The Role of Hepcidin in Iron Metabolism. Part 2

Structurally, the hepcidin peptide resembles a bent hairpin held together by four disulfide bonds. The disulfide connectivity was recently revised. NMR spectroscopy, partial reductive alkylation and Fourier transform mass spectroscopy were used to resolve ambiguities arising from the proximity of the four disulfides. The new model indicates that two bonds stabilize the antiparallel β-sheet, and two tether the bent conformation of the peptide. Our recent data indicate that hepcidin binding to its receptor requires the involvement of one of the disulfide bonds. However, considering that removal of individual bonds does not dramatically decrease hepcidin activity in vitro, multiple disulfide bonds must be capable of forming a contact with ferroportin.

The disulfide bonding pattern is strictly conserved across species that produce hepcidin – fish, amphibians, reptiles and mammals. Moreover, hepcidin from one species can bind to the receptor from an evolutionarily distant species, e.g. human and zebrafish hepcidin were active against mouse ferroportin. Apart from the disulfide bonds, structure-function studies also revealed that the N-terminus of hepcidin is important for its iron-regulatory activity. The N-terminally truncated human 20-aa peptide was inactive both in vitro and in vivo indicating that this region may also contain contact residues for hepcidin interaction with its receptor.

The amphipathic structure of hepcidin and its extensive disulfide bonding are common characteristics of antimicrobial and antifungal peptides. However, hepcidin has only displayed modest antimicrobial properties in vitro at very high concentrations (10–30 μM), and the significance of its antimicrobial properties in vivo remains to be determined. Patients with hereditary hemochromatosis, a disease generally resulting from relative hepcidin deficiency, are reported to develop infections caused by unusual microorganisms (Vibrio, Yersinia and Listeria), but this susceptibility could be related to the bacteria benefitting from increased iron levels rather than from the loss of any direct antibacterial effect of hepcidin.

Hepcidin is the main regulator of plasma iron concentrations. Injection of hepcidin into mice resulted in a dramatic drop in serum iron within just 1 h. Even though hepcidin is rapidly cleared from the plasma, the effect of a single dose was apparent for up to 72 h, likely because of the time required to resynthesize sufficient amounts of the hepcidin receptor, ferroportin.

Chronic overexpression of hepcidin causes iron-restricted anemia in mice and humans, typically manifested as microcytic, hypochromic anemia. Conversely, hepcidin deficiency in mice and humans results in iron overload with iron deposition in the liver and other parenchyma, and sparing of the macrophage-rich spleen. Complete absence of hepcidin in humans causes juvenile hemochromatosis, the most severe form of hereditary hemochromatosis.

The phenotypes of hepcidin excess and deficiency indicate that hepcidin inhibits intestinal iron uptake and the release of iron from macrophages recycling old red blood cells. When hepcidin was overexpressed during embryonic