Receptor-modulated iron release from transferrin: differential effects on N- and C-terminal sites

Abstract

Iron release to PPi from N- and C-terminal monoferric transferrins and their complexes with transferrin receptor has been studied at pH 7.4 and 5.6 in 0.05 M HEPES or MES/0.1 M NaCl/0.01 M CHAPS at 25 degrees C. The two sites exhibit kinetic heterogeneity in releasing iron. The N-terminal form is slightly less labile than its C-terminal counterpart at pH 7.4, but much more facile in releasing iron at pH 5.6. At pH 7.4, iron removal by 0.05 M pyrophosphate from each form of monoferric transferrin complexed to the receptor is considerably slower than from the corresponding free monoferric transferrin. However, at pH 5.6, complexation of transferrin to its receptor affects the two forms differently. The rate of iron release to 0.005 M pyrophosphate by the N-terminal species is substantially the same whether transferrin is free or bound to the receptor. In contrast, the C-terminal form releases iron much faster when complexed to the receptor than when free. Urea/PAGE analysis of iron removal from free and receptor-complexed diferric transferrin at pH 5.6 reveals that its C-terminal site is also more labile in the complex, but its N-terminal site is more labile in free diferric transferrin. Thus, the newly discovered role of transferrin receptor in modulating iron release from transferrin predominantly involves the C-terminal site. This observation helps explain the prevalence of circulating N-terminal monoferric transferrin in the human circulation.

The Crystal Structure of Iron-free Human Serum Transferrin Provides Insight into Inter-lobe Communication and Receptor Binding

“…However, this helix undergoes no discernable movement between the apo, monoferric, or diferric states of TF (using pig serum TF as a model for the diferric form). The absence of a significant change in the position of this helix between the conformations may simply reflect the absence of the TFR, which we believe may be critical in inducing such a change (see below) (69,70).…”

Hepatic iron overload and hepatocellular carcinoma

“…It is characterized by increased absorption of dietary iron that results in progressive deposition of the metal in multiple organs, but particularly in the liver [11]. The inherited predisposition to excessive absorption of the metal is the consequence of mutations in a number of genes concerned with iron absorption [7,[9][10][11][12][13]. HCC complicating HH is most commonly attributed (in 70-95% of patients) to homozygosity for the C282Y mutation of the HFE gene [7,[9][10][11][12][13].…”

“…The inherited predisposition to excessive absorption of the metal is the consequence of mutations in a number of genes concerned with iron absorption [7,[9][10][11][12][13]. HCC complicating HH is most commonly attributed (in 70-95% of patients) to homozygosity for the C282Y mutation of the HFE gene [7,[9][10][11][12][13]. This mutation impairs the expression of HFE protein on cell membranes, thereby compromising iron sensing by hepatocytes and the transcription and release of hepcidin [14].…”

“…attributable to the iron molecule per se, by virtue of its capacity to generate oxidative stress and to form mutagenic hydroxyl radicals and suppress host defense, or whether this effect occurs indirectly through the induction of chronic inflammation leading to cirrhosis and hence to HCC [10,[21][22][23]. Oxidative stress leads to lipid peroxidation of unsaturated fatty acids in membranes of cells and organelles.…”

“…No biological mechanisms exist for the excretion of iron in excess of physiological requirements [9], and sloughing of intestinal mucosal cells and menstrual blood loss are normally the main processes by which the metal is lost from the body [1][2][3]. Body iron stores accumulate insidiously with aging [9,10].…”