Peptidylarginine deiminase (PAD): A promising target for chronic diseases treatment.

In 1958, the presence of citrulline in the structure of the proteins was discovered for the first time. Several years later they found that Arginine converted to citrulline during a post-translational modification process by PAD enzyme. Each PAD is expressed in a certain tissue developing a series of diseases such as inflammation and cancers. Among these, PAD2 and PAD4 play a role in the development of rheumatoid arthritis (RA) by producing citrullinated autoantigens and increasing the production of inflammatory cytokines. PAD4 is also associated with the formation of NET structures and thrombosis. In the crystallographic structure, PAD has several calcium binding sites, and the active site of the enzyme consists of different amino acids. Various PAD inhibitors have been developed divided into pan-PAD and selective PAD inhibitors. F-amidine, Cl-amidine, and BB-Cl-amidine are some of pan-PAD inhibitors. AFM-30a and JBI589 are selective for PAD2 and PAD4, respectively. There is a need to evaluate the effectiveness of existing inhibitors more accurately in the coming years, as well as design and production of novel inhibitors targeting highly specific isoforms.

Isolated auto-citrullinated regions of PADI4 associate to the intact protein without altering their disordered conformation.

PADI4 is one of the human isoforms of a group of enzymes intervening in the conversion of arginine to citrulline. It is involved in the development of several types of tumors, as well as other immunological illnesses, such as psoriasis, multiple sclerosis, or rheumatoid arthritis. PADI4 auto-citrullinates in several regions of its sequence, namely in correspondence of residues Arg205, Arg212, Arg218, and Arg383. We wanted to study whether the citrullinated moiety affects the conformation of nearby regions and its binding to intact PADI4. We designed two series of synthetic peptides comprising either the wild-type or the relative citrullinated versions of such regions - i.e., a first series of peptides comprising the first three arginines, and a second series comprising Arg383. We studied their conformational properties in isolation by using fluorescence, far-ultraviolet (UV) circular dichroism (CD), and 2D1H NMR. Furthermore, we characterized the binding of the wild-type and citrullinated peptides in the two series to the intact PADI4, by using isothermal titration calorimetry (ITC), fluorescence, and biolayer interferometry (BLI), as well as by molecular docking simulations. We observed that citrullination did not alter the local conformational propensities of the isolated peptides. Nevertheless, for all the peptides in the two series, citrullination slowed down the kinetic koff rates of the binding reaction to PADI4, probably due to differences in electrostatic effects compared to the presence of arginine. The affinities of PADI4 for unmodified peptides were slightly larger than those of the corresponding citrullinated ones in the two series, but they were all within the same range, indicating that there were no relevant variations in the thermodynamics of binding due to sequence effects. These results highlight details of the self-citrullination of PADI4 and, more generally, of possible auto-catalytic mechanisms taking place in vivo for other citrullinating enzymes or, alternatively, in proteins undergoing citrullination passively.

Exploring the effects of saccharides on the solvation behaviour of L-Citrulline in aqueous medium: Volumetric, transition state theory, transfer parameters and spectroscopic approach.

This study examines the volumetric, viscometric and UV-vis characteristics of L-Citrulline in water and aqueous saccharides at atmospheric pressure across the whole concentration range and the absorber operating temperature range of 293.15 K -313.15 K. Density, partial molar volume, apparent molar isobaric expansion, Hepler's constant and hydration number were among the examined volumetric parameters, and viscosity coefficients, viscosity deviation and free energy for viscous flow activation were among the examined viscometric parameters. Stronger interactions between L-Citrulline and L-Arabinose were indicated by the increase in the transfer characteristics in the following order: L-Arabinose > D-xylose. A comparison of the taste behaviours of L-Citrulline in water and in an aqueous solution of saccharides has also been attempted. L-Citrulline interacts with all solvents in a significant way, as evidenced by the UV-visible spectra suggested by the shift in UV-visible absorption maxima that correspond with a rise in L-Citrulline content in the solvent systems chosen.

Collision-Induced Dissociation of Citrullinated Peptide Anions.

Peptide identification by positive electrospray ionization (ES+) tandem mass spectrometry (MS/MS) is a well-established strategy in proteomics. Several research groups reported the usefulness of negative electrospray ionization (ES-) for gaining complementary structural information on peptides and their post-translational modifications (PTM) compared to ES+. Fragmentation of citrullinated peptides has not been previously explored in ES-. In this study, 9 peptides containing citrulline residues were investigated in ES- by stepwise collision energy-dependent measurements on a QTOF instrument and a Q-Orbitrap instrument. Our results of high resolution and mass accuracy show the favored citrulline-selective loss of HNCO from these peptide precursors and their fragments─similarly to that in ES+─along with y-NH3/z, c, c-NH3/b sequence ions. Loss of HNCO from citrullinated peptides in ES- and a proposed mechanism for the reaction have been described here for the first time. HNCO loss intensities from precursors were generally even higher than that in ES+. Interestingly, the most intense fragments corresponded to neutral losses from sequence ions while intact sequence ions were usually minor components of the spectra. High-intensity ions related to cleavages N-terminal to Asp and Glu residues that have been previously reported were also observed. On the other hand, a relatively high number of peaks were observed, possibly due to internal fragmentation and/or scrambling events. While (ES-) MS/MS spectra always require manual inspection and the annotation may be ambiguous, the favorable loss of HNCO and the preferable cleavage N-terminal to Asp residues can be used to differentiate between citrullinated/deamidated sequences.

Differential Protein Citrullination in Human ER- and ER+ Tumor and Adjacent Healthy Breast Tissue.

Post-translational modification of arginine to citrulline is catalyzed by members of the peptidylarginine deiminase (PAD) family. Dysregulation of this catalysis is a significant driver of the pathogenesis of numerous inflammatory diseases, including cancer. However, dysregulation of PAD activity has not been examined in breast cancer with respect to hormone receptor status. In this study, we measured PAD enzyme levels using Western blotting and investigated protein citrullination using a mass spectrometry-based proteomics approach in primary estrogen receptor negative (ER-) or positive (ER+) breast tumor and matched adjacent normal tissue. Our findings reveal 72 and 41 citrullinated proteins in ER- tumor and adjacent healthy tissue, respectively, where 20 of these proteins are common between the two groups. We detected 64 and 49 citrullinated proteins in ER+ tumor and adjacent healthy tissue, respectively, where 32 proteins are common. Interestingly, upon comparison of ER- and ER+ tumor tissue, only 32 citrullinated proteins are shared between the two and the rest are unique to the tumor's receptor status. Using the STRING database for protein-protein interaction network analysis, these proteins are involved in protein-folding events (i.e., heat shock proteins) in ER- samples and blood-clotting events (i.e., fibulin) in ER+ samples. Constituents of the extracellular matrix structure (i.e., collagen and fibrinogen) were found in both. Herein, we establish evidence that supports the role of this unique post-translational modification in breast cancer biology. Finally, to aid drug discovery against citrullination, we developed a liquid chromatography-ultraviolet method to measure PAD enzymatic activity and optimized glucagon-like peptide II to quantitatively measure the ability of PADs to citrullinate its substrate.

Gastric ultrasound, citrulline, and intestinal fatty acid-binding protein as markers of gastrointestinal dysfunction in critically ill patients: A pilot prospective cohort study.

BACKGROUND: Gastrointestinal (GI) dysfunction is common in the intensive care unit (ICU), although there is no consensus on biomarkers of GI dysfunction. We aimed to evaluate ultrasound-based gastric antrum measurements and serum intestinal fatty acid-binding protein (IFABP) and citrulline levels in relation to GI dysfunction in critically ill patients. METHODS: Adult critically ill patients receiving enteral nutrition and stayed for in the ICU for ≥48 h was included. GI dysfunction was described using Gastrointestinal Dysfunction Score (GIDS). Gastric antrum measurements, including craniocaudal (CC) diameter, anteroposterior diameter, and antral-cross sectional area (CSA), as well as serum levels for IFABP and citrulline, were prospectively recorded at baseline and on day 3 and day 5 of enteral nutrition. The receiver operating characteristic (ROC) analysis was performed to evaluate gastric ultrasound parameters, serum IFABP, and citrulline concentrations in predicting GI dysfunction. RESULTS: Thirty-nine participants with a median age of 60 years were recruited and 46.2% of participants had GI dysfunction. ROC analysis revealed that the cutoff value of CSA score to predict GI dysfunction was 4.48 cm2 , which provided 72.7% sensitivity and 77.2% specificity (area under the curve = 0.768, 95% CI: 0.555-0.980). At baseline, gastric residual volume was highly correlated with CC diameter and CSA (r = 0.764, P < 0.001 and r = 0.675, P < 0.001, respectively). Serum IFABP and citrulline levels had no correlation with GI dysfunction or gastric ultrasound parameters (P > 0.05). CONCLUSION: CSA was associated with GI dysfunction in critically ill patients. Serum IFABP and citrulline concentrations were poor in predicting GI dysfunction.

Characterization of a recombinant arginine deiminase from Halothermothrix orenii and its application in citrulline production.

In recent years, arginine deiminase (ADI, EC 3.5.3.6) has attracted much attention as a biocatalyst that produces the functional amino acid l-citrulline from l-arginine and also as an anticancer enzyme. Here, we identified and characterized a putative ADI from the thermophilic bacterium Halothermothrix orenii. The H. orenii ADI (H-ADI) protein was expressed in Escherichia coli BL21(DE3) with a specific activity of 91.8 U/mg protein at 55°C and pH 6.5. The enzyme remained at 74% relative activity after incubation at 45°C for 180 min, only 25% at 50°C. The melting temperature was 56°C. H-ADI is not a metal-requiring enzyme; Ni2+ slightly improved the catalytic activity. The Km and Vmax for l-arginine were 55.5 mM and 156.8 µmol/min/mg protein, respectively. Moreover, three residues (Arg183, Arg237, and His273) were key to the formation of l-citrulline, as analyzed by alanine-scanning mutagenesis. Finally, the enzymatic synthesis of l-citrulline was carried out at 50°C with a conversion ratio reaching 99.03%. Together, these findings show that H-ADI is a promising biocatalyst for the production of l-citrulline.

Enhancing the Safety and Efficacy of PSMA-Based Small-Molecule Drug Conjugates by Linker Stabilization and Conjugation to Transthyretin Binding Ligand.

This work describes the enhancement of a novel antitumor therapeutic platform that combines advantages from small-molecule drug conjugates (SMDCs) and antibody drug conjugates (ADCs). Valine-citrulline (VCit) dipeptide linkers are commonly used cathepsin B cleavable linkers for ADCs. However, the instability of these linkers in mouse serum makes translating efficacy data from mouse to human more challenging. Replacing the VCit linker with glutamic acid-valine-citrulline (EVCit) has been reported to enhance the stability of ADCs in mouse serum. However, the effect of EVCit linker on the stability of SMDCs has not been reported. Here, we report that incorporating the EVCit linker in prostate-specific membrane antigen-targeting SMDCs, equipped with the transthyretin ligand AG10, resulted in conjugates with lower toxicity, an extended half-life, and superior therapeutic efficacy to docetaxel in a xenograft mouse model of prostate cancer. This should make SMDCs' preclinical toxicity and efficacy data from mice more reliable for predicting human results.

Preface to Nitric Oxide Modulators in Health and Disease I.

Nitric oxide (NO) is a small free radical molecule biosynthesized by nitric oxide synthases (NOS), a family of oxidoreductases responsible for the conversion of the natural substrate L-arginine into L-citrulline and NO [...].

Boron-Assisted Selective Citrulline Modification under Mild Conditions.

Protein citrullination is one type of protein post-translational modification. Previous methods entail the use of a strongly acidic condition (pH <1), which impedes its exploration under physiological and pathological conditions. Here, we developed a biocompatible method based on o-boron-assisted citrulline modification. We demonstrated that this method enables selective and mainly irreversible modification of citrulline residues under neutral conditions. We expect that it will provide a valuable tool for the study of protein citrullination.

Stepwise Collision Energy-Resolved Tandem Mass Spectrometric Experiments for the Improved Identification of Citrullinated Peptides.

The use of tandem mass spectrometry (MS/MS) is a fundamental prerequisite of reliable protein identification and quantification in mass-spectrometry-based proteomics. In bottom-up and middle-down proteomics, proteins are identified by the characteristic fragments of their constituting peptides. Post-translational modifications (PTMs) often further complicate proteome analyses. Citrullination is an increasingly studied PTM converting arginines to citrullines (Cit, X) and is implicated in several autoimmune and neurological diseases as well as different types of cancer. Confirmation of citrullination is known to be very challenging since it results in the same molecular mass change as Asn/Gln deamidation. In this study, we explore which MS/MS characteristics can be used for the reliable identification of citrullination. We synthesized several peptides incorporating Cit residues that model enzymatic cleavages of different proteins with verified or putative citrullination. Collision-induced dissociation was used to investigate the energy dependence of Byonic and Mascot scores and confirmed sequence coverage (CSC) along with the neutral loss of HNCO characteristic to citrulline side chains. We found that although the recommended values (19-45 V) for ramped collision energy settings cover the optimal Mascot, Byonic, or %CSC scores effectively, the diagnostic HNCO loss from precursors and fragments may reach their maximum intensities at lower and higher collision energies, respectively. Therefore, we suggest broadening the ramp range to ∼5-60 V to obtain more favorable identification rates for citrullinated peptides. We also found that Byonic was more successful in correctly identifying citrullinated peptides with deamidated residues than Mascot.

Nitric Oxide Synthase activity in major depressive episodes before and after antidepressant treatment: Results of a large case-control treatment study.

BACKGROUND: Nitric oxide synthase (NOS) activity, an enzyme potentially involved in the major depressive episodes (MDE), could be indirectly measured by the L-Citrulline/L-Arginine ratio (L-Cit/L-Arg). The aim of this study was: (1) to compare the NOS activity of patients with a MDE to that of healthy controls (HC); (2) to assess its change after antidepressant treatment. METHODS: A total of 460 patients with a current MDE in a context of major depressive disorder (MDD) were compared to 895 HC for NOS activity (L-Cit/L-Arg plasma ratio). L-Arg and L-Cit plasma levels were measured using a MS-based liquid chromatography method. Depressed patients were assessed at baseline, and after 3 and 6 months of antidepressant treatment for depression severity and clinical response. RESULTS: Depressed patients had a lower NOS activity than HC at baseline [0.31 ± 0.09 v. 0.38 ± 0.12; 95% confidence interval (CI) -0.084 to -0.062, p < 0.0001]. Lower NOS activity at baseline predicted a higher response rate [odds ratio (OR) = 29.20; 95% CI 1.58-536.37; p = 0.023]. NOS activity in depressed patients increased significantly up to 0.34 ± 0.08 after antidepressant treatment (Est = 0.0034; 95% CI 0.0002-0.0067; p = 0.03). CONCLUSIONS: Depressed patients have a decreased NOS activity that improves after antidepressant treatment and predicts drug response. NOS activity may be a promising biomarker for MDE in a context of MDD.

Citrullinated myelin induces microglial TNFα and inhibits endogenous repair in the cuprizone model of demyelination.

BACKGROUND: Microglia are the primary phagocytes of the central nervous system and are responsible for removing damaged myelin following demyelination. Previous investigations exploring the consequences of myelin phagocytosis on microglial activation overlooked the biochemical modifications present on myelin debris. Such modifications, including citrullination, are increased within the inflammatory environment of multiple sclerosis lesions. METHODS: Mouse cortical myelin isolated by ultracentrifugation was citrullinated ex vivo by incubation with the calcium-dependent peptidyl arginine deiminase PAD2. Demyelination was induced by 6 weeks of cuprizone (0.3%) treatment and spontaneous repair was initiated by reversion to normal chow. Citrullinated or unmodified myelin was injected into the primary motor cortex above the cingulum bundle at the time of reversion to normal chow and the consequent impact on remyelination was assessed by measuring the surface area of myelin basic protein-positive fibers in the cortex 3 weeks later. Microglial responses to myelin were characterized by measuring cytokine release, assessing flow cytometric markers of microglial activation, and RNAseq profiling of transcriptional changes. RESULTS: Citrullinated myelin induced a unique microglial response marked by increased tumor necrosis factor α (TNFα) production both in vitro and in vivo. This response was not induced by unmodified myelin. Injection of citrullinated myelin but not unmodified myelin into the cortex of cuprizone-demyelinated mice significantly inhibited spontaneous remyelination. Antibody-mediated neutralization of TNFα blocked this effect and restored remyelination to normal levels. CONCLUSIONS: These findings highlight the role of post-translation modifications such as citrullination in the determination of microglial activation in response to myelin during demyelination. The inhibition of endogenous repair induced by citrullinated myelin and the reversal of this effect by neutralization of TNFα may have implications for therapeutic approaches to patients with inflammatory demyelinating disorders.

Improving the flame retardancy and accelerating the degradation of poly (lactic acid) in soil by introducing fully bio-based additives.

In this study, a novel bio-based flame retardant LC-PA is prepared by the Mannich reaction between phytic acid (PA) and L-citrulline (LC). LC-PA is combined with tannic acid (TA) and introduced into PLA to improve fire performance and accelerate biodegradability. Compared with control PLA, the PLA composite containing 10% LC-PA/TA increases the LOI value to 26.9%, reaches a V-0 rating in the UL-94 test, and reduces the peak heat release rate and total heat release by 24.5% and 21.1%, respectively. More importantly, the introduction of LC-PA/TA accelerates the degradation rate of PLA in soil, which is of significance for biodegradable materials. The addition of LC-PA/TA can attract water and provide a suitable energy source for microbial proliferation, accelerating the hydrolysis and microbial degradation of PLA. This work provides a practical approach for high flame retardancy and rapid biodegradability in the soil to the bio-based polymer.

A Conserved Nonribosomal Peptide Synthetase in Xenorhabdus bovienii Produces Citrulline-Functionalized Lipopeptides.

The entomopathogenic bacterium Xenorhabdus bovienii exists in a mutualistic relationship with nematodes of the genus Steinernema. Free-living infective juveniles of Steinernema prey on insect larvae and regurgitate X. bovienii within the hemocoel of a host larva. X. bovienii subsequently produces a complex array of specialized metabolites and effector proteins that kill the insect and regulate various aspects of the trilateral symbiosis. While Xenorhabdus species are rich producers of secondary metabolites, many of their biosynthetic gene clusters remain uncharacterized. Here, we describe a nonribosomal peptide synthetase (NRPS) identified through comparative genomics analysis that is widely conserved in Xenorhabdus species. Heterologous expression of this NRPS gene from X. bovienii in E. coli led to the discovery of a family of lipo-tripeptides that chromatographically appear as pairs, containing either a C-terminal carboxylic acid or carboxamide. Coexpression of the NRPS with the leupeptin protease inhibitor pathway enhanced production, facilitating isolation and characterization efforts. The new lipo-tripeptides were also detected in wild-type X. bovienii cultures. These metabolites, termed bovienimides, share an uncommon C-terminal d-citrulline residue. The NRPS lacked a dedicated chain termination domain, resulting in product diversification and release from the assembly line through reactions with ammonia, water, or exogenous alcohols.

A Streamlined Data Analysis Pipeline for the Identification of Sites of Citrullination.

Citrullination is an enzyme-catalyzed post-translational modification (PTM) that is essential for a host of biological processes, including gene regulation, programmed cell death, and organ development. While this PTM is required for normal cellular functions, aberrant citrullination is a hallmark of autoimmune disorders as well as cancer. Although aberrant citrullination is linked to human pathology, the exact role of citrullination in disease remains poorly characterized, in part because of the challenges associated with identifying the specific arginine residues that are citrullinated. Tandem mass spectrometry is the most precise method for uncovering sites of citrullination; however, due to the small mass shift (+0.984 Da) that results from citrullination, current database search algorithms commonly misannotate spectra, leading to a high number of false-positive assignments. To address this challenge, we developed an automated workflow to rigorously and rapidly mine proteomic data to unambiguously identify the sites of citrullination from complex peptide mixtures. The crux of this streamlined workflow is the ionFinder software program, which classifies citrullination sites with high confidence on the basis of the presence of diagnostic fragment ions. These diagnostic ions include the neutral loss of isocyanic acid, which is a dissociative event that is unique to citrulline residues. Using the ionFinder program, we have mapped the sites of autocitrullination on purified protein arginine deiminases (PAD1-4) and mapped the global citrullinome in a PAD2-overexpressing cell line. The ionFinder algorithm is a highly versatile, user-friendly, and open-source program that is agnostic to the type of instrument and mode of fragmentation that are used.

Citrullinated Antigens with Multiple Citrulline Similar Motif in the Diagnosis of Rheumatoid Arthritis: A Preliminary Single-Center Study.

The presence of anti-citrullinated protein antibodies (ACPAs) in the serum is one of the immunological features of rheumatoid arthritis (RA). Anti-cyclic citrullinated peptide (CCP) assay has been widely used in clinic for the diagnosis of RA. However, up to 40% of RA patients are anti-CCP negative and the diagnostic sensitivity in this population needs to be improved for better clinical management. In this study, peptides with Multiple Citrulline Similar Motif (MCSM) were synthesized and a new ELISA system, which we called RA_CP, was developed to detect citrullinated antigens with MCSM present in the serum. 106 RA,48 other arthritis patients and 41 sex- and age-matched healthy controls (HCs) were included in this study. Patients with RA have a significantly higher amount of citrullinated antigens with MCSM than other arthritis patients and HCs. RA patients with positive anti-CCP are also MCSM positive, whereas 75% anti-CCP negative patients are positive for MCSM. The diagnostic sensitivity for anti-CCP and MCSM was 81.1% and 95.3%, while the specificity was 100% and 94.4%, respectively. ROC curve analyses showed that the area under the curve (AUC) values were 0.906 (95% CI: 0.860-0.951) for anti-CCP and 0.948 (95% CI: 0.912-0.985) for MCSM while the combination of MCSM and anti-CCP test has the highest AUC (0.971, 95% CI: 0.946-0.996). Our results suggest that detection of citrullinated antigens with MCSM has improved sensitivity compared with anti-CCP assay and could serve as a biomarker in diagnosis of RA patients.

A fructosylated peptide derived from a collagen II T cell epitope for long-term treatment of arthritis (FIA-CIA) in mice.

Rheumatoid arthritis (RA) is a systemic inflammatory autoimmune disease which affects primarily the joints. Peptides of several proteins have shown an effect in some experimental animal models of RA. We investigated arthritis development in male DBA/1 mice which were injected with bovine collagen II (bCII) and human fibrinogen (hFib) on days 0 and 21, leading to stable and reproducible disease induction in 100% of immunized mice (FIA-CIA). In a second study, two bCII-derived peptides were given three times in the course of 6 weeks after FIA-CIA induction to test for impact on arthritis. Mice were scored weekly for arthritis and anti-citrullinated peptide antibodies (ACPAs) were determined in the sera taken on days 0, 14, 35, 56 and 84. Histology of the hind paws was performed at the end of the experiment. Intravenous administration of peptide 90578, a novel fructosylated peptide derived from the immunodominant T cell epitope of bCII, at a dosage of 1 mg/kg resulted in significant beneficial effects on clinical outcome parameters and on the arthritis histology scores which was sustained over 12 weeks. Survival tended to be improved in peptide 90578-treated mice. Intravenous administration of pure soluble peptide 90578 without adjuvants is a promising approach to treat RA, with treatment starting at a time when ACPAs are already present. The results complement existing data on peptide "vaccination" of healthy animals, or on treatment using recombinant peptide expressing virus or complex biological compounds.

Progesterone stimulates histone citrullination to increase IGFBP1 expression in uterine cells.

Peptidylarginine deiminases (PAD) enzymes were initially characterized in uteri, but since then little research has examined their function in this tissue. PADs post-translationally convert arginine residues in target proteins to citrulline and are highly expressed in ovine caruncle epithelia and ovine uterine luminal epithelial (OLE)-derived cell line. Progesterone (P4) not only maintains the uterine epithelia but also regulates the expression of endometrial genes that code for proteins that comprise the histotroph and are critical during early pregnancy. Given this, we tested whether P4 stimulates PAD-catalyzed histone citrullination to epigenetically regulate expression of the histotroph gene insulin-like growth factor binding protein 1 (IGFBP1) in OLE cells. 100 nM P4 significantly increases IGFBP1 mRNA expression; however, this increase is attenuated by pre-treating OLE cells with 100 nM progesterone receptor antagonist RU486 or 2 µM of a pan-PAD inhibitor. P4 treatment of OLE cells also stimulates citrullination of histone H3 arginine residues 2, 8, and 17 leading to enrichment of the ovine IGFBP1 gene promoter. Since PAD2 nuclear translocation and catalytic activity require calcium, we next investigated whether P4 triggers calcium influx in OLE cells. OLE cells were pre-treated with 10 nM nicardipine, an L-type calcium channel blocker, followed by stimulation with P4. Using fura2-AM imaging, we found that P4 initiates a rapid calcium influx through L-type calcium channels in OLE cells. Furthermore, this influx is necessary for PAD2 nuclear translocation and resulting citrullination of histone H3 arginine residues 2, 8, and 17. Our work suggests that P4 stimulates rapid calcium influx through L-type calcium channels initiating PAD-catalyzed histone citrullination and an increase in IGFBP1 expression.

GC-MS Discrimination of Citrulline from Ornithine and Homocitrulline from Lysine by Chemical Derivatization: Evidence of Formation of N5-Carboxy-ornithine and N6-Carboxy-lysine.

Derivatization of amino acids by 2 M HCl/CH3OH (60 min, 80 °C) followed by derivatization of the intermediate methyl esters with pentafluoropropionic anhydride (PFPA) in ethyl acetate (30 min, 65 °C) is a useful two-step derivatization procedure (procedure A) for their quantitative measurement in biological samples by gas chromatography-mass spectrometry (GC-MS) as methyl ester pentafluoropropionic (PFP) derivatives, (Me)m-(PFP)n. This procedure allows in situ preparation of trideutero-methyl esters PFP derivatives, (d3Me)m-(PFP)n, from synthetic amino acids and 2 M HCl/CD3OD for use as internal standards. However, procedure A converts citrulline (Cit) to ornithine (Orn) and homocitrulline (hCit) to lysine (Lys) due to the instability of their carbamide groups under the acidic conditions of the esterification step. In the present study, we investigated whether reversing the order of the two-step derivatization may allow discrimination and simultaneous analysis of these amino acids. Pentafluoropropionylation (30 min, 65 °C) and subsequent methyl esterification (30 min, 80 °C), i.e., procedure B, of Cit resulted in the formation of six open and cyclic reaction products. The most abundant product is likely to be N5-Carboxy-Orn. The second most abundant product was confirmed to be Orn. The most abundant reaction product of hCit was confirmed to be Lys, with the minor reaction product likely being N6-Carboxy-Lys. Mechanisms are proposed for the formation of the reaction products of Cit and hCit via procedure B. It is assumed that at the first derivatization step, amino acids form (N,O)-PFP derivatives including mixed anhydrides. At the second derivatization step, the Cit-(PFP)4 and hCit-(PFP)4 are esterified on their C1-Carboxylic groups and on their activated Nureido groups. Procedure B also allows in situ preparation of (d3Me)m-(PFP)n from synthetic amino acids for use as internal standards. It is demonstrated that the derivatization procedure B enables discrimination between Cit and Orn, and between hCit and Lys. The utility of procedure B to measure simultaneously these amino acids in biological samples such as plasma and urine remains to be demonstrated. Further work is required to optimize the derivatization conditions of procedure B for biological amino acids.