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Download Author Version (PDF) Organic & Biomolecular Chemistry Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/obc Page 1 of Journal8 Name Organic & Biomolecular Chemistry Dynamic Article Links► Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx ARTICLE TYPE Stable selones in glutathione peroxidase like catalytic cycle of selenonicotinamide derivative Parashiva Prabhu,a Beena G. Singh,b* Masato Noguchi,c Prasad P. Phadnis,a Vimal K. Jain,a Michio Iwaoka,c K. Indira Priyadarsinib 5 Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Selenonicotinamide, 2,2′-diselenobis[3-amidopyridine] (NictSeSeNict), exhibits glutathione peroxidase (GPx) like activity of catalyzing reduction of hydrogen peroxide (H2O2) by glutathione (GSH). Estimated reactivity parameters for the reaction of selenium species, according to Dalziel kinetic model, towards 10 GSH (GSH) and H2O2 (H2O2), indicated that the rate constant for the reaction of NictSeSeNict with GSH 77 is higher as compared to that with H2O2, indicating that the activity is initiated by reduction. Se NMR spectroscopy, HPLC analysis, mass spectrometry (MS) and absorption spectroscopy were employed for understanding the nature of selenium intermediates responsible for the activity. The 77Se NMR resonance at 525 ppm due to NictSeSeNict disappeared in the presence of GSH with initial appearance of signals at 15 364 and 600 ppm, assigned to selone (NictC=Se) and selenenylsulfide (NictSeSG), respectively. Reaction of H2O2 with NictSeSeNict produced a mixture of selenenic acid (NictSeOH) and seleninic acid 77 (NictSeO2H) with Se NMR resonances appearing at 1069 and 1165 ppm, respectively. Addition of three equivalents of GSH to this mixture produced characteristic 77Se NMR signal of NictSeSG. HPLC analysis of the product formed by the reaction of NictSeSeNict with GSH confirmed the formation of NictC=Se 20 absorbing at 375 nm. Stopped-flow kinetic studies with global analysis revealed a bimolecular rate constant of 4.8 0.5 x 103 M-1s-1 and 1.7 0.6 x 102 M-1s-1 for the formation of NictC=Se produced in two consecutive reactions of NictSeSeNict and NictSeSG with GSH, respectively. Similarly the rate -1 -1 constant for the reaction of NictC=Se with H2O2 was estimated to be 18 1.8 M s . These studies clearly indicated that the GPx activity of NictSeSeNict is initiated by reduction to form NictSeSG and a 25 stable selone, which is responsible for its efficient GPx activity. Introduction Inspired by the biological function of selenocysteine in selenoenzymes like GPx as an antioxidant, attempts have been Nicotinamide is an important moiety present in a wide range of made during the past 10-15 years to synthesize a wide variety of biomolecules associated with processes such as energy 8 organoselenium compounds that mimic the activity of GPx. The production, synthesis of fatty acids, etc. It is a precursor for the 50 first compound evaluated for GPx activity extensively was 30 coenzymes NAD (nicotinamide adenine dinucleotide) and NADP 9 ebselen, an aromatic selenoamide. Accordingly many new (nicotinamide adenine dinucleotide phosphate),1and an important 2 synthetic organoselenium compounds have been examined for cofactor in numerous enzymatic redox reactions. Nicotinamide 10 3 4 GPx activity and a number of them showed promising results. exhibits anti-inflammatory and antioxidant activities. Further research in the development of selenium compounds Antioxidants are required to minimize the oxidative stress in 55 indicated that weak non-bonding interactions between selenium 35 cellular systems. Oxidative stress is defined as a state in which an atom and nearby heteroatoms either intramolecularly or imbalance occurs in favour of reactive oxygen species (ROS) intermolecularly play decisive role in the effectiveness of GPx formation and as a result the biological system's ability to 11 like activity of ebselen and related compounds. Since last one detoxify the reactive intermediates or to repair the resulting decade our group is involved in design and development of low damage decreases.5 ROS include molecular species like hydrogen 60 molecular weight organoselenium compounds as antioxidants and 40 peroxide (H2O2). Living organisms have developed several 12 radioprotectors. With this background, nicotinoyl based Chemistry &Biomolecular AcceptedOrganic Manuscript enzymatic systems such as catalases, superoxide dismutase and organoselenium compounds were conceptulaized and glutathione peroxidase (GPx) to detoxify these ROS.6 GPx synthesized. Preliminary investigations on the GPx like activity belongs to oxidoreductase family that catalyzes reduction of of NictSeSeNict (Scheme 1) revealed its superior activity as hydroperoxides by glutathione (GSH). The enzymatic activity of 13 7 65 compared to ebselen and related derivatives. Therefore to 45 the GPx is due to the redox property of selenocysteine. understand the factors responsible for its higher activity, detailed This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 |1 Organic & Biomolecular Chemistry Page 2 of 8 catalytic mechanism was investigated by applying enzyme k4 NictSeOH GSH NictSeSG H O kinetics and the resultant intermediates were characterized by 2 (4) 77 Se NMR spectroscopy, HPLC, mass spectrometry (MS) and k5 NictSeOH HO NictSeOH H O absorption spectroscopy. 2 2 2 2 (5) k O NictSeSG GSH6 NictSeH GSSG 35 (6) NH2 k7 NictSeSG H O NictSeOH GSOH N Se 2 2 (7) 5 2 Scheme 1 Structure of 2,2′-diselenobis[3-amidopyridine] (NictSeSeNict) k8 NictSeO H 3GSH NictSeSG GSSG 2H O 2 2 (8) Results and Discussion Depending upon its reactivity with the two substrates, i.e. GSH or a. GPx activity by NADPH assay H2O2, the GPx cycle is either initiated by the reaction of 14 40 NictSeSeNict with H2O2 (eqn. 1) or GSH (eqn. 2). In order to know the individual reactions with these substrates, the kinetic 1.0 (a) data was treated with Dalziel equation as applied in our earlier studies with selenocystine.14b The enzyme kinetic parameters were estimated using eqn. 9. 0.8 2 NictSeSeNict GSHHO2 2 GH 0.6 (c) 0 GSH H2 O 2 GSH H 2 O 2 45 (9) 0.4 (b) Here [NictSeSeNict] is the total enzyme concentration. NictSeSeNict being a diselenide produces two reactive species 0.2 either during oxidation or reduction that can independently participate in the GPx-like cycle. Therefore, in this case the total 0.0 50 enzyme concentration is twice the concentration of NictSeSeNict. Nornalized absorbance, 340 nm 340 absorbance, Nornalized 0 50 100 150 200 In eqn. 9, GSH is the reciprocal of the total reactivity of the Time, s enzyme and its intermediates with GSH, i.e. [GSH = 1/kG = 1/(k2 + k4 + k6+ k8)], H2O2 is the reciprocal of the total reactivity of the 10 Fig. 1 Plots showing change in absorbance at 340 nm as a function of enzyme and its intermediates with H2O2, i.e. [H2O2 = 1/kH = 1/(k1 time due to NADPH oxidation. (a = blank; b = NictSeSeNict, c = 55 + k3+ k5+ k7)]. Here, kG and kH are the apparent rate constants for ebselen). Concentration of NADPH, GSH, H O , glutathione reductase, 2 2 the overall reactions involving GSH and H2O2, respectively. The and selenium catalyst were fixed at 0.34 mM, 1 mM, 1 mM, 5.0 Units/mL term is the parameter related to the rate of formation of the and 10 µM, respectively GH ternary complex. A non-zero value of GH indicates the formation 15 GPx like catalytic activity of NictSeSeNict studied by of a ternary complex between the enzyme and the two substrates. using NADPH-GSSG reductase coupled assay indicated that the 60 The term 0 is equal to the reciprocal of the turnover number decay of NADPH at 340 nm (figure.1), increased significantly in which corresponds to the maximum catalytic rate at unit enzyme 15 the presence of NictSeSeNict (Fig. 1b). The initial decay (υ) concentration (0 = 1/kcat). To estimate these parameters, a under identical experimental conditions was compared with series of experiments were performed and the initial reduction 20 ebselen (Fig. 1c), and was used as a reference in the present rate (υ) of H2O2 which is directly related to the decay of NADPH study. The results indicated that, υ for NictSeSeNict (12.00.6 x 65 at 340 nm, was measured in the presence of NictSeSeNict (10 10-7 Ms-1) is two-folds higher than that of ebselen (6.0 0.3 x 10- µM) at a fixed initial concentration of GSH and different 7 -1 Ms ). Considering that the former is a diselenide and the latter concentrations of H2O2 (0.1 mM –1.2 mM). A primary linear plot is a monoselenide the observed increase in the rate constant may was obtained for the variation of [2NictSeSeNict]/υ as a function 25 indicate similar activity by the two compounds have similar GPx of reciprocal concentration of H2O2. The slope and intercept of activity. To analyze this, a detailed enzyme kinetic study was 70 the plot are represented by eqns.
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