Unraveling Hepcidin Plasma Protein Binding: Evidence from Peritoneal Equilibration Testing

Abstract

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.

Hepcidin-Ferroportin Interaction Controls Systemic Iron Homeostasis

“…One is efficient filtration of hepcidin as a small peptide through the renal glomerular membrane [ 27 ], followed by reuptake and degradation of filtered hepcidin in the proximal tubule, utilizing a generic mechanism for recycling the amino acid content of filtered proteins. Efficient filtration of hepcidin in the glomerulus is facilitated by the relatively low fraction of hepcidin that is bound by plasma protein, estimated at about 40% [ 28 ]. A small percentage of filtered hepcidin survives the uptake and degradation in the proximal tubule and can be detected in urine, similarly to other peptide hormones.…”

Iron and Chronic Kidney Disease: Still a Challenge

“…PD is a continuous technique with diffusive and convective transport of small-and middle-molecular solutes; however, individual properties of peritoneal membrane may influence efficacy of hepcidin removal. So far, peritoneal hepcidin clearance using peritoneal equilibration test was evaluated only in one small study (36). The efficacy of PD removal should be confirmed in further studies.…”

Hepcidin is described as the master regulator of iron: could its removal by CRRT lead to iron dysmetabolism in the critically ill?

“…Fifty percent of critically ill patients develop AKI and 25% require RRT [ 3 ]. Hepcidin and pro-hepcidin have molecular weights of 2700 Da and 10,000 Da, respectively, and, therefore, may be removed by continuous RRT (CRRT), which uses membranes with a cut-off of 35,000 Da [ 4 ]. The protein-bound fraction of hepcidin is about 40% [ 4 ] which does not impede its elimination by convection or diffusion [ 5 ].…”

“…Hepcidin and pro-hepcidin have molecular weights of 2700 Da and 10,000 Da, respectively, and, therefore, may be removed by continuous RRT (CRRT), which uses membranes with a cut-off of 35,000 Da [ 4 ]. The protein-bound fraction of hepcidin is about 40% [ 4 ] which does not impede its elimination by convection or diffusion [ 5 ]. It has been demonstrated that maintenance dialysis with both super-flux polysulphone (PS) and acrylonitrile 69 (AN69) membranes similarly removed hepcidin [ 5 ].…”