Iron uptake and transfer from ceruloplasmin to transferrin

Abstract

BackgroundDietary and recycled iron are in the Fe2+ oxidation state. However, the metal is transported in serum by transferrin as Fe3+. The multi-copper ferroxidase ceruloplasmin is suspected to be the missing link between acquired Fe2+ and transported Fe3+. MethodsThis study uses the techniques of chemical relaxation and spectrophotometric detection. ResultsUnder anaerobic conditions, ceruloplasmin captures and oxidizes two Fe2+. The first uptake occurs in domain 6 (<1ms) at the divalent iron-binding site. It is accompanied by Fe2+ oxidation by Cu2+D6. Fe3+ is then transferred from the binding site to the holding site. Cu+D6 is then re-oxidized by a Cu2+ of the trinuclear cluster in about 200ms. The second Fe2+ uptake and oxidation involve domain 4 and are under the kinetic control of a 200s change in the protein conformation. With transferrin and in the formed ceruloplasmin–transferrin adduct, two Fe3+ are transferred from their holding sites to two C-lobes of two transferrins. The first transfer (~100s) is followed by conformation changes (500s) leading to the release of monoferric transferrin. The second transfer occurs in two steps in the 1000–10,000second range. ConclusionFe3+ is transferred after Fe2+ uptake and oxidation by ceruloplasmin to the C-lobe of transferrin in a protein–protein adduct. This adduct is in a permanent state of equilibrium with all the metal-free or bounded ceruloplasmin and transferrin species present in the medium. General significanceCeruloplasmin is a go-between dietary or recycled Fe2+ and transferrin transported Fe3+.