Synthesis, characterisation and catalase-like activity of dimanganese(III) complexes of 1,5-bis(5-X-salicylidenamino)pentan-3-ol (X=nitro and chloro)

The dimanganese(III,III) complexes [Mn(2)(III)(5-NO(2)-salpentO)(mu-AcO)(mu-MeO)(methanol)(2)]Y (1: Y=Br, 2a: Y=I, 2b: Y=I(3)), [Mn(2)(III)(5-NO(2)-salpentO)(mu-AcO)(mu-MeO)(methanol)(ClO(4))] (3) and [Mn(2)(III)(5-Cl-salpentO)(mu-AcO)(mu-MeO)(methanol)(2)]Br (4), where salpentOH is the symmetrical Schiff base ligand 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. Complex 2b crystallises in the monoclinic system, space group P2(1)/c, and exhibits Mn. . .Mn separation of 2.911 A. This Mn. . .Mn separation is very close to the other characterized (mu-alkoxo)(2)(mu-acetato)Mn(2)(III) complexes of X-salpentOH (X=MeO, Br and H) and reveals that the aromatic substituent has little influence on the geometric parameters of the bimetallic core. A correlation between the electronic character of the different ring substituents, the redox potentials of the dinuclear complexes and their catalase activity was evidenced. Complexes 1-4 show saturation kinetics with [H(2)O(2)] and the H(2)O(2) disproportionation involves redox cycling between the Mn(2)(III)/Mn(2)(IV) levels. The catalytic activity studies show that bound acetate is required for catalase activity and that the acetato and alkoxo bridges serve as internal bases facilitating the proton transfer coupled to oxidation of the metal centre.     

Antitumor activity of Mn(III) complexes in combination with phototherapy and antioxidant therapy

Manganese (Mn) is an essential trace element and trivalent Mn complexes have been used as oxidation catalysts and enzyme mimetics. We studied the cytotoxicity of Mn(III) derivatives of citrate, pyrophosphate and salicylene diamine (respectively, MnCit, MnPPi and EUK8) toward HeLa cells stressed by ultraviolet irradiation and the effect of the co-administration of ascorbate and para-amino salicylate (PAS) on cell viability. Metal complexes enhanced the lethality of irradiated cells, and this effect was even more pronounced when ascorbate was co-administered with Mn(III) species. The active role of Mn(III) compounds in the antitumor activity was demonstrated by the treatment of the cells with the chelator PAS, which restored the viability of both non-irradiated and UV-irradiated cells. The association of the Mn(III) metallodrugs with radiation and an antioxidant proved to be a very effective approach to chemotherapy.     

Inactivation of glutamine synthetase by a purified rabbit liver microsomal cytochrome P-450 system

Several mixed-function oxidation systems catalyze inactivation of Escherichia coli glutamine synthetase and other key metabolic enzymes. In the presence of NADPH and molecular oxygen, highly purified preparations of cytochrome P-450 reductase and cytochrome P-450 (isozyme 2) from rabbit liver microsomes catalyze enzyme inactivation. The inactivation reaction is stimulated by Fe(III) or Cu(II) and is inhibited by catalase, Mn(II), Zn(II), histidine, and the metal chelators o-phenanthroline and EDTA. The inactivation of glutamine synthetase is highly specific and involves the oxidative modification of a histidine in each glutamine synthetase subunit and the generation of a carbonyl derivative of the protein which forms a stable hydrazone when treated with 2,4-dinitrophenylhydrazine. We have proposed that the mixed-function oxidation system (the cytochrome P-450 system) produces Fe(II) and H2O2 which react at the metal binding site on the glutamine synthetase to generate an activated oxygen species which oxidizes a nearby susceptible histidine. This thesis is supported by the fact that (a) Mn(II) and Zn(II) inhibit inactivation and also interfere with the reduction of Fe(III) to Fe(II) by the P-450 system; (b) Fe(II) and H2O2 (anaerobically), in the absence of a P-450 system, catalyze glutamine synthetase inactivation; (c) inactivation is inhibited by catalase; and (d) hexobarbital, which stimulates the rate of H2O2 production by the P-450 system, stimulates the rate of glutamine synthetase inactivation. Moreover, inactivation of glutamine synthetase by the P-450 system does not require complex formation because inactivation occurs when the P-450 components and the glutamine synthetase are separated by a semipermeable membrane. Also, if endogenous catalase is inhibited by azide, rabbit liver microsomes catalyze the inactivation of glutamine synthetase.     

New conjugates of superoxide dismutase/catalase mimetics with cyclodestrins

Mimetics of antioxidant enzymes such as superoxide dismutases (SOD) or catalases are reported as potential new drugs able to reduce oxidative stress damage. In particular, manganese(III) complexes of salen-type ligands have been studied as both SOD and catalase mimetics. In this paper, we report the synthesis of two novel conjugates of salen-type ligands with the β-cyclodextrin, the 6-deoxy-6-[(S-cysteamidopropyl(1,2-diamino)N,N′-bis(salicylidene))]-β-cyclodextrin and the 6-deoxy-6-[(S-cysteamidopropyl(1,2-diamino)N,N′-bis(3-methoxysalicylidene))]-β-cyclodextrin, their spectroscopic characterization, and the synthesis and the characterization of their manganese(III) complexes. The SOD-like activity of the metal complexes was investigated by the indirect Fridovich method. The catalase like activity was tested using a Clark-type oxygen electrode. The peroxidase activity was tested using the ABTS (2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid)) assay. The glycoconjugation of salen-manganese(III) complexes yields compounds with enhanced SOD activity. These complexes also show catalase and peroxidase activities higher than the simple salen complexes (EUK 113 and EUK 108).     

New dimanganese(III) complexes of pentadentate (N2O3) Schiff base ligands with the [Mn2(mu-OAc)(mu-OR)2]3+ core: synthesis, characterization and mechanistic studies of H2O2 disproportionation

Two new diMn(III) complexes [Mn(2)(III)L(1)(mu-AcO)(mu-MeO)(methanol)(2)]Br (1) and [Mn(2)(III)L(2)(mu-AcO)(mu-MeO)(methanol)(ClO(4))] (2) (L(1)H(3)=1,5-bis(2-hydroxybenzophenylideneamino)pentan-3-ol; L(2)H(3)=1,5-bis(2-hydroxynaphtylideneamino)pentan-3-ol) were synthesized and structurally characterized. Structural studies evidence that these complexes have a bis(mu-alkoxo)(mu-carboxylato) triply bridged diMn(III) core in the solid state and in solution, with two substitution-labile sites--one on each Mn ion--in cis-position. The two complexes show catalytic activity toward disproportionation of H(2)O(2), with saturation kinetics on [H(2)O(2)], in methanol and dimethyl formamide at 25 degrees C. Spectroscopic monitoring of the H(2)O(2) disproportionation reaction suggests that (i) complexes 1 and 2 dismutate H(2)O(2) by a mechanism involving redox cycling between Mn(2)(III) and Mn(2)(IV), (ii) the complexes retain the dinuclearity during catalysis, (iii) the active form of the catalyst contains bound acetate, and (iv) protons favors the formation of inactive Mn(II) species. Comparison to other dimanganese complexes of the same family shows that the rate of catalase reaction is not critically dependent on the redox potential of the catalyst, that substitution of phenolate by naphtolate in the Schiff base ligand favors formation of the catalyst-substrate adduct, and that, in the non-protic solvent, the bulkier substituent at the imine proton position hampers the binding to the substrate.     

Bridging-type changes facilitate successive oxidation steps at about 1 V in two binuclear manganese complexes--implications for photosynthetic water-oxidation

The redox behavior of two synthetic manganese complexes illustrates a mechanistic aspect of importance for light-driven water oxidation in Photosystem II (PSII) and design of biomimetic systems (artificial photosynthesis). The coupling between changes in oxidation state and structural changes was investigated for two binuclear manganese complexes (1 and 2), which differ in the set of first sphere ligands to Mn (N(3)O(3) in 1, N(2)O(4) in 2). Both complexes were studied by electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) in three oxidation states which had been previously prepared either electro- or photochemically. The following bridging-type changes are suggested. In 1: Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(II)<-->Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(III)-->Mn(III)-(mu-OR)(mu-OCO)(mu-O)-Mn(III). In 2: Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(III)<-->Mn(III)-(mu-OR)(mu-OCO)(2)-Mn(III)-->Mn(III)-(mu-OR)(mu-OCO)(mu-O)-Mn(IV). In both complexes, the first one-electron oxidation proceeds without bridging-type change, but involves a redox-potential increase by 0.5-1V. The second one-electron oxidation likely is coupled to mu-oxo-bridge (or mu-OH) formation which seems to counteract a further potential increase. In both complexes, mu-O(H) bridge formation is associated with a redox transition proceeding at approximately 1V, but the mu-O(H) bridge is observed at the Mn(2)(III,III) level in 1 and at the Mn(III,IV) level in 2, demonstrating modulation of the redox behavior by the terminal ligands. It is proposed that also in PSII bridging-type changes facilitate successive oxidation steps at approximately the same potential.     

Ascorbate does not act as a pro-oxidant towards lipids and proteins in human plasma exposed to redox-active transition metal ions and hydrogen peroxide

The combination of ascorbate, transition metal ions, and hydrogen peroxide (H(2)O(2)) is an efficient hydroxyl radical generating system called "the Udenfriend system." Although the pro-oxidant role of ascorbate in this system has been well characterized in vitro, it is uncertain whether ascorbate also acts as a pro-oxidant under physiological conditions. To address this question, human plasma, used as a representative biological fluid, was either depleted of endogenous ascorbate with ascorbate oxidase, left untreated, or supplemented with 25 microM-1 mM ascorbate. Subsequently, the plasma samples were incubated at 37 degrees C with 50 microM-1 mM iron (from ferrous ammonium sulfate), 60 or 100 microM copper (from cupric sulfate), and/or 200 microM or 1 mM H(2)O(2). Although endogenous and added ascorbate was depleted rapidly in the presence of transition metal ions and H(2)O(2), no cholesterol ester hydroperoxides or malondialdehyde were formed, i.e., ascorbate protected against, rather than promoted, lipid peroxidation. Conversely, depletion of endogenous ascorbate was sufficient to cause lipid peroxidation, the rate and extent of which were enhanced by the addition of metal ions but not H(2)O(2). Ascorbate also did not enhance protein oxidation in plasma exposed to metal ions and H(2)O(2), as assessed by protein carbonyl formation and depletion of reduced thiols. Interestingly, neither the rate nor the extent of endogenous alpha-tocopherol oxidation in plasma was affected by any of the treatments. Our data show that even in the presence of redox-active iron or copper and H(2)O(2), ascorbate acts as an antioxidant that prevents lipid peroxidation and does not promote protein oxidation in human plasma in vitro.     

Oxidation of methyl linoleate in micellar solutions induced by the combination of iron(II)/ascorbic acid and iron(II)/H2O2

The oxidation of methyl linoleate (ML) was studied in the presence of Fe(II) alone and its combination with either ascorbic acid (AsAH(2)) or hydrogen peroxide (H(2)O(2)) at different molar ratios. Reactions were carried out in micellar solutions of TTAB (tetradecyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate), respectively, and were monitored by UV spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Fe(II) alone was able to catalyze the oxidation of ML in micellar solutions of TTAB, but not in those of SDS. The combination of H(2)O(2) with Fe(II) showed catalytic effect only in the TTAB medium, leading to different ML and Fe(II) oxidation kinetics compared to the Fe(II)-only catalyzed reactions. The AsAH(2)/Fe(II) combination demonstrated to be a good catalyst for the oxidation of ML in SDS micellar solutions, but not in TTAB micellar solutions; the activity of the catalyst was dependent on the AsAH(2)/Fe(II) molar ratio. The obtained results confirm that, for the ML oxidation to be initiated, the presence of a Fe(II)/Fe(III) couple is essential, which is related to the pH of micellar solutions. The catalytic properties of the AsAH(2)/Fe(II) combination were explained by taking into account the anti-oxidant and pro-oxidant properties of AsAH(2), as well as the possible formation of an iron/ascorbate complex as the initiator of the ML oxidation.     

NADH oxidation by manganese peroxidase with or without alpha-hydroxy acid

NADH oxidation by manganese peroxidase (MnP) was done in a reaction mixture including either alpha-hydroxy acid or acetate. The oxidation in the former reaction mixture was inhibited by a catalase and was accelerated by exogenous H2O2, while the oxidation in the latter reaction mixture was inhibited by a superoxide dismutase and was not accelerated by the exogenous H2O2. These results indicated that there are significant differences between the two reaction systems, particularly, in the active oxygen species involved in the reactions. Additionally, the experiment of MnP reduction with Mn(II) suggests that MnP has a separate catalytic activity other than an oxidation of Mn(II) to Mn(III) in the reaction mixture including acetate.     

Oxidation of hydroquinones by the versatile ligninolytic peroxidase from Pleurotus eryngii. H2O2 generation and the influence of Mn2+.

Formation of H2O2 during the oxidation of three lignin-derived hydroquinones by the ligninolytic versatile peroxidase (VP), produced by the white-rot fungus Pleurotus eryngii, was investigated. VP can oxidize a wide variety of phenols, including hydroquinones, either directly in a manner similar to horseradish peroxidase (HRP), or indirectly through Mn3+ formed from Mn2+ oxidation, in a manner similar to manganese peroxidase (MnP). From several possible buffers (all pH 5), tartrate buffer was selected to study the oxidation of hydroquinones as it did not support the Mn2+-mediated activity of VP in the absence of exogenous H2O2 (unlike glyoxylate and oxalate buffers). In the absence of Mn2+, efficient hydroquinone oxidation by VP was dependent on exogenous H2O2. Under these conditions, semiquinone radicals produced by VP autoxidized to a certain extent producing superoxide anion radical (O2*-) that spontaneously dismutated to H2O2 and O2. The use of this peroxide by VP produced quinone in an amount greater than equimolar to the initial H2O2 (a quinone/H2O2 molar ratio of 1 was only observed under anaerobic conditions). In the presence of Mn2+, exogenous H2O2 was not required for complete oxidation of hydroquinone by VP. Reaction blanks lacking VP revealed H2O2 production due to a slow conversion of hydroquinone into semiquinone radicals (probably via autooxidation catalysed by trace amounts of free metal ions), followed by O2*- production through semiquinone autooxidation and O2*- reduction by Mn2+. This peroxide was used by VP to oxidize hydroquinone that was mainly carried out through Mn2+ oxidation. By comparing the activity of VP to that of MnP and HRP, it was found that the ability of VP and MnP to oxidize Mn2+ greatly increased hydroquinone oxidation efficiency.     

Generation of hydrogen peroxide, superoxide and hydroxyl radicals during the oxidation of dihydroxyfumaric acid by peroxidase.

1. Dihydroxyfumarate slowly autoxidizes at pH6. This reaction is inhibited by superoxide dismutase but not by EDTA. Mn2+ catalyses dihydroxyfumarate oxidation by reacting with O2 leads to to form Mn3+, which seems to oxidize dihydrofumarate rapidly. Cu2+ also catalyses dihydroxyfumarate oxidation, but by a mechanism that does not involve O2 leads to. 2. Peroxidase catalyses oxidation of dihydroxyfumarate at pH6; addition of H2O2 does not increase the rate. Experiments with superoxide dismutase and catalase suggest that there are two types of oxidation taking place: an enzymic, H2O2-dependent oxidation of dihydroxyfumarate by peroxidase, and a non-enzymic reaction involving oxidation of dihydroxyfumarate by O2 leads to. The latter accounts for most of the observed oxidation of dihydroxyfumarate. 3. During dihydroxyfumarate oxidation, most peroxidase is present as compound III, and the enzymic oxidation may be limited by the low rate of breakdown of this compound. 4. Addition of p-coumaric acid to the peroxidase/dihydroxyfumarate system increases the rate of dihydroxyfumarate oxidation, which is now stimulated by addition of H2O2, and is more sensitive to inhibition by catalase but less sensitive to superoxide dismutase. Compound III is decomposed in the presence of p-coumaric acid. p-Hydroxybenzoate has similar, but much smaller, effects on dihydroxyfumarate oxidation. However, salicylate affects neither the rate nor the mechanism of dihydroxyfumarate oxidation. 5. p-Hydroxybenzoate, salicylate and p-coumarate are hydroxylated by the peroxidase/dihydroxyfumarate system. Experiments using scavengers of hydroxyl radicals shown that OH is required. Ability to increase dihydroxyfumarate oxidation is not necessary for hydroxylation to occur.     

Synthesis and characterization of new chromium(III), vanadium(IV), and titanium(III) complexes with biologically active isonicotinic acid hydrazide

New complexes of the type [Cr(INH)2Cl2]Cl.2H2O, VO(INH)2Cl2 and TiO(INH)2Cl, where INH = isonicotinic acid hydrazide, have been prepared. The complexes were characterized by infrared and UV-vis spectroscopy, proton nuclear magnetic resonance (NMR) and elemental analyses, molar conductivity and x-ray powder diffraction measurements. For the Cr(III)-complex, the ligand was coordinated through its carbonyl group and amino nitrogen atom; for V(IV)-complex and Ti(III)-complex, the ligand was coordinated through its carbonyl oxygen and heterocyclic nitrogen, respectively. Octahedral geometry has been proposed for all the complexes. The complexes of Cr(III) and Ti(III) showed significant tuberculostatic activity.     

Inactivation and reactivation of manganese catalase: oxidation-state assignments using X-ray absorption spectroscopy.

The oxidation states of the Mn atoms in three derivatives of Mn catalase have been characterized using a combination of X-ray absorption near-edge structure (XANES) and EPR spectroscopies. The as-isolated enzyme has an average oxidation state of Mn(III) and contains a Mn(III) form, together with a reduced Mn(II) form and a variable amount (10-25%) of a Mn(III)/Mn(IV) mixed-valence derivative. Treatment with NH2OH rapidly reduces the majority of the enzyme to a Mn(II) derivative with no loss of activity. Inactivation by treatment with NH2OH + H2O2 converts all of the enzyme to a mixed-valence Mn(III)/Mn(IV) form. The inactive, mixed-valence derivative can be completely reactivated by long-term (greater than 1 h) anaerobic incubation with NH2OH, giving a reduced Mn(II)/Mn(II) derivative. These data suggest a catalytic model in which the enzyme cycles between a reduced Mn(II)/Mn(II) state and an oxidized Mn(III)/Mn(III) state.     

Evaluating hydrogen bond interactions in enzymes containing Mn(III)-histidine complexation using manganese-imidazole complexes

It is often difficult to control hydrogen bond interactions in small molecule compounds that model metalloenzyme active sites. The imidazole-containing ligands 4,5-dicarboxyimidazole (H(3)DCBI) and 4,5-dicarboxy- N-methylimidazole (H(2)MeDCBI) allow examination of the effects of internal hydrogen bonding between carboxylate and imidazole nitrogen atoms. A new series of mononuclear manganese imidazole complexes have been prepared using these ligands: Mn(III)(salpn)(H(2)DCBI)(DMF) (1), Mn(III)(salpn)(HMeDCBI) (2), Mn(III)(dtsalpn)(HMeDCBI) (3), [Mn(IV)(dtsalpn)(HMeDCBI)]PF(6) (4), Mn(III)(salpn)(H(2)DCBI) (5), Mn(III)(dtsalpn)(H(2)DCBI) (6), and Mn(IV)(dtsalpn)(H(2)DCBI)PF(6) (8). Complexes 1, 2, 3, 5, and 6 have been prepared by direct reaction of salpn [salpn=(salicylideneaminato)-1,3-diaminopropane)] or dtsalpn [dtsalpn=(3,5-di- t-butylsalicylideneaminato)-1,3-diaminopropane)] and H(3)DCBI and H(2)MeDCBI with Mn(III) acetate, while complexes 4 and 8 were made by bulk electrolysis of complex 3 or 6 in dichloromethane. Complexes 1, 2, and 6 were characterized by X-ray diffraction. The impact of hydrogen bonding interactions of the complexes has been demonstrated by X-ray diffraction, cyclic voltammetry, and EPR spectroscopy. In all complexes the central metal ion is present in a six-coordinate geometry. Magnetic susceptibility measurements confirm the spin and oxidation states of the complexes. The cyclic voltammograms of 3 and 6 in dichloromethane reveal single, reversible redox waves with E(1/2)=600 mV and 690 mV, respectively. The X-band EPR spectrum of 4 shows a broad signal around g=4.4, and the corresponding complex 8 possesses a broad signal at slightly lower field ( g=5.5) than 4. These studies demonstrate that even small changes in the effective charge of the imidazole ligand can have a profound impact on the structure, spectroscopy, and magnetism of manganese(IV) complexes. We use these observations to present a model that may explain the origin of the g=4.1 signal in the S(2) state of photosystem II.     

Mn(II) oxidation is the principal function of the extracellular Mn-peroxidase from Phanerochaete chrysosporium

The manganese peroxidase (MnP), from the lignin-degrading fungus Phanerochaete chrysosporium, an H2O2-dependent heme enzyme, oxidizes a variety of organic compounds but only in the presence of Mn(II). The homogeneous enzyme rapidly oxidizes Mn(II) to Mn(III) with a pH optimum of 5.0; the latter was detected by the characteristic spectrum of its lactate complex. In the presence of H2O2 the enzyme oxidizes Mn(II) significantly faster than it oxidizes all other substrates. Addition of 1 M equivalent of H2O2 to the native enzyme in 20 mM Na-succinate, pH 4.5, yields MnP compound II, characterized by a Soret maximum at 416 nm. Subsequent addition of 1 M equivalent of Mn(II) to the compound II form of the enzyme results in its rapid reduction to the native Fe3+ species. Mn(III)-lactate oxidizes all of the compounds which are oxidized by the enzymatic system. The relative rates of oxidation of various substrates by the enzymatic and chemical systems are similar. In addition, when separated from the polymeric dye Poly B by a semipermeable membrane, the enzyme in the presence of Mn(II)-lactate and H2O2 oxidizes the substrate. All of these results indicate that the enzyme oxidizes Mn(II) to Mn(III) and that the Mn(III) complexed to lactate or other alpha-hydroxy acids acts as an obligatory oxidation intermediate in the oxidation of various dyes and lignin model compounds. In the absence of exogenous H2O2, the Mn-peroxidase oxidized NADH to NAD+, generating H2O2 in the process. The H2O2 generated by the oxidation of NADH could be utilized by the enzyme to oxidize a variety of other substrates.     

Site-specific DNA damage induced by hydrazine in the presence of manganese and copper ions. The role of hydroxyl radical and hydrogen atom.

The mechanism of DNA damage by hydrazine in the presence of metal ions was investigated by DNA sequencing technique and ESR-spin trapping method. Hydrazine caused DNA damage in the presence of Mn(III), Mn(II), Cu(II), Co(II), and Fe(III). The order of inducing effect on hydrazine-dependent DNA damage (Mn(III) greater than Mn(II) approximately Cu(II) much greater than Co(II) approximately Fe(III)) was related to that of the accelerating effect on the O2 consumption rate of hydrazine autoxidation. DNA damage by hydrazine plus Mn(II) or Mn(III) was inhibited by hydroxyl radical scavengers and superoxide dismutase, but not by catalase. On the other hand, bathocuproine and catalase completely inhibited DNA damage by hydrazine plus Cu(II), whereas hydroxyl radical scavengers and superoxide dismutase did not. Hydrazine plus Mn(II) or Mn(III) caused cleavage at every nucleotide with a little weaker cleavage at adenine residues, whereas hydrazine plus Cu(II) induced piperidine-labile sites frequently at thymine residues, especially of the GTC sequence. ESR-spin trapping experiments showed that hydroxyl radical is generated during the Mn(III)-catalyzed autoxidation of hydrazine, whereas hydrogen atom adducts of spin trapping reagents are generated during Cu(II)-catalyzed autoxidation. The results suggest that hydrazine plus Mn(II) or Mn(III) generate hydroxyl free radical not via H2O2 and that this hydroxyl free radical causes DNA damage. A possibility that the hydrogen atom releasing compound participates in hydrazine plus Cu(II)-induced DNA damage is discussed.     

Synthesis, characterization and antitumor studies of Mn(II), Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) complexes of N-salicyloyl-N'-o-hydroxythiobenzhydrazide

A new ligand N-salicyloyl-N'-o-hydroxythiobenzhydrazide (H2Sotbh) forms complexes [Mn(HSotbh)2], [Fe(Sotbh-H)(H2O)2], [M(Sotbh)] [M=Co(II), Cu(II) and Zn(II)] and [Ni(Sotbh)(H(2)O)2], which were characterized by various physico-chemical techniques. M?ssbauer spectrum of [Fe(Sotbh-H)(H2O)2] reveals the quantum admixture of 5/2 and 3/2 spin-states. Mn(II), Cu(II) and Ni(II) complexes were observed to inhibit the growth of tumor in vitro, whereas, Fe(III), Co(II), Zn(II) complexes did not. In vivo administration of Mn(II), Cu(II) and Ni(II) resulted in prolongation of survival of tumor bearing mice. Tumor bearing mice administered with Mn(II), Cu(II) and Ni(II) complexes showed reversal of tumor growth associated induction of apoptosis in lymphocytes. The paper discusses the possible mechanisms and therapeutic implication of the H2Sotbh and its metal complexes in tumor regression and tumor growth associated immunosuppression.     

In vitro interaction between homocysteine and copper ions: Potential redox implications

Homocysteine (Hcys) has been implicated in various oxidative stress-related disorders. The presence of a thiol on its structure allows Hcys to exert a double-edge redox action. Depending on whether Cu2+ ions occur concomitantly, Hcys can either promote or prevent free radical generation and its consequences. We have addressed in vitro the interaction between Hcys and Cu2+ ions, in terms of the consequences that such interaction may have on the free radical scavenging properties of Hcys and on the redox state and redox activity of the metal. To this end, we investigated the free radical-scavenging, O2(*-)-generating, and ascorbate-oxidizing properties of the interacting species by assessing the bleaching of ABTS*+ radicals, the reduction of O2(*-)-dependent cytochrome c, and the copper-dependent oxidation of ascorbate, respectively. In addition, electron paramagnetic resonance and Cu(I)-bathocuproine formation were applied to assess the formation of paramagnetic complexes and the metal redox state. Upon a brief incubation, the Hcys/Cu2+ interaction led to a decrease in the free radical-scavenging properties of Hcys, and to a comparable loss of the thiol density. Both effects were partial and were not modified by increasing the incubation time, despite the presence of Cu2+ excess. Depending on the molar Hcys:Cu2+ ratio, the interaction resulted in the formation of mixtures that appear to contain time-stable and ascorbate-reducible Cu(II) complexes (for ratios up to 2:1), and ascorbate- and oxygen-redox-inactive Cu(I) complexes (for ratios up to 4:1). Increasing the interaction ratio beyond 4:1 was associated with the sudden appearance of an O2(*-)-generating activity. The data indicate that depending on the molar ratio of interaction, Hcys and Cu2+ react to form copper complexes that can promote either antioxidant or pro-oxidant actions. We speculate that the redox activity arising from a large molar Hcys excess may partially underlie the association between hyper-homocysteinemia and a greater risk of developing oxidative-related cardiovascular diseases.     

Kinetics of Pseudomonas aeruginosa cytochrome c551 and cytochrome oxidase oxidation by Co(phen)3(3+) and Mn(CyDTA)(H2O)-

The reaction between reduced Pseudomonas cytochrome c551 and cytochrome oxidase with two inorganic metal complexes, Co(phen)3(3+) and Mn(CyDTA)(H2O)-, has been followed by stopped-flow spectrophotometry. The electron transfer to cytochrome c551 by both reactants is a simple process, characterized by the following second-order rate constant: k = 4.8 X 10(4) M-1 sec-1 in the case of Co(phen)3(3+) and k = 2.3 X 10(4) M-1 sec-1 in the case of Mn(CyDTA)(H2O)-. The reaction of the c-heme of the oxidase with both metal complexes is somewhat heterogeneous, the overall process being characterized by the following second-order rate constants: k = 1.7 X 10(3) M-1 sec-1 with Co(phen)3(3+) and k = 4.3 X 10(4) M-1 sec-1 with Mn(CyDTA)(H2O)- as oxidants; under CO (which binds to the d1-heme) the former constant increases by a factor of 2, while the latter does not change significantly. The oxidation of the d1-heme of the oxidase by Co(phen)3(3+) occurs via intramolecular electron transfer to the c-heme, a direct bimolecular transfer from the complex being operative only at high metal complex concentrations; when Mn(CyDTA)(H2O)- is the oxidant, the bimolecular oxidation of the d1-heme competes successfully with the intramolecular electron transfer.     

Modeling the haloperoxidases: reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin

The manganese meso-dimethylimidazolium porphyrin complex Mn(III)[TDMImP] reacted with HOBr/OBr(-) to generate the corresponding oxo-Mn(V)[TDMImP] species. The rate of this process accelerated with increasing pH. A forward rate constant, k(for), of 1.65x10(6)M(-1)s(-1) was determined at pH 8. Under these conditions, the oxo-Mn(V) species is short-lived and is transformed into the corresponding oxo-Mn(IV) complex. A first-order rate constant, k(obs), of 0.66 s(-1) was found for this reduction process at pH 8. The mechanism of this reduction process, which was dependent on bromide ion, appeared to proceed via an intermediate Mn(III)-O-Br complex. Thus, both a fast, reversible Mn(III)-O-Br bond heterolysis and a slower homolytic pathway occur in parallel in this system. The reverse oxidation reaction between oxo-Mn(V)[TDMImP] and bromide was investigated as a function of pH. The rate of this oxo-transfer reaction (k(rev)=1.4x10(3)M(-1)s(-1) at pH 8) markedly accelerated as the pH was lowered. The observed first-order dependence of the rate on [H(+)] indicates that the reactive species responsible for bromide oxidation is a protonated oxo-hydroxo complex and the stable species present in solution at high pH is dioxo-Mn(V)[TDMImP], [O=Mn(V)=O](-). The oxo-Mn(V) species retains nearly all of the oxidative driving force of the hypohalite. The equilibrium constant K(equi)=k(for)/k(rev) for the reversible process was determined at three different pH values (K(equi)=1.15x10(3) at pH 8) allowing the measurement of the redox potentials E of oxo-Mn(V)/Mn(III) (E=1.01 V at pH 8). The redox potential for this couple was extrapolated over the entire pH scale using the Nernst relationship and compared to those of the manganese 2- and 4-meso-N-methylpyridinium porphyrin couples oxo-Mn(V)[2-TMPyP]/Mn(III)[2-TMPyP], oxo-Mn(V)[4-TMPyP]/Mn(III)[4-TMPyP], OBr(-)/Br(-) and H(2)O(2)/H(2)O. Notably, the redox potential of oxo-Mn(V)/Mn(III) for the imidazolium porphyrin approaches that of H(2)O(2)/H(2)O at low pH.