Vanadyl(IV) conalbumin. I. Metal binding site configurations

Biological sulfur in the blood cells of Ascidia ceratodes: XAS spectroscopy and a cellular-enzymatic hypothesis for vanadium reduction in the ascidians

Two samples of living blood cells and of cleared blood plasma from the Phlebobranch tunicate Ascidia ceratodes from Bodega Bay, California, and one of fresh Henze solution from A. ceratodes of Monterey Bay, California, have been examined using sulfur K-edge x-ray absorption spectroscopy (XAS). Biological sulfur included sulfate esters, sulfate and bisulfate ions, benzothiazole, thianthrene, epi-sulfide, thiol and disulfide. Glutathione dominated reduced sulfur, from which an average intracellular Voltage of -0.21 V was calculated. Sulfate-bisulfate ratios yielded blood cell pH values of 2.0 and 2.8. Total blood cell [sulfur] was 373±9 mM or 296±73 mM from BaSO4 gravimetry. Two plasma samples (pH 6.9 or 7.0; [S] = 33±6 mM or 26±4 mM) were dominated by sulfate and disulfide. Fresh Henze solution evidenced a sulfur inventory similar to blood cells, with calculated pH = 2.7. A V(III)-sulfonate fraction varied systematically with intracellular pH across six independent blood cell samples, implying a vanadium mobilization pathway. Bodega Bay and Monterey Bay A. ceratodes appear to maintain alternative suites of low-valent sulfur. The significance of the vanabins to vanadium metabolism is critically examined in terms of known protein - V(IV) biochemistry. Finally, a detailed hypothesis for the reduction of [VO4]3- to V(III) in ascidians is introduced. A vanadium oxido-reductase is proposed to span the signet ring membrane and to release V(III) into the inner acidic vacuole. The V(V) to V(III) reduction is predicted require an inner-sphere mechanism, a thiol reductant, 7-coordinate V(III), a biologically accessible Voltage, and proton-facilitated release of V(III).

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.

In vivo and in vitro studies of hafnium-binding to rat serum transferrin

The binding of hafnium to rat serum transferrin was studied using the time differential perturbed angular correlation (TDPAC) technique. Hafnium is interesting as a toxic metal binding to transferrin because it behaves metabolically similarly to plutonium. The isotope 181Hf offers favorable access to the TDPAC-method. Samples were prepared in vivo by intravenous injection of Hf-NTA, Hf-citrate, and Hf-oxalate solutions, respectively, into Sprague-Dawley rats and in vitro by adding Hf-NTA solution to fresh rat serum. In both cases two specific electric quadrupole interactions were observed, which correspond to two well-defined binding configurations. They may be attributed to the N-terminal and the C-terminal binding site in the transferrin molecule. The 181Hf-distribution between these two binding states depends on pH, salt and hafnium concentrations, temperature, and incubation time. With a fast TDPAC-setup of four BaF2-detectors a time resolution of about 600 ps could be achieved. The specific binding configurations of 181Hf and the comparatively slow relaxation times lead to spectra of considerable accuracy.

Estimation of the ferrous-transferrin binding constants based on thermodynamic studies of nickel(II)-transferrin

The equilibrium constants for the binding of Ni2+ to human serum transferrin in 0.01 M hepes containing 5 mM sodium bicarbonate at 25 degrees C and pH 7.4 have been measured. The effective binding constants are log K1 = 4.10 +/- 0.15 and log K2 = 3.23 +/- 0.31 for the reactions Ni2+ + apoTr (K1) in equilibrium Ni2+-Tr. Ni2+ + Ni2+-Tr (K2) in equilibrium Ni2+-Tr-Ni2+ where the explicit terms for bicarbonate and hydrogen ion have been incorporated into the effective binding constants. Titration of both forms of mono(ferric)transferrin indicates that unlike other metal ions, Ni2+ binds preferentially to the N-terminal binding site, but that the site preference is rather small. A linear-free-energy relationship (LFER) for the complexation of Ni2+ and Fe2+ has been prepared. This LFER has been used to estimate effective binding constants of log K1 = 3.2 and log K2 = 2.5 for the ferrous-transferrin complex. These ferrous constants have been combined with the literature binding constants for ferric-transferrin to estimate formal reduction potentials of -340 mV vs. NHE for the C-terminal site and -280 mV for the N-terminal site.

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.

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.