Vanadyl-transferrin

Cannon, J. C., Chasteen, N. D. The Distinction between Metal Binding Sites in Vanadyl Transferrin Complexes in Proteins of Iron Storage and Transport in Biochemistry and Medicine, (ed.) Crichton, R. R, Amsterdam, North Holland Press, 1975... 

In serum, absorbed vanadium is transported mainly bound to transferrin (Lager-kvist etal. 1986, Kustin and Robinson 1995). Vanadium in rat milk was found mainly in the protein fraction, and perhaps also in lactoferrin (Sabbioni and Rade 1980), in which form it is transferred from the mother to the pups. In older rats, vanadium appears to be converted into vanadyl-transferrin and ferritin complexes in the plasma and body fluids (Edel and Sabbioni 1989, Sabbioni and Marafante 1981). 

Vanadium Absorption Vanadate has been suggested to be absorbed through phosphate or other anion transport systems vanadyl has been suggested to use iron transport systems. Absorption occurs in the duodenum <10% Converted into vanadyl-transferrin and vanadyl-ferritin whether transferrin is the transport vehicle and ferritin is the storage vehicle for vanadium remains to be determined. Bone is a respository for excess vanadium... 

Vanadate transport in the erythrocyte was shown to occur via facilitated diffusion in erythrocyte membranes and was inhibited by 4,4 -diisothiocyanostilbene-2,2 -disulfonic acid (DIDS), a specific inhibitor of the band 3 anion transport protein [23], Vanadium is also believed to enter cells as the vanadyl ion, presumably through cationic facilitated diffusion systems. The divalent metal transporter 1 protein (called DMT1, and also known as Nramp2), which carries iron into cells in the gastrointestinal system and out of endosomes in the transferrin cycle [24], has been proposed to also transport the vanadyl cation. In animal systems, specific transport protein systems facilitate the transport of vanadium across membranes into the cell and between cellular compartments, whereas the transport of vanadium through fluids in the organism occurs via binding to proteins that may not be specific to vanadium. 

Vanadium can bind to transferrins in three different oxidation states. V(III) has been reported (131) as giving an air-stable complex with transferrin, probably as the simple cation V3+ the corresponding lactoferrin complex of V(III) is, however, extremely air sensitive, being oxidized in minutes (156), and the transferrin result has been questioned (133). V(IV) forms a colorless complex with transferrins, shown by EPR spectroscopy to incorporate the vanadyl (V02+) ion (155). Displacement studies show that the V02+ ions occupy the specific Fe3+ sites... 

While the affinity of transferrin see Iron Proteins for Storage Transport their Synthetic Analogs) for vanadyl is 10-fold greater than that of albumin, the latter can bind up to 20 vanadyl ions including a specific interaction with cys-34, the only reduced cysteine residue in the protein. 

Vanadium is an element, and as such, is not metabolized. However, in the body, there is an interconversion of two oxidation states of vanadium, the tetravalent form, vanadyl (V+4), and the pentavalent form, vanadate (V+5). Vanadium can reversibly bind to transferrin protein in the blood and then be taken up into erythrocytes. These two factors may affect the biphasic clearance of vanadium that occurs in the blood. Vanadate is considered more toxic than vanadyl, because vanadate is reactive with a number of enzymes and is a potent inhibitor of the Na+K+-ATPase of plasma membranes (Harris et al. 1984 Patterson et al. 1986). There is a slower uptake of vanadyl into erythrocytes compared to the vanadate form. Five minutes after an intravenous administration of radiolabeled vanadate or vandadyl in dogs, 30% of the vanadate dose and 12% of the vanadyl dose is found in erythrocytes (Harris et al. 1984). It is suggested that this difference in uptake is due to the time required for the vanadyl form to be oxidized to vanadate. When V+4 or V+5 is administered intravenously, a balance is reached in which vanadium moves in and out of the cells at a rate that is comparable to the rate of vanadium removal from the blood (Harris et al. 1984). Initially, vanadyl leaves the blood more rapidly than vandate, possibly due to the slower uptake of vanadyl into cells (Harris et al. 1984). Five hours after administration, blood clearance is essentially identical for the two forms. A decrease in glutathione, NADPH, and NADH occurs within an hour after intraperitoneal injection of sodium vanadate in mice (Bruech et al. 1984). It is believed that vanadate requires these cytochrome P-450 components for oxidation to the vanadyl form. A consequence of this action is the diversion of electrons from the monooxygenase system resulting in the inhibition of drug dealkylation (Bruech et al. 1984). 

Vanadium in the plasma can exist in a bound or unbound form (Bruech et al. 1984). Vanadium as vanadyl (Patterson et al. 1986) or vanadate (Harris and Carrano 1984) reversibly binds to human serum transferrin at two metalbinding sites on the protein. With intravenous administration of vanadate or vanadyl, there is a short lag time for vanadate binding to transferrin, but, at 30 hours, the association is identical for the two vanadium forms (Harris et al. 1984). The vanadium-transferrin binding is most likely to occur with the vanadyl form as this complex is more stable (Harris et al. 

In Table 2.5 (Section 2.2.1), the high molecular mass blood constituents albumin and transferrin are listed along with the main low molecular mass constituents. Transferrin in particular is a very strong binder for the vanadyl ion [log A = 14.3(6)] and more or less replaces other ligands. The coordination of vanadyl (and vanadate) to transferrin will be taken up in the more general context of vanadium-protein interaction (Section 5.2). 

The (metabolic) pathways of dietary vanadium, such as vanadate [H2V04], can be expressed as illustrated in Scheme 5.1 after oral uptake, vanadate reaches the gastrointestinal tract, where it is partially reduced and precipitated to vanadyl (VO ) hydroxides, which are excreted with the faeces. Another portion is absorbed and circulated in the blood, where it undergoes redox speciation and complexation by the serum proteins transferrin and albumin. Vanadate and vanadyl are finally incorporated into cells, mainly those of the liver, spleen and kidney. Excretion is achieved via the urine. Part of the vanadium is taken up by bones, where the mean retention time is comparatively long. 

Human serum transferrin is an iron(m) tr ispoTt protein which gives up Fe + ions to bone marrow and placental tissues in preference to other cells such as liver cells the protein can bind two iron atoms per molecule but an anion such as HCOa" must be present for binding to occur. Vanadyl ion has been used (as an e.s.r. spin label) to show that the two metal-binding sites are non-equivalent and the same conclusion has been drawn from studies on the dissociation of labelled protein-bound iron. On the other hand, Harris and Aisen have obtained evidence from radiotracer experiments that the two sites are kinetically equivalent. The kinetics of iron(in) (in the form of ferric citrate) binding to the analogous apoprotein from hen s egg (apo-ovotransferrin) have been reported and the following minimum mechanistic scheme has been proposed ... 

A likely difference in the metabolism of free and chelated sources lies in post-absorptive processes, particularly transport in the bloodstream. The study of the interactions of insulin-enhancing compounds with serum proteins has been almost exclusively studied by EPR. Apo-transferrin and albumin have been implicated in the transport of vanadyl ions in the blood, and these proteins represent a significant metal-binding capability in the blood. Considerable interest in the role of these proteins in the transport and biotransformation of administered vanadium compounds has been evident in the recent literature. 

Cannon JC, Chasteen ND. 1975. Nonequivalence of metal binding sites in vanadyl-labeled human serum transferrin. Biochemistry 14 4573—4577. 

Campbell R, Chasteen ND. 1977. Anion binding study of vanadyl(IV) human sero-transferrin evidence for direct linkage to metal. JR/o/ Chem 252 5996-6001. 

Eaton SS, Dubach J, More KM, Eaton GR, Thurman G, Ambruso DR. 1989. Comparison of the electron spin echo envelope modulation (ESEEM) for human lactoferrin and transferrin complexes of copper(II) and vanadyl ion. JBiol Chem 264 4776-4781. 

Figure /. EPR spectra of (top) apo-transferrin, 0.29 mM, vanadyl sulfate, 1.11 mM (middle) apo-transferrin, 0.13 mM, BMOV, 0.11 mM and (bottom) BMOV, L10mM(T==298K,pH7.4, 0.16MNaCl).

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