A Synthetic Peptide with the Putative Iron Binding Motif of Amyloid Precursor Protein (APP) Does Not Catalytically Oxidize Iron

Abstract

The β-amyloid precursor protein (APP), which is a key player in Alzheimer's disease, was recently reported to possess an Fe(II) binding site within its E2 domain which exhibits ferroxidase activity [Duce et al. 2010, Cell 142: 857]. The putative ligands of this site were compared to those in the ferroxidase site of ferritin. The activity was indirectly measured using transferrin, which scavenges the Fe(III) product of the reaction. A 22-residue synthetic peptide, named FD1, with the putative ferroxidase site of APP, and the E2 domain of APP were each reported to exhibit 40% of the ferroxidase activity of APP and of ceruloplasmin. It was also claimed that the ferroxidase activity of APP is inhibited by Zn(II) just as in ferritin. We measured the ferroxidase activity indirectly (i) by the incorporation of the Fe(III) product of the ferroxidase reaction into transferrin and directly (ii) by monitoring consumption of the substrate molecular oxygen. The results with the FD1 peptide were compared to the established ferroxidase activities of human H-chain ferritin and of ceruloplasmin. For FD1 we observed no activity above the background of non-enzymatic Fe(II) oxidation by molecular oxygen. Zn(II) binds to transferrin and diminishes its Fe(III) incorporation capacity and rate but it does not specifically bind to a putative ferroxidase site of FD1. Based on these results, and on comparison of the putative ligands of the ferroxidase site of APP with those of ferritin, we conclude that the previously reported results for ferroxidase activity of FD1 and – by implication – of APP should be re-evaluated.

Decrease in APP and CP mRNA expression supports impairment of iron export in Alzheimer's disease patients

“…In this same study, a REXXE consensus motif found also in ferritin heavy chain (FtH) was described in APP sequence and it was then proposed that APP possessed ferroxidase activity, thus explaining its putative effect on Fpn-mediated iron export in neurons of APP-null mice [20]. However, posterior studies demonstrated that APP does not possess ferroxidase activity [21,22]. Still, it was shown that APP indeed interacted with Fpn, promoting its stabilization at cell surface, thus influencing cellular iron export in the presence of extracellular apo-transferrin [23].…”

The role of iron in brain ageing and neurodegenerative disorders

“…83,84 Microtubule-associated protein tau (MAPT) deficiency induces intracellular iron accumulation, which causes degeneration of dopaminergic neurons, leading to parkinsonism with dementia in mice. 85 Tau deficiency impairs ferroportin iron export by retaining APP in the endoplasmic reticulum so that it can no longer be trafficked to the neuronal surface, where it can couple its ferroxidase activity to ferroportin.…”

Iron transport across the blood–brain barrier: development, neurovascular regulation and cerebral amyloid angiopathy

“…They described APP as being ferroxidase-active and assigned this activity to a 21 amino acid sequence (named FD1 for ferroxidase domain 1) within the E2 domain of the protein [159]. The proposition that APP is ferroxidase active has been refuted [160-162], however the conclusion drawn by Duce et al. regarding APP's ability to modulate iron efflux from neurons remains sound.…”