Lanthanides, transferrin binding

Even though the specific protein that binds the lanthanides may or may not be transferrin, the behavior of high-specific activity lanthanides (< 1 jtM/kg7) compared with that of lanthanides administered with significant amounts of stable carrier strongly suggests the kind of break-through one would expect if a protein transport system were overloaded. As long as the amount of lanthanide introduced into the... 

The lanthanides modify the UV absorption spectra of proteins when aromatic chro-mophores are present at the binding site. Sharp maxima at 245 and 295 nm in the difference spectrum of transferrin saturated with Tb3+ ion as opposed to that of metal free protein were observed [20]. The shape of the spectrum is suggestive of lanthanide induced... 

For lactoferrin, which binds metal ions more strongly than transferrin (166), all the lanthanide ions, from Yb3+ to La3+, bind with two metal ions per molecule (147), with the log K2 values ranging from 6.4... 

Differences between the two sites become more pronounced for metal ions other than Fe " and anions other than COa . The differences are most pronounced for larger metal ions such as lanthanides. For transferrin some of the larger lanthanides appear to bind in only one of the two sites (Section IV.B.3), and for lactoferrin, although binding occurs in both sites, the second metal ion binds much more weakly, as shown by the curvature of the UV difference titration graph (Fig. 18) the biphasic release of Ce from lactoferrin contrasts with that of Fe (Fig. 28). Even metal ions of the first transition series, of similar size to Fe "", enhance the differences between the two sites. When Cr is bound to either transferrin 134) or lactoferrin (154), different EPR signals are seen for the two sites, and one Cr " ion is much more readily displaced by Fe than the other. Likewise, the EPR spectra of VO " -substituted transferrin indicate different metal configurations in the two sites (207), as do NMR studies of Co -substituted ovotransfer-rin (139). In these cases one metal ion is also released much more readily than the other as the pH is lowered. 

The mechanisms of actinide/lanthanide binding to transferrin may not be the same as that for ironflll)... 

In addition to binding and transporting Fe + ions, transferrin is also responsible for binding certain of the actinide and lanthanide ions in viva, much evidence having been gleaned from animal studies by Taylor et al. (1987) and Taylor (1991). These studies have shown transferrin to bind Pu, Np, Pa in rat plasma. Evidence for Th(IV) binding to transferrin has been obtained (Duilield and Taylor 1986) with indications that this protein also binds Cm ", Eu " Gd " and Yb " in plasma (Taylor... 

UV-difference spectroscopy, and other studies in vitro with plutonium (IV), tho-rium(IV) and a number of trivalent lanthanides, have demonstrated that, like iron, two lanthanide or actinide metal atoms are bound per transferrin molecule (Duffield and Taylor 1986, Taylor et al. 1991, Zak and Aisen 1988). Saturation of the lanthanide- or actinide-containing transferrin with excess iron results in a liberation of the f element, thus suggesting that these metals are binding to the two iron-binding sites on the transferrin molecule (Taylor et al. 1991). These studies have also shown that bicarbonate is necessary, as a synergistic anion, for the binding of actinides and lanthanides to transferrin. 

The observations described above have important implications with regard to actinide/lanthanide distributions within the body. It is apparent that in binding to transferrin, the f elements are participating in certain aspects of the iron transport pathways in vivo. However, no plutonium, e.g., is found within red blood cells following incorporation and there is no unequivocal evidence that plutonium and the other actinides or lanthanides are transported into cells via transferrin-receptor-mediated endocytosis (Duffield and Taylor 1986). This, too, is a puzzling aspect of f-element-transferrin chemistry and biochemistry which needs more study. 

The mechanisms of actinide/lanthanide binding to transferrin may not be the same as that for iron( III). Spectroscopic studies by Duffield and Taylor(1987) have shown the fine details of Pu(IV) and Th(IV) binding to transferrin to be very similar to that of Fe(III). 

The physiological result of the binding of the non-essential lanthanide and actinide metals to albumin, or transferrin, in the blood plasma may well be that the metals are held in a form in which they are virtually unable to penetrate the cell membrane in ionic form thus limiting cellular uptake and also the ability to cause harmful effects by interaction with essential enzymes or other proteins. Thus for the non-essential f elements, protein binding may be regarded as part of a protective mechanism against chemical toxicity,... 

Transferrin and lactoferrin belong to a family of Fe cation buffer and carrier proteins. X-ray crystallography has shown that both proteins include two distorted octahedral Fe + sites, each of which is coordinated with one aspartate, one histidine, and two tyrosinate residues as well as one exogenous carbonate. Luminescent lanthanides such as Tb + and Eu + cations can be incorporated along with Ee cations into these binding sites to provide water-soluble luminescent lanthanide complexes in sophisticated three-dimensional cages. 

The Job method has been used to show that horse-liver alcohol dehydrogenase contains zinc atoms bound in two different environments, and it appears that Zn + plays a functional role in the interaction of DNA with DNA polymerases from E. coli and sea urchins. The nature of the two metal-binding sites in transferrin has been investigated, using tervalent lanthanide ions as fluorescent probes the two sites are different and the distance between them is 43 A or greater. The kinetics of the reaction between Fe -nta and transferrin have been measured by the stopped-flow method and a reaction sequence has been proposed. The same technique, with C1 n.m.r. detection, has been used to study the extraction of the metal from the Hg -bovine serum albumin complex by various ligands. 

---