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Biomedical subjects

G Czapski

Publications and source records attributed to G Czapski.

At least 55 records · Page 3Linked to original sources
PubMed1987

Oxidation-reduction reactions of iron bleomycin in the absence and presence of DNA.

Using the pulse radiolysis technique, we have demonstrated that bleomycin-Fe(III) is stoichiometrically reduced by CO2- to bleomycin-Fe(II) with a rate of (1.9 +/- 0.2) x 10(8) M-1s-1. In the presence of calf thymus DNA, the reduction proceeds through free bleomycin-Fe(III) and the binding constant of bleomycin-Fe(III) to DNA has been determined to be (3.8 +/- 0.5) x 10(4) M-1. It has also been demonstrated that in the absence of DNA O2- reacts with bleomycin-Fe(III) to yield bleomycin-Fe(II)O2, which is in rapid equilibrium with molecular oxygen, and decomposes at room temperature with a rate of (700 +/- 200) s-1. The resulting product of the decomposition reaction is Fe(III) which is bound to a modified bleomycin molecule. We have demonstrated that during the reaction of bleomycin-Fe(II) with O2, modification or self-destruction of the drug occurs, while in the presence of DNA no destruction occurs, possibly because the reaction causes degradation of DNA.

Bleomycin↗
PubMed1986

The role and mechanism of metal ions and their complexes in enhancing damage in biological systems or in protecting these systems from the toxicity of O2-.

Copper complexes of 1,10-phenanthroline and some substituted 1,10-phenanthroline cleave DNA in the presence of a reducing agent and molecular oxygen. Generally, the damage is attributed to hydroxyl radicals which are formed through the Haber-Weiss reaction. It is assumed that this reaction occurs with the ternary metal complexes with the biological target and the mechanism is defined as the "site specific mechanism." In these systems, O2- drives the cycle through the reduction of copper(II). On the other hand, these same copper complexes catalyze the dismutation of O2- and thus should protect the systems from O2- toxicity. In this article, the toxicity of these complexes is explained on kinetic grounds. A general discussion on the various factors which could cause the metal ions or their complexes to act either as protectors from O2- toxicity or as sensitizers of toxic effects of O2- is given.

Biological Transport, Active↗
PubMed1986

When do metal complexes protect the biological system from superoxide toxicity and when do they enhance it?

Many copper and iron complexes can be reduced by O2- as well as by H2O2. According to the rates of reduction and the concentration of O2- and H2O2, the metal complexes may serve either as catalyst of O2- dismutation or as catalysts of the reaction between O2- and H2O2 to form OH. radical (Haber-Weiss reaction). Various factors which influence whether metal complexes protect the biological systems from superoxide toxicity or enhance it are discussed.

Chelating Agents↗
PubMed1985

Mechanism and reaction products of the oxidation of Cu(I)-phenanthroline by H2O2.

We have suggested a possible reaction mechanism for the oxidation of the cuprous phenanthroline complex by H2O2 in the presence of formate and methanol. The cuprous phenanthroline complex was generated by pulse and gamma radiolysis. We measured the decay kinetics of this complex as well as the chain length of this reaction. Our observations indicate that in this reaction OH. is not formed directly, but through the decomposition of a metal-peroxo complex. This mechanism does not necessarily operate with other copper compounds, especially with copper complexes bound to a biological target.

Chemical Phenomena↗
PubMed1985

The contribution of endogenous and exogenous effects to radiation-induced damage in the bacterial spore.

Radical scavengers such as polyethylene glycol 400 and 4000 and bovine albumin have been used to define the contribution of exogenous and endogenous effects to the gamma-radiation-induced damage in aqueous buffered suspensions of Bacillus pumilus spores. The results indicate that this damage in the bacterial spore is predominantly endogenous both in the presence of 1 atmosphere of oxygen, and in anoxia.

Animals↗
PubMed1984

Reaction of .OH.

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Animals↗
PubMed1984

Roles of copper and O(2) in the radiation-induced inactivation of T7 bacteriophage.

The effect of copper on the radiation damage induced in T7 bacteriophage has been investigated. The phages were gamma-irradiated and the effects of copper(II) ions in the presence of various additives and radical scavengers were examined in an attempt to better understand the effect of transition metal ions on the role of free radicals, particularly superoxide, in biological damage. The present work extends a study previously done on isolated enzyme to a whole biological entity. Copper(II) ions even at very low concentrations enhanced the lethal effect of radiation. This sensitization was observed in both the presence and the absence of oxygen. The effect of copper could be reverted by chelating agents such as EDTA or 1,10-phenanthroline. Hydrogen peroxide enhanced the sensitizing effect of copper, though little if any protection was provided by catalase or SOD. High molecular weight scavengers of free radicals in the presence of both copper(II) and hydrogen peroxide had no protective effect. (This is in contrast to metal-free systems where, although such scavengers are incapable of penetrating the phages, they protect them against inactivation.) These scavengers, without added H2O2, afforded only slight protection to the irradiated phages in the presence of Cu. Low molecular weight scavengers of free radicals reduced but did not eliminate the sensitizing effect of copper. The sensitizing effect of copper was also observed with other T-odd phages, but not with the T-even series. Copper(II) ions under similar experimental conditions did not sensitize T4 or T2 phages but rather had a protective effect. The results are interpreted in terms of a site-specific Fenton mechanism according to which the binding of the metal ion to the phages is a prerequisite for the occurrence of the biological damage. The results also indicate that most of the copper effect is endogenous. This is in accord with the failure of copper to sensitize the T-even phages, which differ by the rigidity and permeability of their outer coat structures.

Cesium Radioisotopes↗
PubMed1983

On the cytotoxicity of vitamin C and metal ions. A site-specific Fenton mechanism.

The toxicity of ascorbate towards phage lambda and the phages T2-T7 has been investigated. At room temperature the T-odd and lambda bacteriophages are highly susceptible to ascorbate-induced damage, whereas the T-even phages are practically resistant. The toxicity of ascorbate is dependent on the presence of copper (or iron) and oxygen, although oxygen is not required in the presence of H2O2. Hydrogen peroxide is essential for the ascorbate-induced phage inactivation and the damage is prevented by catalase. At the concentrations used, most of the copper ions are bound to the phage particles. Chelating agents such as EDTA or histidine fully protect the phages, whereas salicylate only reduces the rate of phage inactivation. OH scavengers such as sucrose, formate, mannitol, tert-butyl alcohol or poly(ethylene glycol) have no protective effect. Experiments with DNA labeled phages indicate that both phage adsorption and DNA injection are impaired as a result of the exposure to ascorbate and copper. The failure to express the viral genetic information as a result of single and double-strand breaks in the DNA, probably also contribute to the loss of the plaque-forming ability of the phages. The results are interpreted in terms of a 'site-specific' Fenton mechanism according to which the binding of the transition metal ions to the biological target is a prerequisite for the production of damage. The bound metal ion is reduced either by O(2), ascorbate or other reductants and is subsequently reoxidized by H2O2 yielding OH. radicals. This cyclic redox reaction of the metal generates OH. radicals which react with vital macromolecules with a high probability of causing 'multi-hit' damage. This 'site-specific' formation of OH. radicals, which takes place near the target molecules, accounts both for the high damaging efficiency and for the failure of OH. scavengers to protect against it.

Ascorbic Acid↗
PubMed1980

Chain inequivalence in bovine methemoglobin.

Using pulse radiolysis, a single heme in the tetramer of bovine methemoglobin was reduced within a few microseconds to the ferro state, producing a valence intermediate. The kinetics of oxygen binding to the valence intermediate as well as the re-oxidation of the ferro-heme to the ferric state were studied as a function of pH. The kinetics of the oxygenation revealed the existence of two species, characterized by high and low affinities for oxygen that are associated with two quaternary structures (R and T, respectively). A sigmoidal curve representing a transition between the two states as a function of pH was derived. Above pH 7.7 only the R state could be observed, while below pH 6.5 the T state was dominant. The reaction between the valence intermediate and ferricyanide at pH 7.75 (R state) consisted of two (about) equal contributions (k1 = 23 x 10(4) M-1 S-1; k2 = 2.1 x 10(4) M-1 S-1) attributed to the beta and alpha subunits within the tetramer, respectively. At pH 6.3 (T state) a similar phenomenon was observed (k1 = 69 x 10(4) M-1 S-1; k2 = 3.7 x 10(4) M-1 S-1), indicating chain inequivalences both in the T and the R states of methemoglobin. In the presence of inositol hexakisphosphate the T leads to R transition, as monitored by oxygenation of the valence intermediate, was shifted up to a higher pH by about 0.35. Yet similar rate constants exhibiting similar chain inequivalences have been measured.

Animals↗
PubMed1979

Quaternary structure of methemoglobin. Pulse radiolysis study of the binding of oxygen to the valence hybrid.

The pulse radiolysis of solutions of adult human methemogolbin was used in order to reduce a single heme iron within the protein tetramers. The valence hybrids thus formed were reacted with oxygen. Kinetics of the reactions were studied. The effects of pH and inositol hexaphosphate were examined. The kinetics of the ligation of oxygen to stripped valence hybrids showed a single phase behavior at the pH range 6.5 to 9. As the pH was lowered below 6.5, a second, slower phase became apparent. In the presence of inositol hexaphosphate, above pH 8, the kinetics of oxygen binding was of a single phase. As the pH was lowered, a transition to a second, slower phase was noticed. Below pH 7, the slower phase was the only detectable one. The analysis of the relative contribution of the faster phase to the total reaction as a function of the pH showed a typical transition curve characterized by a pK = 7.5 and a Hill parameter n = 2.9. On this basis, it is concluded that human adult stripped methemoglobin resides in an R quarternary structure, while the presence of IHP stabilizes the T structure at pH below 7.5. This transition between the quaternary structures of methemoglobin cannot be accounted for by the switch between the high spin and the low spin states of the ferric iron. This switch of spin state takes place at pH greater than 8.2.

Heme↗
PubMed1978

Do copper ions influence the reduction of ferricytochrome C by O-2?

Recently, it was suggested that the measured rate of reduction of ferricytochrome C by O-2 below pH 8, was too high in the presence of high concentrations of formate (Koppenol, W.H., Van Buuren, K.J.H., Butler J. and Braams, R. (1976) Biochim. Biophys. Acta 449, 157-168). The high values were attributed to the presence of impurities of copper, which compete for O-2. This assumption is consistent with either a decrease in the reduction yield of ferricytochrome C in the presence of copper, or with a very fast reaction of Cu(I) with ferricytochrome C. It was previously shown by us and by others that the reduction yield of ferricytochrome C by O-2 IS 100%. We measured the rate of reduction of ferricytochrome C by Cu(I), and found that this reaction is slow: k = (1.5 +/- 0.5) . 10(3) M-1 . s-1. Therefore, our results rule out the possibility that below pH 8 copper impurities affect the measured rate constant of the reduction of ferricytochrome C by O-2.

Copper↗
PubMed1978

Quaternary states of methemoglobin and its valence-hybrid. A pulse radiolysis study.

Using the pulse radiolysis technique on solutions of stripped adult human methemoglobin, we found that the heme-iron within a single subunit in the tetramer was reduced to iron(II). The valence-hybrid thus formed was reacted with oxygen and with carbon monoxide. Kinetics of the reactions were studied. The effects of pH, inositol hexaphosphate, and temperature on these reactions were examined. The kinetics of the ligation of O2 and CO were used to characterize the affinity states of the valence-hybrid and its parent methemoglobin. Our results support the description of stripped methemoglobin A as residing in an R state. In the presence of inositol hexaphosphate methemoglobin is stabilized in a T state, but it switches into a high affinity state when the pH is raised a0ove 8.0. This structural transition was not found to coincide with the switch of spin state of the heme-iron that accompanies the ionization of water in aquomethemoglobin A.

Adult↗
PubMed1977

The reaction of superoxide radical with iron complexes of EDTA studied by pulse radiolysis.

The reactions of Fe3+-EDTA and Fe2+-EDTA with O2- and CO2- were investigated in the pH range 3.8--11.8. Around neutral pH O2- reduces Fe3+-EDTA with a rate constant which is pH dependent kpH 5.8--8.1 = 2 - 10(6)--5 - 10(5) M-1 - s-1. At higher pH values this reaction becomes much slower. The CO2- radical reduces Fe3+-EDTA with kpH 3.8--1- = 5 +/- 1 - 10(7) M-1 - s-1 independent of pH. At pH 9--11.8, Fe2+-EDTA forms a complex with O2- with kFe2+-EDTA + O2 = 2 - 10(6)--4 - 10(6) M-1 - s-1 which is pH dependent. We measured the spectrum of Fe2+-EDTA-O2- and calculated epsilon 290 over max = 6400 +/- 800 M-1 - cm-1 in air-saturated solutions. In O2-saturated solutions another species is formed with a rate constant of 7 +/- 2 s-1. This intermediate absorbs around 300 nm but we were not able to identify it.

Edetic Acid↗