Transferrins iron removal

The aim of treatment of iron overload is to remove all potentially toxic iron from the body. In hereditary haemochromatosis this can be achieved by weekly phlebotomies of 500 ml until the desired serum ferritin concentration (mostly <50 gg/1) or a normal transferrin iron saturation is reached (Brissot et ah, 2000). 

Matsumoto et al. demonstrated that the removal of iron from diferric transferrin by the tris-hydroxamate siderophore mimic TAGE occurs in two discreet steps (90). The slower step corresponds to iron removal from the more stable C-lobe site on transferrin and the faster step to removal from the N-lobe. The rates of removal are similar to the rates of removal of iron from diferric transferrin by desferrioxamine B (4), signifying similar mechanisms of removal between the two systems (90). 

Fig. 2.12 Examples of non-linear Arrhenius (or Eyring) plots (a) 1u(A oh)7 " ) vs T for the base hydrolysis of trans-Co(en)2ClJ. Curvature may result when k, k2 and A// , not equalling A// in the conjugate-base mechanism (Sec. 4.3.4). Reprinted with permission from C. Blakeley and M. L. Tobe, J. Chem. Soc. Dalton Trans. 1775 (1987). (b) nk vs T for iron removal from C- and N-terminal monoferric transferrin (lower and upper scales respectively). Transferrin contains two iron binding sites = 35 A apart. Either of the two sites, designated C- and N-terminal, can be exclusively labelled by Fe(lll) ions and these may be removed by a strong ligand such as a catechol (see Sec. 4.11). Reprinted with permission from S. A. Kretschmar and K. N. Raymond, J. Amer. Chem. Soc. 108, 6212 (1986). (1986) American Chemical Society.

In Figure 2.12(b) is shown the temperature dependence of the rate constant for iron removal from N-terminal monoferric transferrin. There is an obvious break between 12 and 20 °C and this is ascribed to a temperature-induced conformational change. The effect becomes less distinct when the ionic strength is increased from 0.13 to 2.0 M,See also Sec. 4.11. 

Schade reviewed (114) the earlier studies on the role of serum transferrin in iron transport. Various early investigators had observed that the blood serum transferrin rapidly bound iron administered either through the gastrointestinal tract or by intravenous injection. There was a rapid turnover of iron in the blood serum and the degree of saturation of the transferrin was related to the amount of iron administered. In no instances, however, was the blood serum transferrin ever saturated with iron. Jandl et al. (71) have shown that both ovotransferrin and serum transferrin can transport plasma iron into red cells and that the transport is dependent on the concentration of transferrin. Iron taken up by the blood cells could not be eluted by subsequent incubation with iron-free transferrin solutions. More recently Morgan and Laurel (99) reported that iron uptake in reticulocytes is independent of the transferrin concentration. The iron complex of serum transferrin has a higher affinity for immature red cells than does the iron-free protein (72). Both bind specifically to immature red cells and the attachment permits the cells to remove the iron. Once the iron is removed, however, the iron-free transferrin can be replaced by an iron-transferrin complex. 

Iron removal Increased iron and ferritin levels are found in approx. 30% of patients with chronic hepatitis B or C. Several studies have shown that the success rate of interferon therapy is reduced in the presence of elevated liver iron values. This is attributed to the fact that iron overload inhibits not only lymphocyte proliferation, but also the function of killer cells and B cells as well as the production of antibodies. Iron plays a role in the formation of free radicals and the occurrence of dangerous lipid peroxidations, (s. pp 68, 401) Furthermore, iron, like oxygen radicals, promotes fibrogenesis. Iron removal leads to an improvement in laboratory parameters and better response to interferon-a therapy. (217, 243) On the other hand, the iron level is reduced as a result of successful IFN therapy. In the case of a higher serum iron status before the initiation of interferon therapy, venesections at one week intervals should be considered, if necessary until normal laboratory values (iron, ferritin, transferrin saturation) have been restored. During interferon therapy, a low-iron diet is advisable, as is the consumption of 2 x 1 cup of black tea (in the morning and at noon) to reduce iron absorption through chelate formation ( cheap, free of side effects and useful )- (s. p. 625) Silymarin also leads to iron mobilization due to chelate formation. 

Deferoxamine is a highly selective chelator of iron that theoretically binds ferric (Fe +) iron in a 1 1 molar ratio (100 mg deferoxamine to 8.5 mg ferric iron) that is more stable than the binding of iron to transferrin. Deferoxamine removes excess iron from the circulation and some iron from transferrin by chelating ferric complexes in equilibrium with transferrin. The resulting iron-deferoxamine complex, ferrioxamine, is then excreted in the urine. Its action on intracellular iron is unclear, but it may have a protective intracellular effect or may chelate extramitochondrial iron. The parenteral administration of deferoxamine produces an orange-red-colored urine within 3 to 6 hours because of the presence of ferrioxamine in the urine. For mild to moderate cases of iron poisoning, where its use is unclear, the presence of discolored urine indicates the persistent presence of chelatable iron and the need to continue deferoxamine. The reliance on discolored urine as a therapeutic end point has been challenged because it is not sensitive and is difficult to detect. ... 

Deferoxamine is isolated as the iron chelate from Streptomyces pilosus and is treated chemically to obtain the metal-free ligand. Deferoxamine has the desirable properties of a remarkably high affinity for ferric iron K = 10 M ) coupled with a very low affinity for calcium K =10 M" ). Studies in vitro have shown that it removes iron from hemosiderin and ferritin and, to a lesser extent, from transferrin. Iron in hemoglobin or cytochromes is not removed by deferoxamine. 

The biphasic kinetic pattern described for the removal of iron from transferrin by pyrophosphate can be ascribed to the two different iron-containing sites in transferrin/ Various anions and acids can assist such removal/ Details of iron removal have been probed by studying the kinetics of metal removal from transferrin derivatives containing iron and cobalt variously distributed between the two inequivalent binding sites, and from transferrins containing iron in only one of the two sites. The kinetics of iron removal from the Fee sites show a first-order dependence on pyrophosphate concentration, from the FeN sites show saturation kinetics. The current situation with respect to mechanisms of iron removal from, and incorporation into, transferrin have been reviewed. ... 

Figure 8. Spectral changes accompanying iron removal from transferrin. The bottom curve represents the unreacted transferrin ami the top curve the final product,...

The pM values of most of the catecholate ligands are well above that of transferrin, indicating that iron removal is thermodynamically favored. The question remains, however, as to the rate of this exchange reaction. Therefore we have investigated the kinetics of iron removal from transferrin by these types of catecholate ligands. The addition of 3,4-LICAMS to diferric transferrin results in the series of spectra shown in Figure 8. 

Previous results have indicated that iron removal from transferrin might Involve the formation of a ternary iron-transferrin-llgand intermediate, which dissociates into FeL and apotransferrin (12, 13). Such a scheme is outlined in Eq. 9. 

Figure 9. Iron removal from 0.2mM diferric transferrin by various concentrations of 3,4-LICAMS...

Figure 10. Plot of the observed rate constant for iron removal from transferrin 0.4mM) vs. the concentration of 3,4-LICAMS. The points represent the experimental data, the line is calculated from the derived rate constants.

Relative kinetics of iron removal from human transferrin by several iron sequestering agents. 

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