Unraveling Hepcidin Plasma Protein Binding: Evidence from Peritoneal Equilibration Testing

Peptide hormone hepcidin regulates systemic iron metabolism and has been described to be partially bound to α2-macroglobulin and albumin in blood. However, the reported degree of hepcidin protein binding varies between <3% and ≈89%. Since protein-binding may influence hormone function and quantification, better insight into the degree of hepcidin protein binding is essential to fully understand the biological behavior of hepcidin and interpretation of its measurement in patients. Here, we used peritoneal dialysis to assess human hepcidin protein binding in a functional human setting for the first time. We measured freely circulating solutes in blood and peritoneal fluid of 14 patients with end-stage renal disease undergoing a peritoneal equilibration test to establish a curve describing the relation between molecular weight and peritoneal clearance. Calculated binding percentages of total cortisol and testosterone confirmed our model. The protein-bound fraction of hepcidin was calculated to be 40% (±23%). We, therefore, conclude that a substantial proportion of hepcidin is freely circulating. Although a large inter-individual variation in hepcidin clearance, besides patient-specific peritoneal transport characteristics, may have affected the accuracy of the determined binding percentage, we describe an important step towards unraveling human hepcidin plasma protein binding in vivo including the caveats that need further research

Intra-individual variability of serum hepcidin-25 in haemodialysis patients using mass spectrometry and ELISA

Background. Measurement of serum hepcidin levels may provide a useful alternative to the current methods of determining iron status in chronic haemodialysis (HD) patients. However, the biological variability of this pivotal regulator of iron homeostasis is unclear, and the impact of inflammation, dialysis clearance and iron therapy on hepcidin variability has not been established.Methods. Two independent studies in chronic HD patients were conducted; serum hepcidin levels were measured at the start of dialysis sessions in 20 UK patients and in 43 Dutch patients by mass spectrometry (MS). Samples from UK patients were also analysed by a competitive enzyme-linked immunosorbent assay (cELISA). Coefficient of variance (CV1) was calculated and potential factors affecting CV1 were also examined.Results. The median CV1 (inter-quartile range) was 23% (17-28) for the UK MS, 26% (17-48) for the Dutch MS and 23% (17-39) for the UK cELISA. The CV1 was similar in those patients receiving and those not receiving regular intravenous iron. The CV1 was not associated with the degree of inflammation. Hepcidin levels were higher following an inter-dialytic period of 3 versus 2 days (P = 0.02).Conclusions. These findings suggest considerable variability of serum hepcidin levels in HD patients. Inflammation and the use of iron did not impact on the degree of variability, and hepcidin levels were higher after an inter-dialytic period of 3 versus 2 days. These findings need to be taken into account in future studies assessing the utility of serum hepcidin as a guide to the use of iron or erythropoiesis-stimulating agents therapy.</p