Alberto Bindoli

The role of selenium peroxidases in the protection against oxidative damage of membranes

The present review deals with the chemical properties of selenium in relation to its antioxidant properties and its reactivity in biological systems. The interaction of selenite with thiols and glutathione and the reactivity of selenocompounds with hydroperoxides are described. After a short survey on distribution, metabolism and organification of selenium, the role of this element as a component of the two seleno-dependent glutathione peroxidases is described. The main features of glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase are also reviewed. Both enzymes reduce different hydroperoxides to the corresponding alcohols and the major difference is the reduction of lipid hydroperoxides in membrane matrix catalyzed only by the phospholipid hydroperoxide glutathione peroxidase. However, in spite of the different specificity for the peroxidic substrates, the kinetic mechanism of both glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase seems identical and proceeds through a tert-uni ping pong mechanism. In the reaction cycle, indeed, as supported by the kinetic data, the oxidation of the ionized selenol by the hydroperoxide yields a selenenic acid that in turn is reduced back by two reactions with reduced glutathione. Special emphasis has been given to the role of selenium-dependent glutathione peroxidases in the prevention of membrane lipid peroxidation. While glutathione peroxidase is able to reduce hydrogen peroxide and other hydroperoxides possibly present in the soluble compartment of the cell, this enzyme fails to inhibit microsomal lipid peroxidation induced by NADPH or ascorbate and iron complexes. On the other hand, phospholipid hydroperoxide glutathione peroxidase, by reducing the phospholipid hydroperoxides in the membranes, actively prevents lipid peroxidation, provided a normal content of vitamin E is present in the membranes. In fact, by preventing the free radical generation from lipid hydroperoxides, phospholipid hydroperoxide glutathione peroxidase decreases the vitamin E requirement necessary to inhibit lipid peroxidation. Finally, the possible regulatory role of the selenoperoxidases on the arachidonic acid cascade enzymes (cyclooxygenase and lipoxygenase) is discussed.

Biochemical and toxicological properties of the oxidation products of catecholamines

The normal catabolism of catecholamines proceeds through enzymatic pathways (monoaminooxidase, catechol-o-methyltranserase, and phenolsulphotransferase). In addition, nonenzymatic oxidative pathways might take place since catechols are readily oxidized. In this review article, the pathways of formation of the oxidation products of catecholamines and their reactions are described. The interactions of these products with different biological systems and their toxicity are examined. Among the reactions known to occur is that with sulfhydryls, which results in either a covalently linked adduct or disulfide production. Another interesting pathway to toxicity involves the oxidation of these catecholamine products by oxygen, with the formation of damaging oxygen-derived species. The action of the oxidation products of catecholamines is outlined, with special attention to the nervous and cardiac systems.

Emerging Protein Targets for Anticancer Metallodrugs: Inhibition of Thioredoxin Reductase and Cathepsin B by Antitumor Ruthenium(II)−Arene Compounds

A series of ruthenium(II)-arene (RAPTA) compounds were evaluated for their ability to inhibit thioredoxin reductase (either cytosolic or mitochondrial) and cathepsin B, two possible targets for anticancer metallodrugs. In general, inhibition of the thioredoxin reductases was lower than that of cathepsin B, although selected compounds were excellent inhibitors of both classes of enzymes in comparison to other metal-based drugs. Some initial structure-activity relationships could be established. On the basis of the obtained data, different mechanisms of binding/inhibition appear to be operative; remarkably the selectivity of the ruthenium compounds toward solid metastatic tumors also correlates to the observed trends. Notably, docking studies of the interactions of representative RAPTA compounds with cathepsin B were performed that provided realistic structures for the resulting protein-metallodrug adducts. Good agreement was generally found between the inhibiting potency of the RAPTA compounds and the computed stability of the corresponding cat B/RAPTA adducts.

Lipid peroxidation in mitochondria

The present review article takes into consideration the most important aspects of lipid peroxidation in mitochondria. Firstly the various ways by which lipid peroxidation is induced and the relevant mechanisms are described and discussed. After examining the major effects of lipid peroxidation on mitochondrial enzymes and bioenergetic functions, some aspects of the pathophysiology of lipid peroxidation are considered in connection with maturation of reticulocytes, alternative oxidase of plant mitochondria, aging, and ischemia-reperfusion syndrome. The final part of the article is devoted to the regulation and control of lipid peroxidation in mitochondria with particular emphasis to the role of the respiratory substrates.

Gold(I) Carbene Complexes Causing Thioredoxin 1 and Thioredoxin 2 Oxidation as Potential Anticancer Agents

Gold(I) complexes with 1,3-substituted imidazole-2-ylidene and benzimidazole-2-ylidene ligands of the type NHC-Au-L (NHC = N-heterocyclic carbene L = Cl or 2-mercapto-pyrimidine) have been synthesized and structurally characterized. The compounds were evaluated for their antiproliferative properties in human ovarian cancer cells sensitive and resistant to cisplatin (A2780S/R), as well in the nontumorigenic human embryonic kidney cell line (HEK-293T), showing in some cases important cytotoxic effects. Some of the complexes were comparatively tested as thioredoxin reductase (TrxR) and glutathione reductase (GR) inhibitors, directly against the purified proteins or in cell extracts. The compounds showed potent and selective TrxR inhibition properties in particular in cancer cell lines. Remarkably, the most effective TrxR inhibitors induced extensive oxidation of thioredoxins (Trxs), which was more relevant in the cancerous cells than in HEK-293T cells. Additional biochemical assays on glutathione systems and reactive oxygen species formation evidenced important differences with respect to the classical cytotoxic Au(I)-phosphine compound auranofin.

Induction of mitochondrial permeability transition by auranofin, a Gold(I)-phosphine derivative

Gold(I)‐thiolate drugs are compounds that specifically interact with thiol and/or selenol groups and are essentially utilized in the treatment of rheumatoid arthritis. Considering the importance of thiol groups in regulating mitochondrial membrane permeability, the effects of auranofin (S‐triethylphosphinegold(I)‐2,3,4,6‐tetra‐O‐acetyl‐1‐thio‐β‐D‐glucopyranoside), a second‐generation gold drug, were studied on mitochondria isolated from rat liver. Auranofin, at submicromolar concentrations, was able to induce the mitochondrial membrane permeability transition observed as swelling and loss of membrane potential. Both events are completely inhibited by cyclosporin A, the specific inhibitor of mitochondrial permeability transition. Calcium ions and energization by succinate are required for the occurrence of permeability transition. By interacting with the active site selenol group, auranofin results as an extremely potent inhibitor of mitochondrial thioredoxin reductase, both isolated and in its mitochondrial environment. It is concluded that auranofin, in the presence of calcium ions, is a highly efficient inducer of mitochondrial membrane permeability transition, potentially referable to its inhibition of mitochondrial thioredoxin reductase.

Antioxidant effect of manganese

The antioxidant effects of manganese and other transition metals were studied as the inhibition of microsomal lipid peroxidation and crocin bleaching by peroxyl radicals. The peroxyl radical scavenging capacity was measured by competition kinetics analysis. While Zn(II), Ni(II), and Fe(II) were almost completely ineffective, Mn(II) and Co(II) showed a free radical scavenging capacity, exhibiting relative rate constant ratios respectively of 0.513 and 0.287. This indicates that Mn(II) is by far the most active. Therefore, the chain-breaking antioxidant capacity of Mn(II) seems to be related to the rapid quenching of peroxyl radicals according to the reaction R-OO. + Mn(II) + H(+)-->ROOH+Mn(III). The antioxidant mechanism is discussed considering the different reduction potentials of the examined cations.

Anticancer therapeutics that target selenoenzymes: synthesis, characterization, in vitro cytotoxicity, and thioredoxin reductase inhibition of a series of gold(I) complexes containing hydrophilic phosphine ligands.

Gold(I) complexes bearing water‐soluble phosphine ligands, including 1,3,5‐triaza‐7‐phosphaadamantane (PTA), 3,7‐diacetyl‐1,3,7‐triaza‐5‐phosphabicyclo[3.3.1]nonane (DAPTA), and sodium triphenylphosphine trisulfonate (TPPTS), in combination with thionate ligands, were screened for their antiproliferative activities against human ovarian cancer cell lines A2780 either sensitive or resistant to cisplatin. In addition, the compounds were screened for their inhibition of mammalian thioredoxin reductases (TrxR), enzymes that are overexpressed in many tumor cells and contribute to drug resistance. The gold(I)–phosphine complexes efficiently inhibited cytosolic and mitochondrial TrxRs at concentrations that did not affect the related oxidoreductase glutathione reductase (GR). Additional complementary information on the enzyme metallation process and potential gold binding sites was obtained through the application of a specific biochemical assay using a thiol‐tagging reagent, BIAM (biotin‐conjugated iodoacetamide).

Principles in Redox Signaling: From Chemistry to Functional Significance

Reactive oxygen and nitrogen species are currently considered not only harmful byproducts of aerobic respiration but also critical mediators of redox signaling. The molecules and the chemical principles sustaining the network of cellular redox regulated processes are described. Special emphasis is placed on hydrogen peroxide (H(2)O(2)), now considered as acting as a second messenger, and on sulfhydryl groups, which are the direct targets of the oxidant signal. Cysteine residues of some proteins, therefore, act as sensors of redox conditions and are oxidized in a reversible reaction. In particular, the formation of sulfenic acid and disulfide, the initial steps of thiol oxidation, are described in detail. The many cell pathways involved in reactive oxygen species formation are reported. Central to redox signaling processes are the glutathione and thioredoxin systems controlling H(2)O(2) levels and, hence, the thiol/disulfide balance. Lastly, some of the most important redox-regulated processes involving specific enzymes and organelles are described. The redox signaling area of research is rapidly expanding, and future work will examine new pathways and clarify their importance in cellular pathophysiology.

Inhibitory action of quercetin on xanthine oxidase and xanthine dehydrogenase activity

Quercetin is an equally good inhibitor of xanthine oxidase (type O, oxygen-reducing enzyme) and xanthine dehydrogenase (type D, NAD+-reducing enzyme) activity of a preparation of the xanthine-oxidizing enzyme partially purified from rat liver. The inhibition seems competitive with the oxidase form and non-competitive (mixed-type) with the dehydrogenase form of the enzyme. These inhibitory properties should be referred to the flavonoid structure of quercetin rather than to its antioxidant power. The antioxidant properties of quercetin and its inhibitory effect on the xanthine-oxidizing enzyme are discussed with reference to hyperuricemic and ischemic states.