Sukhvinder S. Bansal

Results of the first international round robin for the quantification of urinary and plasma hepcidin assays: need for standardization

This study indicates that hepcidin levels reported by various methods vary considerably but analytical variance is generally low and similar for all methods. See related perspective article on page 1631. The recently discovered iron regulatory peptide hormone hepcidin holds promise as a novel biomarker in iron metabolism disorders. To date, various mass spectrometry and immunochemical methods have been developed for its quantification in plasma and urine. Differences in methodology and analytical performance hinder the comparability of data. As a first step towards method harmonization, several hepcidin assays were compared. Worldwide eight laboratories participated in a urinary and plasma round robin in which hepcidin was analyzed. For both urine and plasma: (i) the absolute hepcidin concentrations differed widely between methods, (ii) the between-sample variation and the analytical variation of the methods are similar. Importantly, the analytical variation as percentage of the total variance is low for all methods, indicating their suitability to distinguish hepcidin levels of different samples. Spearman correlations between methods were generally high. The round robin results inform the scientific and medical community on the status and agreement of the current hepcidin methods. Ongoing initiatives should facilitate standardization by exchanging calibrators and representative samples.

Current status of the measurement of blood hepcidin levels in chronic kidney disease.

Hepcidin is a small defensin-like peptide produced in the liver in response to anemia, hypoxia, or inflammation. In addition to its anti-microbial properties, it has also been found to be a key regulator of iron utilization, providing increased understanding of why chronic kidney disease patients absorb iron poorly from the gut and also why many hemodialysis patients develop functional iron deficiency in the presence of inflammation. Hepcidin synthesis is upregulated in uremia, as in other inflammatory states. The ability to measure hepcidin in biologic fluids has stimulated interest in the potential applicability of this measurement as a more informative marker of iron status than the traditional iron indices such as serum ferritin and transferrin saturation. Until recently, however, the assays for measuring hepcidin have lacked precision, accuracy, and internal validation. Over the last few years, however, several assays have become available that address these limitations. Broadly speaking, these can be divided into radioimmunoassays, ELISAs, and mass spectrometry methods. The purpose of this review is to outline the various assays available at the present time, to critique their advantages and limitations, and to report comparative data in patients with chronic kidney disease. A concern with the immunoassays is that they detect more than biologically active hepcidin-25. Mass spectrometric assays are specific for hepcidin-25 but are labor intensive and require more costly and sophisticated instrumentation. Thus, although mass spectrometry is more accurate, it is less practical for routine clinical use at the present time.

Quantitation of hepcidin in serum using ultra‐high‐pressure liquid chromatography and a linear ion trap mass spectrometer

Hepcidin is a peptide hormone that functions as a key regulator of mammalian iron metabolism. Biological levels are increased in end-stage renal disease and during inflammation but suppressed in hemochromatosis. Thus hepcidin levels have diagnostic importance. This study describes the development of an analytical method for the quantitative determination of the concentration of hepcidin in clinical samples. The fragmentation of hepcidin was investigated using triple quadrupole and linear ion trap mass spectrometers. A standard quantity of a stable isotopically labelled hepcidin internal standard was added to serum samples. Extraction was performed by protein precipitation and weak cation-exchange magnetic nanoparticles. Chromatography was carried out on sub 2 microm particle stationary phase, using ultra-high-pressure liquid chromatography and a linear ion trap for quantitation. The lower limit of quantitation was 0.4 nmol/L with less than 20% accuracy and precision. The mean hepcidin concentration in sera for controls was 4.6 +/- 2.7 nmol/L, in patients with sickle cell disease, 7.0 +/- 8.9 nmol/L; in patients with end-stage renal disease, 30.5 +/- 15.7 nmol/L; and patients with penetrant hereditary hemochromatosis, 1.4 +/- 0.8 nmol/L.

Quantification of hepcidin using matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

Hepcidin is known to be a key systemic iron-regulatory hormone which has been demonstrated to be associated with a number of iron disorders. Hepcidin concentrations are increased in inflammation and suppressed in hemochromatosis. In view of the role of hepcidin in disease, its potential as a diagnostic tool in a clinical setting is evident. This study describes the development of a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) assay for the quantitative determination of hepcidin concentrations in clinical samples. A stable isotope labeled hepcidin was prepared as an internal standard and a standard quantity was added to urine samples. Extraction was performed with weak cation-exchange magnetic nanoparticles. The basic peptides were eluted from the magnetic nanoparticles using a matrix solution directly onto a target plate and analyzed by MALDI-TOF MS to determine the concentration of hepcidin. The assay was validated in charcoal stripped urine, and good recovery (70-80%) was obtained, as were limit of quantitation data (5 nmol/L), accuracy (RE <10%), precision (CV <21%), within -day repeatability (CV <13%) and between-day repeatability (CV <21%). Urine hepcidin levels were 10 nmol/mmol creatinine in healthy controls, with reduced levels in hereditary hemochromatosis (P < 0.000005) and elevated levels in inflammation (P < 0.0007). In summary a validated method has been developed for the determination of hepcidin concentrations in clinical samples.

Solid phase synthesis of [18F]Labelled peptides for positron emission tomography

A strategy for the solid phase synthesis of [18F]labelled peptides has been developed. The peptides were prepared on solid support and acylated with 4-[18F]fluorobenzoic acid using HATU within 3 min and the labelled peptide was released from the solid support within 7 min. The [18F]labelled peptides were produced in good purity with a specific activity of 20-25 GBq/micromol.

Quantitation of hepcidin in human urine by liquid chromatography–mass spectrometry

Hepcidin is a peptide hormone that functions as a key regulator of mammalian iron metabolism. Serum and urine levels are increased in inflammation and suppressed in hemochromatosis, and they may have diagnostic importance. This study describes the development and validation of an analytical method for the quantitative determination of the concentration of hepcidin in clinical samples. A stable, isotopically labeled internal standard, [15N,13C2]Gly12,20-hepcidin, was synthesized and a standard quantity was added to urine samples. Extraction was performed using weak cation exchange magnetic nanoparticles. An ion trap mass spectrometer was used to quantify hepcidin in the samples. The hepcidin assay was validated, and good recovery of hepcidin was obtained. The assay is accurate and precise. Urinary hepcidin levels of 3 to 9 nmol/mmol creatinine(-1) were found in healthy controls, with reduced levels in hemochromatosis (P<0.00006) and elevated levels in inflammation (P<0.00035). In sickle cell disease, a wide range was found, with the mean value not differing significantly from controls (P<0.26). In summary, a validated method has been developed for the quantitation of hepcidin using a stable, isotopically labeled internal standard and applied to determine the concentrations of hepcidin in the low nanomolar range in urine samples from patients and controls.

Conformational flexibility determines selectivity and antibacterial, antiplasmodial, and anticancer potency of cationic α-helical peptides.

Background: Antimicrobial peptides (AMPs) have the potential to act against multiple pathogenic targets. Results: AMPs that maintain conformational flexibility are more potent against multiple pathogens and less hemolytic. Conclusion: Antimicrobial action and hemolysis proceed via differing mechanisms. Significance: The potency, selectivity, and ability of AMPs to reach intracellular pathogens can be modulated using general principles. We used a combination of fluorescence, circular dichroism (CD), and NMR spectroscopies in conjunction with size exclusion chromatography to help rationalize the relative antibacterial, antiplasmodial, and cytotoxic activities of a series of proline-free and proline-containing model antimicrobial peptides (AMPs) in terms of their structural properties. When compared with proline-free analogs, proline-containing peptides had greater activity against Gram-negative bacteria, two mammalian cancer cell lines, and intraerythrocytic Plasmodium falciparum, which they were capable of killing without causing hemolysis. In contrast, incorporation of proline did not have a consistent effect on peptide activity against Mycobacterium tuberculosis. In membrane-mimicking environments, structures with high α-helix content were adopted by both proline-free and proline-containing peptides. In solution, AMPs generally adopted disordered structures unless their sequences comprised more hydrophobic amino acids or until coordinating phosphate ions were added. Proline-containing peptides resisted ordering induced by either method. The roles of the angle subtended by positively charged amino acids and the positioning of the proline residues were also investigated. Careful positioning of proline residues in AMP sequences is required to enable the peptide to resist ordering and maintain optimal antibacterial activity, whereas varying the angle subtended by positively charged amino acids can attenuate hemolytic potential albeit with a modest reduction in potency. Maintaining conformational flexibility improves AMP potency and selectivity toward bacterial, plasmodial, and cancerous cells while enabling the targeting of intracellular pathogens.

Mechanisms of plasma non-transferrin bound iron generation: insights from comparing transfused diamond blackfan anaemia with sickle cell and thalassaemia patients

In transfusional iron overload, extra‐hepatic iron distribution differs, depending on the underlying condition. Relative mechanisms of plasma non‐transferrin bound iron (NTBI) generation may account for these differences. Markers of iron metabolism (plasma NTBI, labile iron, hepcidin, transferrin, monocyte SLC40A1 [ferroportin]), erythropoiesis (growth differentiation factor 15, soluble transferrin receptor) and tissue hypoxia (erythropoietin) were compared in patients with Thalassaemia Major (TM), Sickle Cell Disease and Diamond‐Blackfan Anaemia (DBA), with matched transfusion histories. The most striking differences between these conditions were relationships of NTBI to erythropoietic markers, leading us to propose three mechanisms of NTBI generation: iron overload (all), ineffective erythropoiesis (predominantly TM) and low transferrin‐iron utilization (DBA).

Toward Worldwide Hepcidin Assay Harmonization: Identification of a Commutable Secondary Reference Material.

BACKGROUND Absolute plasma hepcidin concentrations measured by various procedures differ substantially, complicating interpretation of results and rendering reference intervals method dependent. We investigated the degree of equivalence achievable by harmonization and the identification of a commutable secondary reference material to accomplish this goal. METHODS We applied technical procedures to achieve harmonization developed by the Consortium for Harmonization of Clinical Laboratory Results. Eleven plasma hepcidin measurement procedures (5 mass spectrometry based and 6 immunochemical based) quantified native individual plasma samples (n = 32) and native plasma pools (n = 8) to assess analytical performance and current and achievable equivalence. In addition, 8 types of candidate reference materials (3 concentrations each, n = 24) were assessed for their suitability, most notably in terms of commutability, to serve as secondary reference material. RESULTS Absolute hepcidin values and reproducibility (intrameasurement procedure CVs 2.9%-8.7%) differed substantially between measurement procedures, but all were linear and correlated well. The current equivalence (intermeasurement procedure CV 28.6%) between the methods was mainly attributable to differences in calibration and could thus be improved by harmonization with a common calibrator. Linear regression analysis and standardized residuals showed that a candidate reference material consisting of native lyophilized plasma with cryolyoprotectant was commutable for all measurement procedures. Mathematically simulated harmonization with this calibrator resulted in a maximum achievable equivalence of 7.7%. CONCLUSIONS The secondary reference material identified in this study has the potential to substantially improve equivalence between hepcidin measurement procedures and contributes to the establishment of a traceability chain that will ultimately allow standardization of hepcidin measurement results.

L-(6,7-dimethoxy-4-coumaryl) alanine: an intrinsic probe for the labelling of peptides

The asymmetric synthesis, spectral properties and incorporation into solid phase peptide synthesis are described for L-(6,7-dimethoxy-4-coumaryl)alanine (Dca). Dca has great utility as a specific and highly sensitive intrinsic probe for fluorescence labelling and quantitation of peptides and proteins.