Copper complexes at N- and C-site of ovotransferrin: quantitative determination and visible absorption spectrum of each complex

Copper binding selectivity of N- and C-sites in serum (human)- and ovo-transferrin

Copper binding selectivity of the N- and C-sites in serum (human)- and ovo-transferrin was investigated through copper binding constants, copper dissociation rate constants, and EPR spectra. At pH 7.4, stepwise copper binding constants of serum (human)-transferrin were K1 = 1.8 (+/- 0.6) x 10(12) M-1 and K2 = 1.2 (+/- 0.5) x 10(11) M-1, and those of ovo-transferrin were K1 = 1.9 (+/- 0.5) x 10(11) M-1 and K2 = 2.1 (+/- 0.4) x 10(11) M-1. Absorbance changes resulting from copper binding to the C- or N-site at various ratios of Cu2+/apo-transferrin were separated by a kinetic method. It was clearly indicated that, in serum (human)-transferrin, the copper binding affinity for the C-site was much larger than that for the N-site, whereas in ovo-transferrin, the C- and N-sites have almost the same affinity for copper ions. In the presence of anions (0.1 M KCl or 0.1 M NaClO4), the stepwise copper binding constants of serum (human)-transferrin were almost 10-times smaller than those in the absence of the anions. The selectivity in binding the copper ions to both sites of serum (human)-transferrin in the presence of 0.1 M NaClO4 is much smaller than that in the presence of 0.1 M KCl or in the absence of the anions (0.1 M KCl and 0.1 M NaClO4). EPR spectra of the copper ions of the N-site in dicupric serum-transferrin are dramatically changed respectively by the addition of 0.1 M KCl, 0.1 M NaCl, and 0.1 M NaClO4. This suggests that the change in the coordination geometry of the copper ions occurs at the N-site.

Malondialdehyde formation in liver mitochondrial and microsomal fractions of vitamin E- and selenium-deficient rats as a function of alpha-tocopherol level: the effect of treatment in vitro with iron

Weanling rats were given diets with adequate vitamin E and selenium or deprived of one or the other or both, nutrients. After 28 days, liver mitochondrial and microsomal fractions were prepared and alpha-tocopherol (alpha-T) and selenium measured. alpha-Tocopherol fell by eight-fold in the doubly deficient rats and selenium fell three-fold. Malondialdehyde (MDA) was found to be undetectable by a sensitive HPLC method. The fractions were subjected to peroxidative stress in in vitro using 0.5 mM Fe2+/10 mM ADP, and MDA and alpha-T were measured at intervals during 30 min. The results showed that in the mitochondrial fractions there was a lag time of at least 2 min before peroxidation became significant, during which time most of the alpha-T was consumed. In the microsomal fraction the lag phase was very short prior to the establishment of a linear rate of peroxidation, although little alpha-T was used up. It was concluded that the mitochondrial fraction withstood the peroxidative challenge better than the microsomal fraction even though the initial level of alpha-T in the microsomal fraction was about double that in the mitochondrial fraction. Selenium deficiency had no effect on the length of the lag phase of the fractions which therefore appears to be a characteristic of mitochondrial or microsomal fractions.

Metal substitution in transferrins: the crystal structure of human copper-lactoferrin at 2.1-A resolution

The structural consequences of binding a metal other than iron to a transferrin have been examined by crystallographic analysis of human copper-lactoferrin, Cu2Lf. X-ray diffraction data were collected from crystals of Cu2Lf, using a diffractometer, to 2.6-A resolution, and oscillation photography on a synchrotron source, to 2.1-A resolution. The structure was refined crystallographically, by restrained least-squares methods, starting with a model based on the isomorphous diferric structure from which the ligands, metal ions, anions, and solvent molecules had been deleted. The final model, comprising 5321 protein atoms (691 residues), 2 Cu2+ ions, 2 (bi)carbonate ions, and 308 solvent molecules has good stereochemistry (rms deviation of bond lengths from standard values of 0.018 A) and gives a crystallographic R value of 0.196 for 43,525 reflections in the range 7.5-2.1-A resolution. The copper coordination is different in the two binding sites. In the N-terminal site, the geometry is square pyramidal, with equatorial bonds to Asp 60, Tyr 192, His 253, and a monodentate anion and a longer apical bond to Tyr 92. In the C-terminal site, the geometry is distorted octahedral, with bonds to Asp 395, Tyr 435, Tyr 528, and His 597 and an asymmetrically bidentate anion. The protein structure is the same as for the diferric protein, Fe2Lf, demonstrating that the closure of the protein domains over the metal is the same in each case irrespective of whether Fe3+ or Cu2+ is bound and that copper could be transported and delivered to cells equally well as iron. The differences in metal coordination are achieved by small movements of the metal ion and anion within each binding site, which do not affect the protein structure.

X.a.f.s. studies of chicken dicupric ovotransferrin

A comparison of Cu K-edge x.a.f.s. spectra with that of the equivalent Fe K-edge for chicken ovotransferrin (COT) indicates that the metal ions occupy essentially the same binding sites in the protein. However, in the case of the Cu2+ complex the metal appears to have reduced co-ordination. Changes are observed in the x.a.f.s. of 90%-saturated COT (Cu1.8COT) on freeze-drying. The three-dimensional X-ray structures of rabbit serum transferrin and human lactoferrin have shown that the ferric cations are co-ordinated by four protein ligands and a bidentate carbonate anion in a distorted octahedral arrangement [Anderson, Baker, Dodson, Norris, Rumball, Waters & Baker (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1768-1774; Anderson, Baker, Norris, Rice and Baker (1989) J. Mol. Biol. 209, 711-734; Bailey, Evans, Garratt, Gorinsky, Hasnain, Horsburgh, Jhoti, Lindley, Mydin, Sarra & Watson (1988) Biochemistry 27, 5804-5812]. This structural information, together with the differences in e.x.a.f.s. spectra for solution and freeze-dried samples of diferric COT [Hasnain, Evans, Garratt & Lindley (1987) Biochem. J. 247, 369-375] suggests that the synergistic carbonate anion may be capable of behaving as a unidentate linkage to the Cu2+ in the dicupric complex. Data for Cu1.8COT are consistent with only three protein ligands bound to Cu2+, monodentate binding of the synergistic anion in one lobe and its bidentate binding in the other lobe. Such flexibility in the anion co-ordination may be a requirement for the uptake and release of metals by the transferrins.

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).

Iron overload and the predisposition of cells to antioxidant consumption and peroxidative damage

We have investigated the effects of iron overload in vivo on the tocopherol levels and the extent of lipid peroxidation in rat liver microsomes and their response to subsequent oxidative stress in vitro. The results demonstrate a direct correlation between consumption of antioxidant defences and the induction and extent of malondialdehyde production in microsomes prepared from iron-loaded rats. The data are consistent with the requirement for iron (II)/iron (III) ratios in lipid peroxidation in control microsomes.

Kinetics of pyrophosphate induced iron release from diferric ovotransferrin

The kinetics of pyrophosphate-induced iron release from diferric ovotransferrin were studied spectrophotometrically at 37 degrees C in 0.1 M HEPES, pH 7.0. At high pyrophosphate concentrations, the kinetics are biphasic, indicating that the rates of iron release from the two, presumably noninteracting iron-binding sites of ovotransferrin are different. The pseudo-first-order rate constants for iron release from both the fast and slow sites exhibit a hyperbolic dependence on pyrophosphate concentrations. The data suggest that pyrophosphate forms complexes with the two iron-binding sites of ovotransferrin prior to iron removal. The stability constants of the complex formed with the fast site (Keqf) and slow site (Keqs) are 8.3 M-1 and 40.4 M-1, respectively. The first-order rate constants for the dissociation of ferric-pyrophosphate from the fast site (k2f) and the slow site (k2s) are 0.062 and 0.0044 min-1, respectively. Results from urea gel electrophoresis studies suggest that iron is released at a much faster rate from the N-terminal binding site of ovotransferrin. At high pyrophosphate concentration, only C-monoferric-ovotransferrin is detected during the course of iron release. At low pyrophosphate concentration, however, a detectable amount of N-monoferric-ovotransferrin is accumulated. This result is consistent with the kinetic finding that the site with a higher k2 (0.062 min-1) has a lower affinity toward pyrophosphate (Keq = 8.3 M-1) whereas the site with a lower k2 (0.0044 min-1) has a higher affinity for pyrophosphate (Keq = 40.4 M-1).