Showing papers by "Govindasamy Mugesh published in 2006"
TL;DR: Although the inhibition of HRP by ebselen is comparable with that of EbTe(2), the inhibitory effect on gamma-radiation induced lipid peroxidation and the GPx activity of ebseleradish peroxidase is found to be much higher than that of EbTe( 2).
43 citations
TL;DR: To understand the role of the second metal ion in hydrolysis, the syntheses and catalytic activities of two mononuclear complexes (3 and 4) that include coordinated water molecules are described, suggesting that the second zinc ion is not crucial for the beta-lactamase activity.
Abstract: Biomimetic systems containing one or two zinc(II) ions supported by phenolate ligands were developed as functional mimics of metallo-beta-lactamase. These
complexes were shown to catalytically hydrolyze beta-lactam substrates, such as oxacillin and penicillin G. The dinuclear zinc complex 1, which has a coordinated water molecule, exhibits high beta-lactamase activity, whereas the dinuclear zinc complex 2, which has no water molecules, but labile chloride ligands, shows a much lower activity. The high beta-lactamase activity of complex 1 can be ascribed to the presence of a zinc-bound water molecule that is activated by being hydrogen bonded to acetate substituents. The kinetics of the hydrolysis of oxacillin by complex I and the effect of pH on the reaction rates are reported in detail. In addition, the kinetic parameters obtained for the synthetic analogues are compared with those of the natural metallo-beta-lactamase from Bacillus cereus (Bell). To understand the role of the second metal ion in hydrolysis, the syntheses and catalytic activities of two mononuclear complexes (3 and 4) that include coordinated water molecules are described. Interestingly, the mononuclear zinc complexes 3 and 4 also exhibit high activity, supporting the assumption that the second zinc ion is not crucial for the beta-lactamase activity.
36 citations
TL;DR: In this paper, the authors carried out biomimetic studies to understand the mechanism by which the antithyroid drugs inhibit the thyroid hormone synthesis and found that the replacement of sulfur with selenium in methimazole leads to an interesting compound that may reversibly block the thyroid peptide synthesis.
Abstract: Propylthiouracil (PTU) and methimazole (MMI) are the most commonly used antithyroid drugs. The available data suggest that these drugs may block the thyroid hormone synthesis by inhibiting the thyroid peroxidase (TPO) or diverting oxidized iodides away from thyroglobulin. It is also known that PTU inhibits the selenocysteine-containing enzyme ID-1 by reacting with the selenenyl iodide intermediate (E-SeI). In view of the current interest in antithyroid drugs, we have recently carried out biomimetic studies to understand the mechanism by which the antithyroid drugs inhibit the thyroid hormone synthesis and found that the replacement of sulfur with selenium in MMI leads to an interesting compound that may reversibly block the thyroid hormone synthesis. Our recent results on the inhibition of lactoperoxidase (LPO)-catalyzed oxidation and iodination reactions by antithyroid drugs are described.
31 citations
TL;DR: A model study with internal amino groups in the selenenyl sulfide state reveals that the basic His residues may play important roles by deprotonating the thiol moiety in theSelenenic acid state and by interacting with the sulfur atom in theselenanyl sulfides state to facilitate the nucleophilic attack of thiol at sulfur rather than at selenium, thereby generating the catalytically active species selenol.
Abstract: The roles of built-in thiol cofactors and the basic histidine (His) residues in the active site of mammalian thioredoxin reductases (TrxRs) are described with the help of experimental and density functional theory calculations on small-molecule model compounds. The reduction of selenenyl sulfides by thiols in selenoenzymes such as glutathione peroxidase (GPx) and TrxR is crucial for the regeneration of the active site. Experimental as well as theoretical studies were carried out with model selenenyl sulfides to probe their reactivity toward incoming thiols. We have shown that the nucleophilic attack of thiols takes place at the selenium center in the selenenyl sulfides. These thiol exchange reactions would hamper the regeneration of the active species selenol. Therefore, the basic His residues are expected to play crucial roles in the selenenyl sulfide state of TrxR. Our model study with internal amino groups in the selenenyl sulfide state reveals that the basic His residues may play important roles by deprotonating the thiol moiety in the selenenic acid state and by interacting with the sulfur atom in the selenenyl sulfide state to facilitate the nucleophilic attack of thiol at sulfur rather than at selenium, thereby generating the catalytically active species selenol. This model study also suggests that the enzyme may use the internal cysteines as cofactors to overcome the thiol exchange reactions.
27 citations
TL;DR: This study provides the first experimental evidence that MSeI not only effectively inhibits the LPO-catalyzed iodination of tyrosine, but also reacts with I(2) to produce novel ionic diselenides.
Abstract: The inhibition of lactoperoxidase (LPO)-catalyzed iodination of L-tyrosine by the anti-thyroid drug methimazole (MMI) and its selenium analogue (MSeI) is described. MSeI inhibits LPO with an IC50 value of 12.4 µM, and this inhibition could be completely reversed by increasing the peroxide concentration. In addition to the inhibition, MSeI reacts with molecular iodine to produce novel ionic diselenides, and the nature of the species formed in this reaction appear to be solvent-dependent. The formation of ionic species in the reaction is confirmed by single-crystal X-ray studies, FT-IR and FT-Raman spectroscopic investigations. This study provides the first experimental evidence that MSeI not only effectively inhibits the LPO-catalyzed iodination of tyrosine, but also reacts with I2 to produce novel ionic diselenides. These results also suggest that MSeI reacts with iodine, even in its oxidized form, to form ionic diselenides containing iodide or polyiodide anions, which might be effective intermediates in the inhibition of thyroid hormones.
21 citations
TL;DR: The studies show that the replacement of sulphur in MMI by selenium leads to a selone, which exists predominantly in its zwitterionic form, which may lead to a reversible inhibition of thyroid hormone biosynthesis.
Abstract: Hydrogen peroxide, generated by thyroid oxidase enzymes, is a crucial substrate for the thyroid peroxidase (TPO)-catalysed biosynthesis of thyroid hormones, thyroxine (T4) and triiodothyronine (T3) in the thyroid gland. It is believed that the H2O2 generation is a limiting step in thyroid hormone synthesis. Therefore, the control of hydrogen peroxide concentration is one of the possible mechanisms for the inhibition of thyroid hormone biosynthesis. The inhibition of thyroid hormone synthesis is required for the treatment of hyperthyroidism and this can be achieved by one or more anti-thyroid drugs. The most widely used anti-thyroid drug methimazole (MMI) inhibits the production of thyroid hormones by irreversibly inactivating the enzyme TPO. Our studies show that the replacement of sulphur in MMI by selenium leads to a selone, which exists predominantly in its zwitterionic form. In contrast to the sulphur drug, the selenium analogue (MSeI) reversibly inhibits the peroxidase-catalysed oxidation and iodination reactions. Theoretical studies on MSeI reveal that the selenium atom in this compound carries a large negative charge. The carbon-selenium bond length in MSeI is found to be close to single-bond length. As the selenium atom exhibits a large nucleophilic character, the selenium analogue of MMI may scavenge the hydrogen peroxide present in the thyroid cells, which may lead to a reversible inhibition of thyroid hormone biosynthesis.
18 citations
TL;DR: The first example of a metal complex derived from the selenium analogue of the anti-thyroid drug methimazole (MSeI) is reported, resulting in the formation of a novel copper (II) complex.
6 citations