A study of the anion binding site of transferrin

Phosphate inhibits in vitro Fe3+ loading into transferrin by forming a soluble Fe(III)-phosphate complex: a potential non-transferrin bound iron species

In chronic kidney diseases, NTBI can occur even when total iron levels in serum are low and transferrin is not saturated. We postulated that elevated serum phosphate concentrations, present in CKD patients, might disrupt Fe(3+) loading into apo-transferrin by forming Fe(III)-phosphate species. We report that phosphate competes with apo-transferrin for Fe(3+) by forming a soluble Fe(III)-phosphate complex. Once formed, the Fe(III)-phosphate complex is not a substrate for donating Fe(3+) to apo-transferrin. Phosphate (1-10mM) does not chelate Fe(III) from diferric transferrin under the conditions examined. Complexed forms of Fe(3+), such as iron nitrilotriacetic acid (Fe(3+)-NTA), and Fe(III)-citrate are not susceptible to this phosphate complexation reaction and efficiently deliver Fe(3+) to apo-transferrin in the presence of phosphate. This reaction suggests that citrate might play an important role in protecting against Fe(III), phosphate interactions in vivo. In contrast to the reactions of Fe(3+) and phosphate, the addition of Fe(2+) to a solution of apo-transferrin and phosphate lead to rapid oxidation and deposition of Fe(3+) into apo-transferrin. These in vitro data suggest that, in principle, elevated phosphate concentrations can influence the ability of apo-transferrin to bind iron, depending on the oxidation state of the iron.

Iron chelators for the treatment of iron overload disease: relationship between structure, redox activity, and toxicity

The success of the iron (Fe) chelator desferrioxamine (DFO) in the treatment of beta-thalassemia is limited by its lack of bioavailability. The design and characterization of synthetic alternatives to DFO has attracted much scientific interest and has led to the discovery of orally active chelators that can remove pathological Fe deposits. However, chelators that access intracellular Fe pools can be toxic by either inhibiting Fe-containing enzymes or promoting Fe-mediated free radical damage. Interestingly, toxicity does not necessarily correlate with Fe-binding affinity or with chelation efficacy, suggesting that other factors may promote the cytopathic effects of chelators. In this review, we discuss the interactions of chelators and their Fe complexes with biomolecules that can lead to toxicity and tissue damage.

Differed preferential iron-binding lobe in human transferrin depending on the presence of bicarbonate detected by HPLC/high-resolution inductively coupled plasma mass spectrometry

The binding of iron (Fe) to human serum transferrin (Tf) was analyzed with an HPLC system equipped with an anion exchange column and directly connected with a high-resolution inductively coupled plasma mass spectrometer for metal detection. The (56)Fe level in the eluate was monitored at resolution m/Deltam=3000. Two monoferric Tfs were assigned based on the results of urea-PAGE and desferrioxamine experiments. When Fe was added as Fe-citrate stepwise to an apo-Tf solution in the presence of bicarbonate, the N-lobe site was the preferential Fe-binding site, while the C-lobe site was preferred in the absence of bicarbonate. In both cases, the Fe-peak areas of the preferential site and Fe(2)-Tf increased up to an Fe/Tf molar ratio of 1, and then the peak area of the monoferric Tf decreased while the peak area of Fe(2)-Tf increased. When the Fe/Tf molar ratio was below 1, the amount of Fe bound to the lobe with a weaker affinity was higher in Fe(2)-Tf than in the monoferric Tf in each case. Namely, Fe(2)-Tf was the preferential binding state of Fe to human serum Tf. The preference is reasonable for transferring Fe ions effectively to Tf-receptors.

Binding of Cu(II), Tb(III) and Fe(III) to chicken ovotransferrin. A kinetic study

The kinetics of binding of Cu(II), Tb(III) and Fe(III) to ovotransferrin have been investigated using the stopped-flow technique. Rate constants for the second-order reaction, k+, were determined by monitoring the absorbance change upon formation of the metal-transferrin complex in time range of milliseconds to seconds. The N and C sites appeared to bind a particular metal ion with the same rate; thus, average formation rate constants k+ (average) were 2.4 x 10(4) M-1 s-1 and 8.3 x 10(4) M-1 s-1 for Cu(II) and Tb(III) respectively. Site preference (N site for Cu(II) and C site for Tb(III] is then mainly due to the difference in dissociation rate constant for the metals. Fe(III) binding from Fe-nitrilotriacetate complex to apo-ovotransferrin was found to be more rapid, giving an average formation rate constant k+ (average) of 5 x 10(5) M-1 s-1, which was followed by a slow increase in absorbance at 465 nm. This slow process has an apparent rate constant in the range 3 s-1 to 0.5 s-1, depending upon the degree of Fe(III) saturation. The variation in the rate of the second phase is thought to reflect the difference in the rate of a conformational change for monoferric and diferric ovotransferrins. Monoferric ovotransferrin changes its conformation more rapidly (3.4 s-1) than diferric ovotransferrin (0.52 s-1). A further absorbance decrease was observed over a period of several minutes; this could be assigned to release of NTA from the complex, as suggested by Honda et al. (1980).

Affinity labels for the anion-binding site in ovotransferrin

Bromopyruvate, a known alkylating agent, has previously been reported to function as an affinity label for the anion-binding site in the iron-binding protein ovotransferrin [Patch, M.G., & Carrano, C. J. (1982) Biochim. Biophys. Acta 700, 217-220]. However, the present results indicate that hydroxypyruvate also functions in an almost identical manner, which implies that alkylation of a susceptible nucleophile cannot be the mechanism responsible for the covalent attachment of the anion. Model complexes and amino acid analysis of labeled ovotransferrin suggest that initial Schiff base formation, followed by reduction of the imine bond between the affinity anion and a lysine within the locus of the anion-binding site, accounts for the irreversible labeling. As expected, the covalently attached anions render the iron in the ovotransferrin-iron-anion ternary complex much more resistant to loss at low pH. It is proposed that the covalently labeled protein be used to test the hypothesis that iron removal from transferrin occurs by protonation and loss of the anion in low-pH lysosomal vesicles.

Characterization of lactoferrin interaction with Streptococcus mutans

Lactoferrin (LF) is an iron-binding glycoprotein common to exocrine secretions and the specific granules of neutrophils. Each molecule is capable of high-affinity coordinate-binding of two ferric ions with two bicarbonate or carbonic anions. The initial aspect of the present study was directed at determining the nature of anion involvement in LF bactericidal activity. It was found that selective anions were capable of inhibiting the expression of bactericidal activity by LF on S. mutans 10449. The ability to block LF expression was directly related to the capacity of the anion to serve as a coordinate ion in iron-binding by the transferrin molecules. These data support the hypothesis that the LF target site on the bacterial surface is anionic. There has been controversy in the literature regarding LF involvement in hydroxy radical generation. The second phase of these studies indicated that treatment of S. mutans with LF under anaerobic conditions abrogated the bactericidal effect of this molecule. LF-killing could be enhanced by the presence of thiocyanate and inhibited by catalase and lactoperoxidase; however, bovine serum albumin was equally effective as an inhibitor. The apparent requirement for oxygen in LF bactericidal effect on S. mutans is not inconsistent with a hydroxy radical mechanism.

The measurement of total and unsaturated iron-binding capacity in serum

An inter-laboratory investigation into the assay of serum iron binding capacity by the magnesium carbonate adsorption method was undertaken. The source of MgCO3, storage temperature of whole serum, lyophilization and storage temperature of lyophilized serum were identified as sources of inter-laboratory variation. Protocols for the clinical assay of UIBC and TIBC are given which have achieved satisfactory inter-laboratory precision.

Transferrin: physiology and function in iron transport

Since its identification about 30 years ago transferrin has attracted the interest of many investigators concerned with its structure, metal-binding properties, genetic polymorphism, and especially its role in the transport of iron within the body. Transferrin's two iron-binding sites appear to be structurally identical with equivalent iron binding after addition of iron in vitro. However, since the experiments of Fletcher and Huehns, the functional homogeneity of transferrin-bound iron has been questioned. Understanding of the precise mechanism of iron release from transferrin to receptor sites on reticulocytes and other tissues active in iron exchange is incomplete. Considerable evidence has been assembled from rats to support the Fletcher-Huehns hypothesis of selective release of A-site iron to erythrocyte precursors and placenta while B-site iron is delivered to hepatocytes. Results of experiments in rabbit and human systems remain controversial. Reasons for this controversy include: variations in the technique of iron addition to transferrin with the possibility of non-specific binding; variations in reticulocytes used in preparing selectively labelled transferrin and in its biological assay; and artifacts introduced in mixed-species experiments. Until methods are more refined and the transferrin-iron receptor interaction is better understood, the controversy about transferrins's iron transport function will persist.

Formation of monoferric ovotransferrins in the presence of chelates

1. When ovotransferrin is partially saturated with iron, endotherms for apo-ovotransferrin, two monoferric ovotransferrins and Fe2-ovotransferrin are observed by differential scanning calorimetry. The relative sizes of the endotherms are changed in the presence of the iron-chelating agents nitrilotriacetic acid and ATP. 2. When iron is added as Fe(III)-nitrilotriacetate, at Fe-nitrilotriacetate: ovotransferrin ratios less than unity, the endotherm for Fe2-ovotransferrin is essentially absent. At Fe-nitrilotriacetate: ovotransferrin ratios of unity the only species present in solution in appreciable concentration as evidenced by their differential-scanning-calorimetry endotherms, are two monoferric ovotransferrins in approximately equal amounts. At Fe-nitrilotriacetate: ovotransferrin ratios greater than unity, the apo-ovotransferrin endotherm is absent, and the endotherms for the two monoferric ovotransferrins decrease in size as the endotherm for Fe2-ovotransferrin increases. 3. In the presence of nitrilotriacetate, binding of iron to the two sites of ovotransferrin is highly anti-co-operative, but essentially indiscriminate. When monoferric ovotransferrin is formed from apo-ovotransferrin, binding at one site is slightly favoured compared with binding at the other site, but once iron has been bound at either site, the binding affinity for iron at the unoccupied site is much decreased.

The effect of serum and experimental variables on the transferrin and reticulocyte interaction

The transfer of iron from transferrin to the developing erythrocyte is a research area of high interest and considerable controversy. We have found that the results of transferrin-reticulocyte incubation studies are quite sensitive to the experimental procedures that are utilized. Reticulocytosis has been induced in rabbits by phelbotomy and phenylhydrazine injections. While the latter gives a higher reticulocyte count, the cells appear to exhibit an altered transferrin-membrane interaction. Transferrin has been iodinated by published methods utilizing chloramine-T and molecular iodine. The iodotransferrin products exhibit the same iron donation ability, however, evidence was found that the chloramine-T treatment leads to a nonspecific binding of transferrin to the reticulocyte. The means of saturating transferrin with 59Fe is also of prime importance. Fe(NH4)2(SO4)2 and especially FeCl3 were found to yield nonspecifically bound iron when added to transferrin or serum. This artifact was reflected in an altered transferrin-reticulocyte interaction. Using what we believe to be optimal conditions, the effect of serum on the transferrin-reticulocyte system was re-examined. The results clearly indicated an enhancement of iron uptake by reticulocytes in the presence of serum, as well as an accelerated incorporation of iron by the cytoplasmic fraction.