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G Taborsky

Publications and source records attributed to G Taborsky.

At least 19 recordsLinked to original sources
PubMed1991

On the interaction of phosvitins with ferric ion: solubility of the Fe(III)-phosphoprotein complex under acidic conditions is a function of the iron/phosphate ratio and the degree of phosvitin phosphorylation.

The interaction of phosvitins, the polyphosphoproteins of the eggs of egg-laying vertebrates, with ferric chloride was investigated under acidic conditions at iron-to-protein phosphorus ratios ranging up to 10. Phosvitins of which all or nearly all serine residues are phosphorylated (P/ser greater than 0.8) precipitate when titrated with the iron salt. As the total Fe/P ratio reaches the value of about 0.5, precipitation becomes maximal. At Fe/P ratios above 0.5, the Fe(III)-phosvitin complex becomes increasingly soluble. At ratios above 2, solubility is essentially fully restored. Phosvitins with an appreciable portion of their serine residues non-phosphorylated (P/ser less than 0.7) show a different dependence of solubility on the Fe/P ratio. The Fe/P ratios of all precipitated complexes themselves vary within a narrow range between about 0.4 and about 1.0; the total Fe/P ratio is varied between 0 and 10. The results imply that phosvitin iron binding sites are non-uniform and that, overall, phosvitin is capable of accommodating iron in different ways, depending on the relative magnitude of the iron load and the availability of phosphoserine clusters in the phosphoprotein.

Animals↗
PubMed1990

Phosphorus nuclear magnetic resonance of diverse phosvitin species.

1. High resolution 31P nuclear magnetic resonance (NMR) spectra, with and without proton decoupling, of the principal egg phosphoproteins--phosvitins--of a bird (Gallus gallus), an amphibian (Xenopus laevis) and a fish (Salmo gairdneri) were obtained. 2. The spectra were evaluated with special reference to available amino acid sequences and the major NMR resonance in all three spectra was assigned to phosphoserine clusters. 3. The resolution of numerous additional phosphorus resonances provides the basis for further investigation of the particular molecular environments of phosvitin-bound phosphoryl groups and their involvement in the diverse binding modes for metal complex formation by phosvitins.

Amino Acids↗
PubMed1987

Iron binding by phosvitins: variable mechanism of iron release by phosvitins of diverse species characterized by different degrees of phosphorylation.

The rate of reductive iron release from Fe(III) complexes of phosvitins of diverse fish species, at varied initial degrees of saturation with iron, was studied with particular attention to the effect of the degree of phosvitin phosphorylation on the kinetics of iron release. The reaction was followed colorimetrically as phosphorprotein-bound iron was transferred to an excess of o-phenanthroline, in the presence of hydroquinone as a reducing agent. The principal finding was the variability of the kinetic order or iron release by phosvitins, depending on their degree of saturation with iron and the extent to which their serine residues were phosphorylated. Highly phosphorylated proteins, especially at high initial degrees of iron saturation, obey first-order kinetics. Partially phosphorylated proteins, especially at low initial degrees of iron saturation, release their iron in a zero-order fashion. First-order rates imply that the iron binding sites are kinetically independent of each other. Zero-order behavior appears to reflect iron release from hypothetical iron-binding clusters serving as kinetically effective reactive centers of unchanging concentration for most of the time course of the reaction. Variations of the initial degree of iron saturation of given phosvitins produced variations in their kinetic behavior. The results are considered in terms of a dynamic model of phosvitin iron binding sites which may constitute themselves diversely, in response to the amount of iron that is to be accommodated, or may reconstitute themselves as their molecular environment becomes altered.

Animals↗
PubMed1986

Iron binding by phosvitin: variation of rate of iron release as a function of the degree of saturation of iron binding sites.

The rate of iron release from Fe(III)-phosvitin complexes, at varied degrees of saturation, was studied. Iron release was induced by reduction in the presence of the ferrous ion chelator, o-phenanthroline. If iron release was induced photochemically (without a chemical reductant), the reactions proceeded in zero order fashion, independently of the degree of saturation but with a strong dependence on the concentration of phenanthroline. When hydroquinone was added and the reactions were conducted in the dark, iron release followed first-order kinetics and the rate constants showed a clear dependence on the degree of saturation of the protein, which was most marked at lower levels of saturation. The results imply control of iron release by binding site differences produced by different intramolecular environments as the protein provides different combinations of its phosphoserine groups as ligands depending on the number of iron atoms to be accommodated per protein molecule.

Animals↗
PubMed1986

Phosvitin isolation from fish eggs: methodological improvements including 'specific' phosvitin precipitation with ferric ion.

Modifications of the method of Wallace et al. [Can. J. Biochem. 44, 1647-1655 (1966)] for phosvitin isolation from vertebrate eggs were devised to enhance the method's general effectiveness. Phosvitins which do not precipitate on dilution from solutions of their lipovitellin complexes may be selectively adsorbed onto, and desorbed from, DEAE-cellulose. Phosvitins which are too small for dialysis or ultrafiltration may be concentrated or desalted by precipitation with stoichiometric amounts of ferric ions, followed by iron removal with EDTA on gel filtration. Since phosvitin distribution among ammonium sulfate fractions depends on initial protein and salt concentrations in a species-specific manner, pilot experiments are needed to establish conditions for optimal fractionation.

Amino Acids↗
PubMed1983

Determination of protein-bound phosphate by continuous flow analysis: automated procedure for the determination of alkali-labile phosphate.

A method is described for the specific, quantitative determination of protein-bound phosphorus by a continuous flow procedure using a Technicon AutoAnalyzer. It is based on the exceptional alkali lability of serine phosphate linkages to beta-elimination when the serine residues are present in a polypeptide chain. The results are reproducible within about 3, 5, or 10%, respectively, when the analytical sample contains about 100, 10, or 3 nmol of protein-bound P. The presence of less than 1 nmol protein-bound P can be detected. The method tolerates wide variations of the pH and ionic composition of the sample, making it suitable for the automatic, serial analysis of chromatographic effluent fractions. Low-molecular-weight phosphomonoesters, ribonucleic acid (phosphodiester), and nucleotide phosphates (pyrophosphate) do not react measurably. Carboxyphenyl phosphate is partially hydrolyzed (10-15%). In contrast, the release of P from various phosphoproteins is quantitative.

Alkalies↗
PubMed1983

Phosvitin.

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Animals↗
PubMed1980

Iron binding by phosvitin and its conformational consequences.

With a view to the potential biological significance of iron binding by the phosphoprotein phosvitin, the interaction of these two electrostatically complementary constituents of egg yolk particles was studied by ultrafiltration, circular dichroism, and sedimentation. Ferric complexes of phosvitin are strong and stable; ferrous complexes are weak and dissociate readily. When saturated, pairs of the approximately 135 phosphate groups of a phosvitin molecule appear to bind 1 iron atom each. These findings confirm and extend previous reports regarding the ferric complex and characterize the ferrous complex for the first time. The iron binding sites are not equivalent. Contrary to previous speculations, iron binding is not accompanied by a conformational change from an unordered structure to one of the beta-type. Apparently, neutralization of negative charges, while necessary, is not a sufficient condition of this transition. Nevertheless, iron affects phosvitin structure. Above pH 2, where the protein is unordered but adjusts its average conformation to changes in its net charge as the pH of its solution is varied, iron mimics the effect of protons quantitatively. Near pH 2, where the beta-type conformation is readily acquired by the protein in the absence of iron, the consequences of iron binding upon conformation are determined by the manner in which the iron-phosvitin interaction is brought about. Either the extent of the transition to the "normal" beta-structure becomes limited or the nature of the resulting conformation becomes modified. It is noteworthy that ellipticity changes in the presence of iron do not necessarily occur in parallel at about 200 and 215 nm as they do when the transconformation is produced by pH changes alone. The binding of iron appears to be mostly intramolecular. Intermolecular cross-links become dominant only if most binding sites are filled and then only as a secondary event subsequent to binding and the initial conformational adjustment.

Animals↗
PubMed1979

Interaction of cytochrome c, ferrous ion, and phosphate. Electron transfer within a stoichiometric complex.

The rate and extent of electron transfer from ferrous ion to ferricytochrome c are enhanced by the presence of inorganic orthophosphate at concentrations comparable to those of reductant and oxidant. Evidence, obtained by the method of continuous variations, shows that the electron transfer occurs within a stoichiometric complex composed of cytochrome c, ferrous ion, and phosphate in molar proportions of about 1:1:1. The incorporation of the anion into this complex appears to result in a modulation of the extent and rate of cytochrome c reduction. The rate of electron transfer obeys a first order rate law, characterized by an apparent first order rate constant of 1.4 min-1. The complex has kinetic significance only; equilibrium dialysis, gel filtration, and sedimentation velocity experiments yielded no evidence for stable binding of phosphate and iron, or of aggregation, on a significant scale. The extent of reduction is limited (for reasons not yet known) to about one-half of the available cytochrome molecules. Reduction in excess of 50% can be achieved only when both, ferrous ion and phosphate, are present in excess of the cytochrome concentration. Kinetic data indicate that reduction to extents over and below 50% occurs by different mechanisms.

Animals↗
PubMed1974

Phosphoproteins.

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Adenosine Triphosphatases↗