Metal site conformational states of vanadyl(IV) human serotransferrin complexes

Separation of iron-free and iron-saturated forms of transferrin and lactoferrin via capillary electrophoresis performed in fused-silica and neutral capillaries

A capillary electrophoresis-based method for the cost-effective and high efficient separation of iron-free and iron-saturated forms of two members of transferrin family: transferrin and lactoferrin has been developed. The proposed qualitative method relying on the SDS application allowed us to separate iron-free and iron-saturated forms of these proteins, as well as human serum albumin, used as an internal standard. Owing to the distinct migration times under established conditions, the combination of transferrin and lactoferrin assays within a single analytical procedure was feasible. The performance of the method using a fused-silica capillary has been compared with the results obtained using the same method but performed with the use of a neutral capillary of the same dimensions. Neutral capillary has been used as an alternative, since the comparable resolution has been achieved with a concomitant reduction of the electroosmotic flow. Despite of this fact, the migration of analytes occurred with similar velocity but in opposite order, due to the reverse polarity application. A quantitative method employing fused-silica capillary for iron saturation study has been also developed, to evaluate the iron saturation in commercial preparations of lactoferrin.

Binding of V(IV) to human transferrin: potential relevance to anticancer activity of vanadocene dichloride

The action mechanism of vanadocene dichloride, Cp2VCl2 (Cp=eta5-C5H5), has been investigated by interaction with human serum transferrin for its promising antitumor activities. Our results have shown that Cp2VCl2 binds to transferrin and form a new complex, and the calculated apparent association constant is 1.37 x 10(5)M(-1) from the fluorescence quenching. Simultaneously, the variation of the secondary structure of transferrin occurs, most probably due to the coordination of the amino residues of protein with VIV. It was evidenced that Cp is released free in solution after VIV binding to transferrin by 1H NMR measurements. These results have shown that Cp2VCl2 forms a complex with transferrin, which may provide a possible pathway in the transport and targeted delivery of the antitumor agent.

Synergistic anion and metal binding to the ferric ion-binding protein from Neisseria gonorrhoeae

The 34-kDa periplasmic iron-transport protein (FBP) from Neisseria gonorrhoeae (nFBP) contains Fe(III) and (hydrogen)phosphate (synergistic anion). It has a characteristic ligand-to-metal charge-transfer absorption band at 481 nm. Phosphate can be displaced by (bi)carbonate to give Fe.CO(3).nFBP (lambda(max) 459 nm). The local structures of native Fe-PO(4)-nFBP and Fe.CO(3).nFBP were determined by EXAFS at the FeK edge using full multiple scattering analysis. The EXAFS analysis reveals that both phosphate and carbonate ligands bind to FBP in monodentate mode in contrast to transferrins, which bind carbonate in bidentate mode. The EXAFS analysis also suggests an alternative to the crystallographically determined position of the Glu ligand, and this in turn suggests that an H-bonding network may help to stabilize monodentate binding of the synergistic anion. The anions oxalate, pyrophosphate, and nitrilotriacetate also appear to serve as synergistic anions but not sulfate or perchlorate. The oxidation of Fe(II) in the presence of nFBP led to a weak Fe(III).nFBP complex (lambda(max) 471 nm). Iron and phosphate can be removed from FBP at low pH (pH 4.5) in the presence of a large excess of citrate. Apo-FBP is less soluble and less stable than Fe.nFBP and binds relatively weakly to Ga(III) and Bi(III) but not to Co(III) ions, all of which bind strongly to apo-human serum transferrin.

Structural origins of the insulin-mimetic activity of bis(acetylacetonato)oxovanadium(IV)

We have investigated the interaction of bis(acetylacetonato)oxovanadium(IV) (VO(acac)(2)) with bovine serum albumin (BSA) by EPR and angle-selected electron nuclear double resonance, correlating results with assays of glucose uptake by 3T3-L1 adipocytes. EPR spectra of VO(acac)(2) showed no broadening in the presence of BSA; however, electron nuclear double resonance titrations of VO(acac)(2) in the presence of BSA were indicative of adduct formation of VO(acac)(2) with albumin of 1:1 stoichiometry. The influence of VO(acac)(2) on uptake of 2-deoxy-d-[1-(14)C]glucose by serum-starved 3T3-L1 adipocytes was measured in the presence and absence of BSA. Glucose uptake was stimulated 9-fold in the presence of 0.5 mm VO(acac)(2), 17-fold in the presence of 0.5 mm VO(acac)(2) plus 1 mm BSA, and 22-fold in the presence of 100 nm insulin. BSA had no influence on glucose uptake, on the action of insulin, or on glucose uptake in the presence of VOSO(4). The maximum insulin-mimetic effect of VO(acac)(2) was observed at VO(acac)(2):BSA ratios less than or equal to 1.0. Similar results were obtained also with bis(maltolato)oxovanadium(IV). These results suggest that the enhanced insulin-mimetic action of organic chelates of VO(2+) may be dependent on adduct formation with BSA and possibly other serum transport proteins.

Effect of vanadium(IV) compounds in the treatment of diabetes: in vivo and in vitro studies with vanadyl sulfate and bis(maltolato)oxovandium(IV)

Vanadyl sulfate (VOSO(4)) was given orally to 16 subjects with type 2 diabetes mellitus for 6 weeks at a dose of 25, 50, or 100 mg vanadium (V) daily [Goldfine et al., Metabolism 49 (2000) 1-12]. Elemental V was determined by graphite furnace atomic absorption spectrometry (GFAAS). There was no correlation of V in serum with clinical response, determined by reduction of mean fasting blood glucose or increased insulin sensitivity during euglycemic clamp. To investigate the effect of administering a coordinated V, plasma glucose levels were determined in streptozotocin (STZ)-induced diabetic rats treated with the salt (VOSO(4)) or the coordinated V compound bis(maltolato)oxovandium(IV) (abbreviated as VO(malto)(2)) administered by intraperitoneal (i.p.) injection. There was no relationship of blood V concentration with plasma glucose levels in the animals treated with VOSO(4), similar to our human diabetic patients. However, with VO(malto)(2) treatment, animals with low plasma glucose tended to have high blood V. To determine if V binding to serum proteins could diminish biologically active serum V, binding of both VOSO(4) and VO(malto)(2) to human serum albumin (HSA), human apoTransferrin (apoHTf) and pig immunoglobulin (IgG) was studied with EPR spectroscopy. Both VOSO(4) and VO(malto)(2) bound to HSA and apoHTf forming different V-protein complexes, while neither V compound bound to the IgG. VOSO(4) and VO(malto)(2) showed differences when levels of plasma glucose and blood V in diabetic rodents were compared, and in the formation of V-protein complexes with abundant serum proteins. These data suggest that binding of V compounds to ligands in blood, such as proteins, may affect the available pool of V for biological effects.

Vanadyl(IV) binding to mammalian ferritins. An EPR study aided by site-directed mutagenesis

During its metabolism, vanadium is known to become associated with the iron storage protein, ferritin. To elucidate probable vanadium binding sites on the protein, VO2+ binding to mammalian ferritins was studied using site-directed mutagenesis and EPR spectroscopy. VO2+-apoferritin EPR spectra of human H-chain (100% H), L-chain (100% L), horse spleen (84% L, 16% H) and sheep spleen (45% L, 55% H) ferritins revealed the presence of alpha and beta VO2+ species in all the proteins, implying that the ligands for these species are conserved between the H- and L-chains. The alpha species is less stable than the beta species and decreases with increasing pH, demonstrating that the two species are not pH-related, a result contrary to earlier proposals. EPR spectra of site-directed HuHF variants of several residues conserved in H- and L-chain ferritins (Asp-131, Glu-134, His-118 and His-128) suggest that His-118 near the outer opening of the three-fold channel is probably a ligand for VO2+ and is responsible for the beta signals in the EPR spectrum. The data indicate that VO2+ does not bind to the Asp-131 and Glu-134 residues within the three-fold channels nor does it bind at the ferroxidase site residues Glu-62 or His-65 or at the putative nucleation site residues Glu-61,64,67. While the ferroxidase site is not a site for VO2+ binding, mutation of residues Glu-62 and His-65 of this site to Ala affects VO2+ binding at His-118, located some 17 A away. Thus, VO2+ spin probe studies provide a window on structural changes in ferritin not seen in most previous work and indicate that long-range effects caused by point mutations must be carefully considered when drawing conclusions from mutagenesis studies of the protein.

Rationalization of the strength of metal binding to human serum transferrin

A wide range of metal ions of natural, therapeutic, diagnostic and toxic interest are transported by serum transferrin (80 kDa). It is therefore important to understand the factors that control the strength of metal binding. We show here that even though Sc3+ has only slightly larger ionic radius than Fe3+ (0.075 nm versus 0.065 nm), it binds to the C-lobe and N-lobe sites much more weakly: logK1* (bicarbonate-independent binding constant) 14.6 +/- 0.2, logK2* 13.3 +/- 0.3, respectively (10 mM Hepes, 5 mM bicarbonate, 310 K). Preferential binding to the C-lobe was established by 1H-NMR spectroscopy. We show that the strength of binding of divalent and trivalent metal ions to human serum transferrin correlates with metal ion acidity [and therefore with the strength of binding to hydroxide, K1(OH)]. The correlations are of predictive value for a range of other metal ions. The plot of logK1* (human serum transferrin) versus logK1(OH) has a negative intercept consistent with unfavorable entropy effects due to lobe closure of apotransferrin on binding of metal ions. This interpretation was tested by comparison with similar correlations of the strength of metal binding to the enzymes carbonic anhydrase and carboxypeptidase with that for the low-M(r) ligand imidazole. These plots have positive intercepts consistent with the preorganized (entatic) state of these metalloenzymes (favorable entropy effects on metal binding).

Anion binding by human lactoferrin: results from crystallographic and physicochemical studies

The anion-binding properties of lactoferrin (Lf), with Fe3+ or Cu2+ as the associated metal ion, have been investigated by physicochemical and crystallographic techniques. These highlight differences between the two sites and in the anion-binding behavior when different metals are bound. Carbonate, oxalate, and hybrid carbonate-oxalate complexes have been prepared and their characteristic electronic and EPR spectra recorded. Oxalate can displace carbonate from either one or both anion sites of Cu2(CO3)2Lf, depending on the oxalate concentration, but no such displacement occurs for Fe2(CO3)2Lf. Addition of oxalate and the appropriate metal ion to apoLf under carbonate-free conditions gives dioxalate complexes with both Fe3+ and Cu2+, except when traces of EDTA remain associated with the protein, when hybrid complexes M2(CO3)(C2O4)Lf can result. The anion sites in the crystal structures of Fe2(CO3)2Lf, Cu2-(CO3)2Lf, and Cu2(CO3)(C2O4)Lf, refined at 2.2, 2.1, and 2.2 A, respectively, have been compared. In every case, the anion is hydrogen bonded to the N-terminus of helix 5, an associated arginine side chain, and a nearby threonine side chain. The carbonate ion binds in bidentate fashion to the metal, except in the N-lobe site of dicupric lactoferrin, where it is monodentate; the difference arises from slight movement of the metal ion. The hybrid complex shows that the oxalate ion binds preferentially in the C-lobe site, in 1,2-bidentate mode, but with the displacement of several nearby side chains. These observations lead to a generalized model for synergistic anion binding by transferrins.

Preliminary crystallographic studies of copper(II)- and oxalate-substituted human lactoferrin

As part of a comparative study on the binding of different metals and anions by human lactoferrin, we have prepared and crystallized: (1) dicupric lactoferrin with Cu2+ and carbonate in each site (Cu2Lf); and (2) a lactoferrin complex with Cu2+ and carbonate in one site, and Cu2+ and oxalate in the other (Cu2oxLf). Crystals of Cu2Lf are orthorhombic: a = 155.9, b = 97.0, c = 56.0 A, space-group P2(1)2(1)2(1); those of Cu2oxLf are also orthorhombici a = 155.9, b = 97.1, c = 56.2 A, space-group P2(1)2(1)2(1). Both are isomorphous with diferric human lactoferrin, Fe2Lf. Diffractometer data to 2.6 A and 2.5 A have been collected for Cu2Lf and Cu2oxLf, respectively. Difference maps show that the main effect of substitution of Cu2+ for Fe3+ is a small shift (0.5 to 1.0 A) in the metal position in each site. For Cu2oxLf the oxalate ion is found to be accommodated in the C-lobe, bound to copper in a bidentate mode, causing only small local changes, in the positions of adjacent Arg and Tyr side-chains.

Oxalate as synergistic anion for Cd(II) binding to ovotransferrin

The Cd(II) derivative of ovotransferrin containing oxalate as synergistic anion was investigated through 113Cd- and 13C-NMR spectroscopy and compared with the analogous derivative obtained in the presence of bicarbonate. Cadmium loaded in the C site gives rise to a 113Cd-NMR signal at 54 ppm, while that bound to the N site is broadened beyond detection. The two resonances at 168.5 and 169.9 ppm observed in the 13C-NMR spectrum of the Cd2 derivative obtained with [13C]oxalate each correspond to slowly exchanging oxalate specifically bound to a single site. No splitting of these resonances due to 113Cd-13C magnetic coupling is observed upon insertion of 113Cd-enriched Cd(II) ion, unlike previous observations for the corresponding derivative obtained with bicarbonate as synergistic anion. It was found that the metal sites in the present derivative are inequivalent, as observed for other metal-transferrin-oxalate adducts. The C site is found to be sensitive to a residue ionization with a pKa of 9.5.

Vanadyl(IV) complexes with pyruvate kinase: activation of the enzyme and electron paramagnetic resonance properties of ternary complexes with the protein

Complexes of the oxocation of vanadyl(IV), VO2+, with pyruvate kinase from rabbit muscle have been investigated by steady-state kinetic assays and by EPR spectroscopy. Pyruvate kinase requires 2 eq of divalent cation for activity. VO2+ alone is a poor activator of the normal physiological reaction catalyzed by the enzyme and of the enzyme-catalyzed exchange of the methyl protons of pyruvate with solvent. VO2+ alone is, however, an activator of the enzyme-catalyzed phosphorylation of glycolate by ATP. VO2+ is more effective than Mg2+ in activation of the bicarbonate-dependent ATPase reaction of pyruvate kinase, and in the enzyme-catalyzed hydrolysis of phosphoenolpyruvate. EPR data show that VO2+ binds to the divalent cation site on the protein competitively with respect to Mg2+. The VO2+-enzyme complex has a high affinity for bicarbonate. Direct coordination of pyruvate, oxalate, and glycolate to the enzyme-bound VO2+ has been established by EPR measurements with specifically 17O-labeled forms of these compounds.

Structure and spatial conformation of the iron-binding sites of transferrins

Transferrins are iron-binding glycoproteins involved in iron metabolism and antibacterial defense mechanisms. Since the discovery of transferrins, many studies have attempted to characterize the iron ligands and to establish the conformation of the iron-binding sites. From chemical and spectroscopic studies, it was generally accepted that iron was hexacoordinated to Tyr and His residues, to a water molecule and to a (bi)carbonate ion, electrostatically linked to an Arg residue. On the basis of these studies, on the one hand, and on the basis of the homologies between the amino acid sequences of transferrins, on the other hand, predicted data have been provided about the number and location of the iron ligands. Recent X-ray crystallography studies of human lactotransferrin have partially confirmed the above-mentioned predicted data and have brought invaluable information about the nature of the ligands and the conformation of the iron-binding site. On the basis of the obtained results, a scheme has been proposed in which the iron is coordinated to 2 Tyr, 1 His and 1 Asp residues, to a (bi)carbonate linked to an Arg residue and probably to a water molecule. The iron-binding site is located at the interface between the two domains which constitute each lobe of the transferrins.

Mössbauer studies of electrophoretically purified monoferric and diferric human transferrin

Electrophoretically purified 57Fe-enriched monoferric and diferric human transferrins and selectively labeled complexes ([C-56Fe,N-57Fe]transferrin and [C-57Fe,N-56Fe]transferrin) were studied by Mössbauer spectroscopy. The data were recorded at 4.2 K over a wide range of applied magnetic fields (0.05-6 T) and were analyzed by a spin-Hamiltonian formalism. Characteristic hyperfine parameters were found and the obtained zero-field splitting parameters (D = 0.25 +/- 0.05 cm-1 and E/D = 0.30 +/- 0.02) agree with previous electron paramagnetic resonance (EPR) findings. The weak-field spectra of the [N-57Fe]transferrin are slightly broader than those of the [C-57Fe]transferrin, indicating that the N-terminal iron site may be more heterogeneous. However, the absorption line positions and the relative intensities of the subspectra originating from the three Kramers doublets of each Fe3+ site are identical. Thus the electronic structures of the two iron sites can be described by the same set of spin-Hamiltonian parameters, indicating that the ligand environments for the two sites are the same, as suggested by the recent X-ray crystallographic studies. This suggestion is further supported by the observation that the strong-field spectra of the two monoferric transferrins are indistinguishable. The selectively labeled mixed-isotope transferrins exhibit spectra that are identical to those of the corresponding monoferric 57Fe-enriched transferrins, implying that the occupation of one iron site has little or no effect on the immediate environment of the other site, a finding that is not surprising since the two sites are separated by approximately 4.2 nm.

Studies on the role of iron in the reversal of vanadium toxicity in chicks

The interaction of dietary iron levels on vanadium toxicity was studied in chicks. Dietary iron levels ranged from a deficiency, ca. 10 ppm, to an adequacy, 100 ppm supplemental iron. to an excess, 1000 ppm supplemental iron. Vanadium was fed at 10, 20, and 40 ppm. Vanadium toxicity as measured by chick growth was more severe in the iron-deficient animals than in those receiving supplemental iron. The increase in degree of toxicity in the iron-deficient animals was accompanied by an increase in the liver vanadium, both total and concentration. The addition, of vanadium to the diet did not influence the iron concentration of the liver or kidney. Radioisotope, studies with(48)V revealed that the absorption of vanadium was not influenced by the iron concentration of the diet, but that the iron-deficient animals retained more vanadium in the blood and liver and less in the bone than did the iron supplemented animals. It is proposed that the degree of iron saturation of transferrin and ferritin to which vanadium can bind is a possible explanation for the results obtained.

Vanadium complexes of transferrin and ferritin in the rat

Vanadium associates with serum transferrin of rats administered vanadyl(IV) sulfate or ammonium metavanadate(V) by gastric intubation. Low molecular weight species account for only 3% of the vanadium present in plasma. The element distributes between the two major isotransferrins in proportion to their concentrations. Rat apotransferrin binds both vanadium(IV) and vanadium(V), forming 2:1 metal-protein complexes in both instances. Although the two isotransferrins apparently differ in their physiological properties, they exhibit identical vanadyl(IV) (VO2+) EPR spectra, indicating identical or very similar metal binding sites for both proteins. In contrast to other transferrins, the two sites of the rat protein are spectroscopically indistinguishable and exhibit a VO2+ EPR spectrum similar to that of the C-terminal metal binding site of human serum transferrin. VO2+ EPR signals are observed with liver, spleen, and kidney tissue samples from animals maintained on a vanadium-supplemented diet. These signals arise from a specific intracellular VO2+ complex with the iron storage protein ferritin.

Spectral characterization of vanadium-transferrin systems

The preparation procedure of vanadium(III) transferrin and its stability are confirmed to be as previously reported. The electronic spectra of vanadium(III), oxovanadium(IV), and vanadium(V) transferrin derivatives are comparatively discussed. A band in the near infrared of the oxovanadium(IV) derivative is observed for the first time.

Preparation and properties of a single-sited fragment from the C-terminal domain of human transferrin

A single-sited iron-binding fragment of human transferrin has been obtained by thermolysin cleavage of the protein, selectively loaded with iron in the C-terminal binding site, in a urea-containing buffer. The fragment contains carbohydrate, and hence derives from the C-terminal half of transferrin. Its metal-binding site accepts Fe3+ and Cu2+ with bicarbonate as accompanying anion, but only Fe3+ with oxalate as anion. EPR spectroscopic properties of the fragment are similar to those of the corresponding site in the intact protein. However, iron-binding by the fragment is weaker than by the C-terminal site of the intact protein, particularly at low pH, suggesting that overall as well as local protein conformation influences the metal-binding functions of the site.

Vanadyl (VO2+) and vanadate (VO-3) ions inhibit the brain microsomal Na,K-ATPase with similar affinities. Protection by transferrin and noradrenaline

The activity of Na,K-ATPase was measured in brain microsomes as the function of increasing concentrations of vanadyl (VOSO4, V4+) and the vanadate (NaVO3, V5+) ions. Both forms of vanadium inhibited the Na,K-ATPase activity with high affinity -Ki (vanadate) = 3 X 10(-7)M and Ki (vanadyl = 1 X 10(-6)M. The stability of V4+ in ATPase reaction media (Tris buffers) was measured by electron spin resonance spectroscopy. Without any reducing agent, V4+ was quickly oxidised by atmospheric oxygen. When a reducing agent such as dithiothreitol was added, the V4+ was stable for at least 30 min and the inhibition pattern of Na,K-ATPase by V4+ was not changed. The blocking effect of V4+ in the presence of dithiothreitol was counteracted by pre-incubation with equimolar concentrations of transferrin or 100 times excess of noradrenaline. The regulation of brain Na,K-ATPase by vanadate may be represented by competition between low-capacity inhibitory binding sites localized on the enzyme molecule and high-capacity sites of intracellular proteins. Preferential binding of vanadyl to the latter type of sites will decrease the intracellular concentration of the free metal and thus eliminate the enzyme inhibition.

Behavior of vanadate and vanadyl ion in canine blood

Radiolabeled vanadium as either vanadyl ion or vanadate ion was injected intravenously into adult beagle dogs, and blood samples were collected at various times up to 48 hr post injection. For each sample, the distribution of vanadium between the cells and the plasma was determined, and the plasma was analyzed by electrophoresis to identify specific vanadium-binding proteins. Initially, vanadyl ion left the bloodstream more rapidly than vanadate, but the rates equalized after about 5 hr. A significant fraction of the vanadium in blood was associated with the cellular component following injection of both forms of vanadium. About 77% of the plasma vanadium was eventually bound by the serum iron transport protein transferrin, regardless of the vanadium species initially injected. For both vanadyl and vanadate, about 30 hr were required to reach the maximum degree of transferrin binding.

Electron-nuclear double resonance of copper complexes of human transferrin

Electron-nuclear double resonance (ENDOR) spectroscopy has been used to study ligand and copper hyperfine interactions in Cu(II) complexes of human transferrin. A nearly isotropic superhyperfine interaction of the Cu(II) spin with a single 14N nucleus was identified, and the principal values of its tensor were estimated. All principal values of the copper hyperfine tensor were also directly measured for the first time. Resonances from at least two exchangeable protons were observed, but their origin could not be ascertained. At physiological pH, and in the presence of bicarbonate, ENDOR spectra of the two metal-binding sites were virtually indistinguishable.

Comparative study of the iron-binding properties of human transferrins. II. Electron paramagnetic resonance of mixed metal complexes of human lactotransferrin

Human lactotransferrin is able to bind two vanadyl(IV) ions in specific metal-binding sites. The EPR signals of the two vanadyl bound ions, however, appear as one. This result suggests that the environments of the binding sites of human lactotransferrin are similar. The binding activity is promoted to pH 4 using carbonate or bicarbonate as synergistic anion. This unusual stability of the anion-binding site, which is destroyed below pH 6 for other transferrins, can explain in part the great stability of the metallic complexes of human lactotransferrin. However, the different sensitivities of the two metal-binding sites towards protonation permit the preparation of mixed vanadyl(IV), iron(III) complexes with VO2+ bound either on the N-terminal (acid-labile or B site) or on the C-terminal (acid-stable or A site) site. Analysis of the spectra of such mixed complexes shows the presence of a third nonspecific VO2+-binding site termed A'. The nonspecific A' site seems to be located on the outer surface of the protein close to the C-terminal site.

Molecular ferrokinetics in the rabbit

Using urea-polyacrylamide gel electrophoresis it has been possible to distinguish the molecular forms of transferrin in rabbit serum. When 59Fe-labelled diferric transferrin is injected into normal, anaemic or hypertransfused, polycythaemic rabbits, iron is removed from diferric transferrin in essentially pairwise fashion. Exchange of iron between transferrin and tissues was also studied using predominantly monoferric transferrin labelled with 59Fe or 125I, and with 125I-labelled apotransferrin. The return of iron from tissue stores to circulating transferrin occurs one atom at a time to either site of the protein and, possibly, in pairwise fashion as well. The rate of clearance of iron from diferric transferrin differs from that of monoferric transferrins, and the rates at which iron is returned to empty sites of transferrin also differ, so that serum iron is not a kinetically homogeneous pool in the rabbit.

Polynuclear iron compounds in human transferrin preparations

During commonly used saturation procedures of transferrin with iron compounds, both as ferri and ferrous, polynuclear iron compounds are easily formed, even when nitrilotriacetate (NTA) is used as a strong iron ligand. The presence of these nonspecific bound irons is demonstrated with Mossbauer spectroscopy and with electronic optical spectroscopy. But no evidence, however, has been found of two different iron binding sites. Because dialysis is not able to remove all polynuclear iron, an easy method with gel filtration has been developed that does remove the polynuclear iron. Some notes are made about the often used method, in transferrin biochemistry, of saturation determination, i.e. the quotient of the absorbances of 470 and 280 nm.

The influence of pH on the equilibrium distribution of iron between the metal-binding sites of human transferrin

The dependence of the metal-binding properties of transferrin on pH in the pH 6--9 range was investigated by urea/polyacrylamide-gel electrophoresis. Equations are presented for calculating the relative values of the four conditional site constants for the stepwise binding of iron to the two sites of transferrin and for calculating the equilibrium distribution of the protein among the four principal forms, apotransferrin, the C-terminal and N-terminal monoferric transferrins and diferric transferrin. The relative affinity of iron for the two sites and the co-operativity of iron-binding follow characteristic "pH titration' curves. A mathematical model that can account for the former behaviour is presented. In both cases the metal-binding sites are affected by the ionization of functional groups with apparent pKa values near physiological pH approx. 7.4. There is strong positive co-operatively in the release of protons from these groups. The results indicate that pH must be accurately controlled in studies of the differential properties of the two sites of the transferrin molecule.

Binding of xylenol orange to transferrin. Demonstration of metal-anion linkage

The iron transport protein, transferrin, binds two metal ions and, concomitantly, two carboxylate anions. The metal ion indicators, xylenol orange and semi-xylenol orange are carboxylate anions which exhibit a characteristic visible spectrum when attached to a metal. We prepared the ternary complexes VO2+-transferrin-xylenol orange and VO2+-transferrin-semi-xylenol orange. The EPR spectra show that the vanadyl ion is attached to the protein and the visible spectra show that the xylenol orange or semi-xylenol orange is attached to the metal. The implication is that in other metal-transferrin-anion complexes, the anion is directly attached to the metal.