The metal bonding domain of the antitumor drug Fe(II)-bleomycin: a DFT investigation

Air pollutants disrupt iron homeostasis to impact oxidant generation, biological effects, and tissue injury

Air pollutants cause changes in iron homeostasis through: 1) a capacity of the pollutant, or a metabolite(s), to complex/chelate iron from pivotal sites in the cell or 2) an ability of the pollutant to displace iron from pivotal sites in the cell. Through either pathway of disruption in iron homeostasis, metal previously employed in essential cell processes is sequestered after air pollutant exposure. An absolute or functional cell iron deficiency results. If enough iron is lost or is otherwise not available within the cell, cell death ensues. However, prior to death, exposed cells will attempt to reverse the loss of requisite metal. This response of the cell includes increased expression of metal importers (e.g. divalent metal transporter 1). Oxidant generation after exposure to air pollutants includes superoxide production which functions in ferrireduction necessary for cell iron import. Activation of kinases and phosphatases and transcription factors and increased release of pro-inflammatory mediators also result from a cell iron deficiency, absolute or functional, after exposure to air pollutants. Finally, air pollutant exposure culminates in the development of inflammation and fibrosis which is a tissue response to the iron deficiency challenging cell survival. Following the response of increased expression of importers and ferrireduction, activation of kinases and phosphatases and transcription factors, release of pro-inflammatory mediators, and inflammation and fibrosis, cell iron is altered, and a new metal homeostasis is established. This new metal homeostasis includes increased total iron concentrations in cells with metal now at levels sufficient to meet requirements for continued function.

Investigation of the effects of alpha-tocopherol on the levels of Fe, Cu, Zn, Mn, and carbonic anhydrase in rats with bleomycin-induced pulmonary fibrosis

This study was designed to examine the effects of vitamin E on the levels of Zn, Mn, Cu, Fe, and carbonic anhydrase in rats with bleomycininduced pulmonary fibrosis. Twenty-one male Wistar albino rats were randomly divided into three groups: bleomycin alone, bleomycin+vitamin E, and saline alone (control group). The bleomycin group was given 7.5 mg/kg body weight (single dose) bleomycin hydrochloride intratracheally. The bleomycin+vitamin E group was also instilled with bleomycin hydrochloride but received injections of alpha-tocopherol twice a week. The control group was treated with saline alone. Animals were sacrified 14 d after intratracheal instillation of bleomycin. Tissue Zn, Mn, Cu, Fe, and carbonic anhydrase activities were measured in the lung and liver. Lung Cu, Fe, and carbonic anhydrase activity increase in both experimental groups. Zn and Mn levels decreased, except for the Mn level in the bleomycin group. Liver Zn, Mn, and Cu levels decreased in both experimental groups compared to the control group, whereas Fe and carbonic anhydrase activity increased in comparison to the control group. However, the liver tissue Fe level decreased compared to the control group. In the histopathologic assesment of lung sections in the bleomycin+vitamin E group, partial fibrotic lesions were observed, but the histopathologic changes were much less severe compared to the bleomycin-treated group.

Proton-shuffle mechanism of O-O activation for formation of a high-valent oxo-iron species of bleomycin

Bleomycins (BLMs) can utilize H2O2 to cleave DNA in the presence of ferric ions. DFT calculations were used to study the mechanism of O-O bond cleavage in the low-spin FeIII-hydroperoxo complex of BLM. The following alternative hypotheses were investigated using realistic structural models: (a) heterolytic cleavage of the O-O bond, generating a Compound I (Cpd I) like intermediate, formally BLM-FeV=O; (b) homolytic O-O cleavage, leading to a BLM-FeIV=O species and an OH* radical; and (c) a direct O-O cleavage/H-abstraction mechanism by ABLM. The calculations showed that (a) is a facile and viable mechanism; it involves acid-base proton reshuffle mediated by the side-chain linkers of BLM, causing thereby heterolytic cleavage of the O-O bond and generation of Cpd I. Formation of Cpd I is found to involve a barrier of 13.3 kcal/mol, which is lower than the barriers in the alternative mechanisms (b and c) that possess respective barriers of 31 and 17 kcal/mol. The so-formed Cpd I species with a radical on the side-chain linker, methylvalerate (V), adjacent to the BLM-FeIV=O complex, resembles the formation of the active species of cytochrome c peroxidase in the Poulos-Kraut proton-shuffle mechanism in heme peroxidases (Poulos, T. L.; Kraut, J. J. Biol. Chem. 1980, 255, 8199-8205). Experimental data are discussed and shown to be in accord with this proposal. It suggests that the high-valence Cpd I species of BLM participates in the DNA cleavage. This is an alternative mechanistic hypothesis to the exclusive reactivity scenario based on ABLM (FeIII-OOH).

DFT analysis of interligand vibrations in a hydroperoxo complex of cobalt bleomycin

Density functional theory (DFT) has been applied to the analysis of interligand vibrations in two chiral isomers of hydroperoxo complex of cobalt bleomycin (BLM-Co(III)-OOH, BLM = bleomycin). The DFT-based normal coordinate analysis reveals that 16O/18O isotope-sensitive modes associated with the Co-OOH moiety uniquely reflect the chiral organization of ligands around the cobalt atom. This study provides an independent probe of cobalt chirality coordinated to BLM and shows that interligand modes associated with the Co-OOH moiety could be used as a structural marker of the chiral isomers.

Oxidative properties of FeO2+: electronic structure and solvation effects

An electronic structure analysis is provided of the action of solvated FeO(2+), [FeO(H(2)O)(5)](2+), as a hydroxylation catalyst. It is emphasized that the oxo end of FeO(2+) does not form hydrogen bonds (as electron donor and H-bond acceptor) with H-bond donors nor with aliphatic C-H bonds, but it activates C-H bonds as an electron acceptor. It is extremely electrophilic, to the extent that it can activate even such poor electron donors as aliphatic C-H bonds, the C-H bond orbital acting as electron donor in a charge transfer type of interaction. Lower lying O-H bonding orbitals are less easily activated. The primary electron accepting orbital in a water environment is the 3sigma*alpha orbital, an antibonding combination of Fe-3d(z(2)) and O-2p(z), which is very low-lying relative to the pi*alpha compared with, for example, the sigma* orbital in O(2) relative to its pi*. This is ascribed to relatively small Fe-3d(z(2)) with O-2p(z) overlap, due to the nodal structure of the 3d(z(2)).The H-abstraction barrier is very low in the gas phase, but it is considerably enhanced in water solvent. This is shown to be due to strong screening effects of the dielectric medium, leading to relative destabilization of the levels of the charged [FeO(H(2)O)(5)](2+) species compared to those of the neutral substrate molecules, making it a less effective electron acceptor. The solvent directly affects the orbital interactions responsible for the catalytic reaction.

Mechanism of Bleomycin Suicide: A Car−Parrinello Molecular Dynamics Investigation

Using first-principles molecular dynamics simulations (Car-Parrinello method) we investigated the possible reaction pathways for decay of the active bleomycin-Fe(III)-OOH complex, so-called bleomycin suicide. The theoretical model of activated bleomycin contains the whole metal bonding domain of the bleomycin ligand. Simulations performed both in a vacuum and in water show that a facile decaying process involves a homolytic O-O bond cleavage with an almost simultaneous hydrogen atom abstraction. The formation of an intra- or intermolecular hydrogen bond appears to be crucial for the decay of the activated bleomycin. We did not observe any evidence of heterolytic cleavage of the O-O bond of the Fe(III)-OOH species.

Direct Hydrogen-Atom Abstraction by Activated Bleomycin: An Experimental and Computational Study

Bleomycin (BLM), a glycopeptide antibiotic chemotherapy agent, is capable of single- and double-strand DNA damage. Activated bleomycin (ABLM), a low-spin Fe(III)-OOH complex, is the last intermediate detected prior to DNA cleavage following hydrogen-atom abstraction from the C-4' of a deoxyribose sugar moiety. The mechanism of this C-H bond cleavage reaction and the nature of the active oxidizing species are still open issues. We have used kinetic measurements in combination with density functional calculations to study the reactivity of ABLM and the mechanism of the initial attack on DNA. Circular dichroism spectroscopy was used to directly monitor the kinetics of the ABLM reaction. These experiments yield a deuterium isotope effect, kH/kD approximately 3 for ABLM decay, indicating the involvement of a hydrogen atom in the rate-determining step. H-atom donors with relatively weak X-H bonds accelerate the reaction rate, establishing that ABLM is capable of hydrogen-atom abstraction. Density functional calculations were used to evaluate the two-dimensional potential energy surface for the direct hydrogen-atom abstraction reaction of the deoxyribose 4'-H by ABLM. The calculations confirm that ABLM is thermodynamically and kinetically competent for H-atom abstraction. The activation and reaction energies for this pathway are favored over both homolytic and heterolytic O-O bond cleavage. Direct H-atom abstraction by ABLM would generate a reactive Fe(IV)=O species, which would be capable of a second DNA strand cleavage, as observed in vivo. This study provides experimental and theoretical evidence for direct H-atom abstraction by ABLM and proposes an attractive mechanism for the role of ABLM in double-strand cleavage.