objective

The goal of the present work was to test the hypothesis that enteroids, a novel ex vivo model, are able to recapitulate the in vivo citrulline production of wild-type (WT) and mutant mice.

Methods

Six-week-old male WT and OTC-deficient mice [sparse fur and abnormal skin (spf-ash) mutation] were studied. Urea and citrulline fluxes were determined in vivo, and OTC abundance was measured in liver and gut tissue. Intestinal crypts were isolated and cultured to develop enteroids. Ex vivo citrulline production and OTC abundance were determined in these enteroids.

Results

Liver OTC abundance was lower (mean ± SE: 0.16 ± 0.01 compared with 1.85 ± 0.18 arbitrary units; P < 0.001) in spf-ash mice than in WT mice, but there was no difference in urea production. In gut tissue, OTC was barely detectable in mutant mice; despite this, a lower but substantial citrulline production (67 ± 3 compared with 167 ± 8 µmol · kg-1 · h-1; P < 0.001) was shown in the mutant mice. Enteroids recapitulated the in vivo findings of a very low OTC content accompanied by a reduced citrulline production (1.07 ± 0.20 compared with 4.64 ± 0.44 nmol · µg DNA-1 · d-1; P < 0.001).

Conclusions

Enteroids recapitulate in vivo citrulline production and offer the opportunity to study the regulation of citrulline production in a highly manipulable system.

The Citrulline Recycling Pathway Sustains Cardiovascular Function in Arginine-Depleted Healthy Mice, but Cannot Sustain Nitric Oxide Production during Endotoxin Challenge

Background

The recycling of citrulline by argininosuccinate synthase 1 (ASS1) and argininosuccinate lyase (ASL) is crucial to maintain arginine availability and nitric oxide (NO) production. Pegylated arginine deiminase (ADI-PEG20) is a bacterial enzyme used to deplete circulating arginine.

Objective

The goal of this research was to test the hypothesis that citrulline is able to sustain intracellular arginine availability for NO production in ADI-PEG20 arginine-depleted mice.

Methods

Six- to 8-wk-old male C57BL/6J mice injected with ADI-PEG20 (5 IU) or saline (control) were used in 4 different studies. Arginine, citrulline, and NO kinetics were determined by using stable isotopes in unchallenged (study 1) and endotoxin-challenged (study 2) mice. Blood pressure was determined by telemetry for 6 d after ADI-PEG20 administration (study 3), and vasomotor activity and ASS1 and ASL gene expression were determined in mesenteric arteries collected from additional mice (study 4).

Results

ADI-PEG20 administration resulted in arginine depletion (<1 compared with 111 ± 37 µmol/L) but in greater plasma citrulline concentrations (900 ± 123 compared with 76 ± 8 µmol/L; P < 0.001) and fluxes (402 ± 17 compared with 126 ± 4 µmol ⋅ kg-1 ⋅ h-1; P < 0.001) compared with controls. Endotoxin-challenged ADI-PEG20-treated mice produced less NO than controls (13 ± 1 compared with 27 ± 2 µmol ⋅ kg-1 ⋅ h-1; P < 0.001). No differences (P > 0.50) were observed for cardiovascular variables (heart rate, blood pressure) between ADI-PEG20-treated and control mice. Furthermore, no ex vivo vasomotor differences were observed between the 2 treatments. ADI-PEG20 administration resulted in greater gene expression of ASS1 (∼3-fold) but lower expression of ASL (-30%).

Conclusion

ADI-PEG20 successfully depleted circulating arginine without any effect on cardiovascular endpoints in healthy mice but limited NO production after endotoxin challenge. Therefore, the citrulline recycling pathway can sustain local arginine availability independently from circulating arginine, satisfying the demand of arginine for endothelial NO production; however, it is unable to do so when a high demand for arginine is elicited by endotoxin.

Ablation of Arginase II Spares Arginine and Abolishes the Arginine Requirement for Growth in Male Mice

Background: Arginine is considered a semiessential amino acid in many species, including humans, because under certain conditions its demand exceeds endogenous production. Arginine availability, however, is determined not only by its production but also by its disposal. Manipulation of disposal pathways has the potential to increase availability and thus abolish the requirement for arginine.Objective: The objective of the study was to test the hypothesis that arginase II ablation increases arginine availability for growth.Methods: In a completely randomized design with a factorial arrangement of treatments, postweaning growth was determined for 3 wk in male and female wild-type (WT) mice and arginase II knockout mice (ARGII) on a C57BL/6J background fed arginine-sufficient [Arg(+); 8 g arginine/kg] or arginine-free [Arg(-)] diets. Tracers were used to determine citrulline and arginine kinetics.Results: A sex dimorphism in arginine metabolism was detected; female mice had a greater citrulline flux (∼30%, P < 0.001), which translated to greater de novo synthesis of arginine (∼31%, P < 0.001). Female mice also had greater arginine fluxes (P < 0.015) and plasma arginine concentrations (P < 0.01), but a reduced arginine clearance rate (P < 0.001). Ablation of arginase II increased plasma arginine concentrations in both sexes (∼27%, P < 0.01) but increased arginine flux only in males (P < 0.01). The absence of arginine in the diet limited the growth of male WT mice (P < 0.01), but had no effect on male ARGII mice (P = 0.12). In contrast, WT females on the Arg(-) diet grew at the same rate and achieved final weight similar to that of female WT mice fed the Arg(+) diet (P = 0.47).Conclusion: The ablation of arginase II in male mice spares arginine that can then be used for growth and to meet other metabolic functions, thus abolishing arginine requirements.

The intestinal-renal axis for arginine synthesis is present and functional in the neonatal pig

The intestinal-renal axis for endogenous arginine synthesis is an interorgan process in which citrulline produced in the small intestine is utilized by the kidney for arginine synthesis. The function of this axis in neonates has been questioned because during this period the enzymes needed for arginine synthesis argininosuccinate synthase (ASS1) and lyase (ASL) are present in the gut. However, evidence of high plasma citrulline concentrations in neonates suggests otherwise. We quantified in vivo citrulline production in premature (10 days preterm), neonatal (7 days old), and young pigs (35 days old) using citrulline tracers. Neonatal pigs had higher fluxes (69 µmol·kg^-1·h^-1, P < 0.001) than premature and young pigs (43 and 45 µmol·kg^-1·h^-1, respectively). Plasma citrulline concentration was also greater in neonatal pigs than in the other age groups. We also determined the site of synthesis and utilization of citrulline in neonatal and young pigs by measuring organ balances across the gut and the kidney. Citrulline was released from the gut and utilized by the kidney in both neonatal and young pigs. The abundance and localization of the enzymes involved in the synthesis and utilization were determined in intestinal and kidney tissue. Despite the presence of ASS1 and ASL in the neonatal small intestine, the lack of colocalization with the enzymes that produce citrulline results in the release of citrulline by the PDV and its utilization by the kidney to produce arginine. In conclusion, the intestinal-renal axis for arginine synthesis is present in the neonatal pig.

Supplemental Citrulline Is More Efficient Than Arginine in Increasing Systemic Arginine Availability in Mice

Background: Arginine is considered to be an essential amino acid in various (patho)physiologic conditions of high demand. However, dietary arginine supplementation suffers from various drawbacks, including extensive first-pass extraction. Citrulline supplementation may be a better alternative than arginine, because its only fate in vivo is conversion into arginine.Objective: The goal of the present research was to determine the relative efficiency of arginine and citrulline supplementation to improve arginine availability.Methods: Six-week-old C57BL/6J male mice fitted with gastric catheters were adapted to 1 of 7 experimental diets for 2 wk. The basal diet contained 2.5 g l-arginine/kg, whereas the supplemented diets contained an additional 2.5, 7.5, and 12.5 g/kg diet of either l-arginine or l-citrulline. On the final day, after a 3-h food deprivation, mice were continuously infused intragastrically with an elemental diet similar to the dietary treatment, along with l-[¹³C₆]arginine, to determine the splanchnic first-pass metabolism (FPM) of arginine. In addition, tracers were continuously infused intravenously to determine the fluxes and interconversions between citrulline and arginine. Linear regression slopes were compared to determine the relative efficiency of each supplement.Results: Whereas all the supplemented citrulline (105% ± 7% SEM) appeared in plasma and resulted in a marginal increase of 86% in arginine flux, supplemental arginine underwent an ∼70% FPM, indicating that only 30% of the supplemental arginine entered the peripheral circulation. However, supplemental arginine did not increase arginine flux. Both supplements linearly increased (P < 0.01) plasma arginine concentration from 109 μmol/L for the basal diet to 159 and 214 μmol/L for the highest arginine and citrulline supplementation levels, respectively. However, supplemental citrulline increased arginine concentrations to a greater extent (35%, P < 0.01).Conclusions: Citrulline supplementation is more efficient at increasing arginine availability than is arginine supplementation itself in mice.

Plasma Glutamine Is a Minor Precursor for the Synthesis of Citrulline: A Multispecies Study

Background: Glutamine is considered the main precursor for citrulline synthesis in many species, including humans. The transfer of ¹⁵N from 2-[¹⁵N]-glutamine to citrulline has been used as evidence for this precursor-product relation. However, work in mice has shown that nitrogen and carbon tracers follow different moieties of glutamine and that glutamine contribution to the synthesis of citrulline is minor. It is unclear whether this small contribution of glutamine is also true in other species.Objective: The objective of the present work was to determine the contribution of glutamine to citrulline production by using nitrogen and carbon skeleton tracers in multiple species.Methods: Humans (n = 4), pigs (n = 5), rats (n = 6), and mice (n = 5) were infused with l-2-[¹⁵N]- and l-[²H₅]-glutamine and l-5,5-[²H₂]-citrulline. The contribution of glutamine to citrulline synthesis was calculated by using different ions and fragments: glutamine M+1 to citrulline M+1, 2-[¹⁵N]-glutamine to 2-[¹⁵N]-citrulline, and [²H₅]-glutamine to [²H₅]-citrulline.Results: Species-specific differences in glutamine and citrulline fluxes were found (P < 0.001), with rats having the largest fluxes, followed by mice, pigs, and humans (all P < 0.05). The contribution of glutamine to citrulline as estimated by using glutamine M+1 to citrulline M+1 ranged from 88% in humans to 46% in pigs. However, the use of 2-[¹⁵N]-glutamine and 2-[¹⁵N]-citrulline as precursor and product yielded values of 48% in humans and 28% in pigs. Furthermore, the use of [²H₅]-glutamine to [²H₅]-citrulline yielded lower values (P < 0.001), resulting in a contribution of glutamine to the synthesis of citrulline of ∼10% in humans and 3% in pigs.Conclusions: The recycling of the [¹⁵N]-glutamine label overestimates the contribution of glutamine to citrulline synthesis compared with a tracer that follows the carbon skeleton of glutamine. Glutamine is a minor precursor for the synthesis of citrulline in humans, pigs, rats, and mice.

Interrelationships between glutamine and citrulline metabolism

PURPOSE OF REVIEW

This article analyzes the contribution of glutamine to the synthesis of citrulline and reviews the evidence that glutamine supplementation increases citrulline production.

RECENT FINDINGS

Glutamine supplementation has been proposed in the treatment of critically ill patients; however, a recent large multicenter randomized controlled trial resulted in increased mortality in the glutamine-supplemented group. Within this context, defining the contribution of glutamine to the production of citrulline, and thus to de-novo arginine synthesis, has become a pressing issue.

SUMMARY

The beneficial effects of glutamine supplementation may be partially mediated by the effects of glutamine on citrulline synthesis by the gut and the de-novo synthesis of arginine by the kidney and other tissues. Although there is no strong evidence to support that glutamine is a major precursor for citrulline synthesis in humans, glutamine has the potential to increase overall gut function and in this way increase citrulline production.

Indian women of childbearing age do not metabolically conserve arginine as do American and Jamaican women

BACKGROUND

In a previous study in pregnant American women, we reported that arginine flux and nitric oxide synthesis increased in trimester 2. More recently, we reported that Indian women do not increase arginine flux during pregnancy as their American or Jamaican counterparts do.

OBJECTIVE

The purpose of this study was to determine whether Indian women of childbearing age are producing less arginine and/or catabolizing more arginine and therefore have less available for anabolic pathways than do Jamaican and American women.

METHODS

Thirty healthy women aged 28.3 ± 0.8 y from the United States, India, and Jamaica (n = 10/group) were given 6 h primed, constant intravenous infusions of guanidino-¹⁵N₂-arginine, 5,5-²H₂-citrulline, ¹⁵N₂-ornithine, and ring-²H₅-phenylalanine, in addition to primed, oral doses of U-¹³C₆-arginine in both the fasting and postprandial states. An oral dose of deuterium oxide was also given to determine fat-free mass (FFM).

RESULTS

Compared with American women, Indian and Jamaican women had greater ornithine fluxes (μmol · kg fat FFM⁻¹ · h⁻¹) in the fasting and postprandial states (27.3 ± 2.5 vs. 39.6 ± 3.7 and 37.2 ± 2.0, respectively, P = 0.01), indicating greater arginine catabolism. However, Jamaican women had a higher endogenous arginine flux than did Indian and American women in the fasting (66.1 ± 3.1 vs. 54.2 ± 3.1 and 56.1 ± 2.1, respectively, P = 0.01) and postprandial (53.8 ± 2.2 vs. 43.7 ± 4.9 and 42.8 ± 3.1, respectively, P = 0.06) states. As a consequence, Indian women had lower arginine bioavailability (μmol · kg FFM⁻¹ · h⁻¹) in the fasting state (42.0 ± 2.6) than did American (49.9 ± 1.3, P = 0.045) and Jamaican (55.5 ± 3.5, P = 0.004) women, as well as in the postprandial state (40.7 ± 3.5 vs. 51.8 ± 1.2 and 57.5 ± 3.2, respectively, P = 0.001).

CONCLUSION

Compared with American and Jamaican women, Indian women of childbearing age have a decreased arginine supply because of increased arginine catabolism without an increase in arginine flux.

Arginine depletion by arginine deiminase does not affect whole protein metabolism or muscle fractional protein synthesis rate in mice

Due to the absolute need for arginine that certain cancer cells have, arginine depletion is a therapy in clinical trials to treat several types of cancers. Arginine is an amino acids utilized not only as a precursor for other important molecules, but also for protein synthesis. Because arginine depletion can potentially exacerbate the progressive loss of body weight, and especially lean body mass, in cancer patients we determined the effect of arginine depletion by pegylated arginine deiminase (ADI-PEG 20) on whole body protein synthesis and fractional protein synthesis rate in multiple tissues of mice. ADI-PEG 20 successfully depleted circulating arginine (<1 μmol/L), and increased citrulline concentration more than tenfold. Body weight and body composition, however, were not affected by ADI-PEG 20. Despite the depletion of arginine, whole body protein synthesis and breakdown were maintained in the ADI-PEG 20 treated mice. The fractional protein synthesis rate of muscle was also not affected by arginine depletion. Most tissues (liver, kidney, spleen, heart, lungs, stomach, small and large intestine, pancreas) were able to maintain their fractional protein synthesis rate; however, the fractional protein synthesis rate of brain, thymus and testicles was reduced due to the ADI-PEG 20 treatment. Furthermore, these results were confirmed by the incorporation of ureido [14C]citrulline, which indicate the local conversion into arginine, into protein. In conclusion, the intracellular recycling pathway of citrulline is able to provide enough arginine to maintain protein synthesis rate and prevent the loss of lean body mass and body weight.

Measurement of (15)N enrichment of glutamine and urea cycle amino acids derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate using liquid chromatography-tandem quadrupole mass spectrometry

6-Aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) is an amino acid-specific derivatizing reagent that has been used for sensitive amino acid quantification by liquid chromatography-tandem quadrupole mass spectrometry (LC-MS/MS). In this study, we aimed to evaluate the ability of this method to measure the isotopic enrichment of amino acids and to determine the positional (15)N enrichment of urea cycle amino acids (i.e., arginine, ornithine, and citrulline) and glutamine. The distribution of the M and M+1 isotopomers of each natural AQC-amino acid was nearly identical to the theoretical distribution. The standard deviation of the (M+1)/M ratio for each amino acid in repeated measurements was approximately 0.1%, and the ratios were stable regardless of the injected amounts. Linearity in the measurements of (15)N enrichment was confirmed by measuring a series of (15)N-labeled arginine standards. The positional (15)N enrichment of urea cycle amino acids and glutamine was estimated from the isotopic distribution of unique fragment ions generated at different collision energies. This method was able to identify their positional (15)N enrichment in the plasma of rats fed (15)N-labeled glutamine. These results suggest the utility of LC-MS/MS detection of AQC-amino acids for the measurement of isotopic enrichment in (15)N-labeled amino acids and indicate that this method is useful for the study of nitrogen metabolism in living organisms.

A novel amino acid analysis method using derivatization of multiple functional groups followed by liquid chromatography/tandem mass spectrometry

We have developed a novel amino acid analysis method using derivatization of multiple functional groups (amino, carboxyl, and phenolic hydroxyl groups).

Extrarenal citrulline disposal in mice with impaired renal function

The endogenous synthesis of arginine, a semiessential amino acid, relies on the production of citrulline by the gut and its conversion into arginine by the kidney in what has been called the "intestinal-renal axis" for arginine synthesis. Although the kidney is the main site for citrulline disposal, it only accounts for ~60-70% of the citrulline produced. Because the only known fate for citrulline is arginine synthesis and the enzymes that catalyze this reaction are widespread among body tissues, we hypothesized that citrulline can be utilized directly by tissues to meet, at least partially, their arginine needs. To test this hypothesis, we used stable and radioactive tracers in conscious, partially nephrectomized (½ and ⅚) and anesthetized acutely kidney-ligated mouse models. Nephrectomy increased plasma citrulline concentration but did not affect citrulline synthesis rates, thus reducing its clearance. Nephrectomy (⅚) reduced the amount of citrulline accounted for as plasma arginine from 88 to 42%. Acute kidney ligation increased the half-life and mean retention time of citrulline. Whereas the rate of citrulline conversion into plasma arginine was reduced, it was not eliminated. In addition, we observed direct utilization of citrulline for arginine synthesis and further incorporation into tissue protein in kidney-ligated mice. These observations indicate that a fraction of the citrulline produced is utilized directly by multiple tissues to meet their arginine needs and that extrarenal sites contribute to plasma arginine. Furthermore, when the interorgan synthesis of arginine is impaired, these extrarenal sites are able to increase their rate of citrulline utilization.

Alterations in glutamine metabolism and its conversion to citrulline in sepsis

In enterocytes, glutamine serves as the major source of energy; another metabolic fate of glutamine is conversion to citrulline. Because sepsis can affect gut function and integrity, alterations in glutamine metabolism may exist and lead to decreased citrulline production. This study aimed to investigate how sepsis affects glutamine metabolism, including its conversion to citrulline, by measuring glutamine and citrulline flux, fractional splanchnic extraction of glutamine and leucine, and the contribution of glutamine nitrogen to citrulline in septic patients and healthy controls. Eight patients with severe sepsis and 10 healthy controls were given primed, constant intravenous infusion of [(2)H2]citrulline and sequential administration of intravenous and enteral [α-(15)N]glutamine and [(13)C]leucine in the postabsorptive state. The results showed that, compared with healthy controls, septic patients had a significantly lower whole body citrulline flux and plasma concentration, higher endogenous leucine flux, and higher glutamine clearance. Fractional splanchnic extraction of leucine was higher in septic patients than in controls, but fractional extraction of glutamine was not different. The majority of the (15)N label transferred from glutamine to citrulline was found at the α-position. These results demonstrate that lower glutamine plasma concentrations in sepsis were a result of increased glutamine clearance. Despite adequate splanchnic uptake of glutamine, there is decreased production of citrulline, suggesting a defect in the metabolic conversion of glutamine to citrulline, decreased uptake of glutamine by the enterocyte but increased uptake by the liver, and/or shunting of glutamine to other metabolic pathways.

De novo synthesis is the main source of ornithine for citrulline production in neonatal pigs

Citrulline is an amino acid synthesized in the gut and utilized for the synthesis of the conditionally essential amino acid arginine. Recently, the origin of the ornithine utilized for citrulline synthesis has become a matter of discussion. Multiple physiological factors may have contributed to the differences found among different researchers; one of these is the developmental stage of the subjects studied. To test the hypothesis that during the neonatal period de novo synthesis is the main source of ornithine for citrulline synthesis, neonatal piglets were infused intravenously or intragastrically with [U-(13)C(6)]arginine, [U-(13)C(5)]glutamine, or [U-(13)C(5)]proline during the fasted and fed periods. [ureido-(15)N]citrulline and [(2)H(2)]ornithine were infused intravenously for the entire infusion protocol. During fasting, plasma proline (13%) and ornithine (19%) were the main precursors for citrulline synthesis, whereas plasma arginine (62%) was the main precursor for plasma ornithine. During feeding, enteral (27%) and plasma (12%) proline were the main precursors for the ornithine utilized in the synthesis of citrulline, together with plasma ornithine (27%). Enteral proline and glutamine were utilized directly by the gut to produce ornithine utilized for citrulline synthesis. Arginine was not utilized by the gut, which is consistent with the lack of arginase activity in the neonate. Arginine, however, was the main source (47%) of plasma ornithine and in this way contributed to citrulline synthesis. In conclusion, during the neonatal period, the de novo pathway is the predominant source for the ornithine utilized in the synthesis of citrulline, and proline is the preferred precursor.

Arginine and ornithine are the main precursors for citrulline synthesis in mice

Recent isotopic tracer studies in mice, piglets, and humans have produced conflicting results as to the main carbon skeleton precursor for citrulline and arginine synthesis. This may be due in part to the different tracers infused and models used to interpret the stable isotope data. Furthermore, previous studies usually investigated a single precursor, which prevented the direct comparison among multiple precursors. To further elucidate the contribution of different precursors to citrulline synthesis, all possible enteral and plasma precursors of citrulline were studied in a mouse model during the postabsorptive and postprandial period using multitracer protocols. In addition, three different models were used to interpret the stable isotope data. The utilization of the classic precursor-product equation, developed for i.v. infused tracers but also used to include i.g. tracers, grossly overestimated the contribution of enteral precursors. Regardless of the model employed, dietary and plasma arginine were the main precursors for citrulline synthesis during feeding and plasma arginine during feed deprivation. The contribution of arginine was directly at the site of citrulline synthesis and through plasma ornithine. The predominant role of arginine and ornithine seen in this study supports the observations in mice, piglets, and humans suggesting that ornithine amino transferase is a pivotal enzyme in this pathway.