The (Bio)Chemistry of Non-Transferrin-Bound Iron

Abstract

In healthy individuals, virtually all blood plasma iron is bound by transferrin. However, in several diseases and clinical conditions, hazardous non-transferrin-bound iron (NTBI) species occur. NTBI represents a potentially toxic iron form, being a direct cause of oxidative stress in the circulating compartment and tissue iron loading. The accumulation of these species can cause cellular damage in several organs, namely, the liver, spleen, and heart. Despite its pathophysiological relevance, the chemical nature of NTBI remains elusive. This has precluded its use as a clinical biochemical marker and the development of targeted therapies. Herein, we make a critical assessment of the current knowledge of NTBI speciation. The currently accepted hypotheses suggest that NTBI is mostly iron bound to citric acid and iron bound to serum albumin, but the chemistry of this system remains fuzzy. We explore the complex chemistry of iron complexation by citric acid and its implications towards NTBI reactivity. Further, the ability of albumin to bind iron is revised and the role of protein post-translational modifications on iron binding is discussed. The characterization of the NTBI species structure may be the starting point for the development of a standardized analytical assay, the better understanding of these species' reactivity or the identification of NTBI uptake mechanisms by different cell types, and finally, to the development of new therapies.

Keywords: LPI; NTBI; albumin; citrate; iron overload; labile plasma pool; non-transferrin-bound iron.

Figure 1

Schematic representation of the structure of citrate and ferric citrate complexes in aqueous solution. (A) Citrate anion. (B) [FeCit₂]⁵⁻. (C) [Fe₂Cit₂]²⁻. (D) [Fe₃Cit₃]³⁻. (E) alternative structure for [Fe₃Cit₃]³⁻. Structures C, D and E are adapted from Gautier-Luneau et al. [71], structure E has been proposed by Fukushima et al. [75].

Figure 2

Human serum albumin structure (PDB: 1BM0 [94]). (A) Known metal ion binding sites in HSA: NTS (N-terminal site), MBS (Metal Binding Site A) and Cys34 (reduced cysteine at position 34). (B) Surface map from HSA, highlighting sidechain oxygen atoms from aspartate and glutamate residues (red) and sidechain nitrogen atoms from arginine and lysine residues (blue). The 4 N-terminal amino acid residues in the protein sequence were absent from the crystallographic structure and were manually modelled for this representation.