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1.
Proton and phosphorus-31 nuclear spin–lattice relaxation times T1 and spin–spin relaxation times T2 have been measured on the single-stranded polyriboadenylic acid [poly(A)]–Mn2+ system in a neutral D2O solution in the temperature range 10°–90°C at 100 and 40.5 MHz, respectively, with the Fourier transform nmr method. Minimum values of T1 have been found for all these nuclei, which have enabled the exact estimation of apparent distances from Mn2+ to H2, H8, H1′, and the phosphorus nucleus to be 4.7, 4.1, 5.2, and 3.0 Å, respectively. The electron spin of Mn2+ penetrates into the phosphorus nucleus, giving 31P hyperfine coupling of more than 106 Hz. Evidence of penetration of the electron spin into H8 and H2 is also obtained, suggesting direct coordination of nitrogen atoms of the adenine ring to the Mn2+ Ion. Combined with the result from proton relaxation enhancement of water, it is concluded that every Mn2+ ion added is bound directly to two phosphate groups with a Mn2+–phosphorus distance of 3.3 Å, while a part of the Mn2+ ions are simultaneouly bound to the adenine ring. It is estimated that 39 ± 13% and 13 ± 5% of Mn2+ are coordinated by N7 and N3 (or N1), respectively. The motional freedom of poly(A) in the environment of the Mn2+ binding site has been found to be quenched to the extent that the rotational motion becomes several times slower than that of the corresponding Mn2+–free poly(A). The activation energies for the molecular motion are, however, practically unchanged from those for Mn2+–free poly(A), and are found to be 8.3, 8.5, 6.1, and 8.7 kcal/mol for H8, H2, H1′, and phosphorus, respectively. T2 of phosphorus is determined by the dissociation rate (k?1) of Mn2+ from the phosphate group for the whole temperature range studied with activation enthalpy of 6.5 kcal/mol. The dissociation rates of Mn2+ from the adenine ring are also estimated from proton T2 values below 50°C.  相似文献   

2.
We have previously proposed a role for calmodulin (CaM) in the regulation of initiation of Ca2+ entry in Jurkat T cells, as well as in the regulation of the current that mediates Ca2+ entry, IT. In this report, we provide evidence for the mechanism of CaM action. We have previously shown that activation-induced Ca 2+ entry into Jurkat T cells is mediated by a current we have called IT. In the whole cell variation, but not the perforated patch variation, of the patch clamp technique, this current is short-lived (under 6 min) suggesting that the current is under the control of a diffusible component of the cytosol. Addition of CaM to the whole cell recording pipette solution maintained IT for up to 20 min, suggesting that CaM may be this diffusible component. Pharmacological inhibitors of CaM blocked the augmentation of IT normally induced by an activating stimulus. Cells electroporated in the presence of anti-CaM antibodies had reduced influx of extracellular Ca2+, with no change in release of Ca2+ from the internal stores. These observations suggest that T cell receptor engagement initiates Ca2+ influx by a pathway that likely includes CaM, which may in turn regulate IT. Influx of extracellular Ca2+ is required for cellular proliferation, and inhibition of CaM by pharmacological inhibitors reduced cellular proliferation. This same inhibition of proliferation was seen in cells electroporated with anti-CaM antibodies. This suggests that inhibition of CaM and/or IT may be a target for therapeutic inhibition of inappropriate T cell proliferation.  相似文献   

3.
The influence of divalent cations, and pH on the behaviour of phosphatidylserine, derived from egg phosphatidylcholine, has been examined employing 31P-NMR techniques. The addition of Ca2+ results in the observation of a “rigid lattice” 31P-NMR spectra and more than an order of magnitude increase in the spin-lattice relaxation time T1. This corresponds to a strong and specific headgroup immobilization by Ca2+, similar to that observed for anhydrous phosphatidylserine. At pH 7.4 the hydrated sodium salt of (egg) phosphatidylserine adopts the bilayer phase, whereas when the pH is decreased through 3.5 a bilayer to hexagonal (HII) polymorphic phase transition is observed at 50°C, which is unaffected by equimolar cholesterol. The same transition is shown to occur at 37°C for phosphatidylserine isolated from human erythrocytes.  相似文献   

4.
In this article, photoluminescence (PL) and thermoluminescence (TL) properties of ZrO2, ZrO2:Dy3+, ZrO2:Dy3+–Gd3+, ZrO2:Dy3+–Yb3+, ZrO2:Dy3+–Er3+, and ZrO2:Dy3+–Sm3+ phosphors synthesized by the Pechini method were investigated. The crystal structure, thermal properties, morphology, PL and TL properties were investigated using X-ray powder diffraction (XRD), differential thermal analysis/thermogravimetric analysis (DTA/TGA), scanning electron microscopy (SEM), PL and TL, respectively. The room temperature emission bands corresponding to 4F9/2 → 6HJ (J = 9/2, 11/2, 13/2 and 15/2) transitions of Dy3+ ions were measured. The phosphors were analysed using TmTSTOP, variable dose, and computerized glow curve fitting methods. Reusability, dose–response, and fading characteristics were investigated. The phosphors have a natural TL emission that vanished by heating treatment. Moreover, new peaks with similar properties to the natural emissions were observed after high-dose irradiation and long-term fading experiments. The glow curves of the phosphors have 13 individual peaks and many low- and high-temperature satellite peaks. The origin of the peaks is ZrO2 host material and doping with rare-earth ions (Gd3+, Dy3+, Yb3+, Er3+ and Sm3+) does not lead to a new glow peak. The dopants cause drastic changes in individual peak intensities of ZrO2.The initial fading rates of all the phosphors are relatively fast, but they slow down as time goes on.  相似文献   

5.
In this paper, we present a simple and rapid colorimetric assay – using the polythymine oligonucleotide T33, citrate-capped gold nanoparticles (AuNPs), and phosphate-buffer saline (PBS) in the presence of Mn2+ – for the highly selective and sensitive detection of Hg2+ in an aqueous solution. Citrate-capped AuNPs adsorbed on randomly coiled T33 were dispersed well in PBS because of strong electrostatic repulsion between DNA molecules. In the presence of Hg2+, the formation of Hg2+–T33 complexes enabled the removal of T33 molecules from the NP surface, resulting in salt-induced NP aggregation. However, the T33-capped AuNPs (T33-AuNPs) were dispersed in PBS solution after the addition of 1.0 μM Hg2+, indicating that T33-AuNPs had poor colorimetric sensitivity toward Hg2+. We uncovered that the addition of Mn2+ to a solution containing 0.75 nM T33-AuNPs and 0.2× PBS resulted in an acceleration of the analysis time (within 5 min) and a 100-fold sensitivity improvement for the detection of Hg2+. As a result, the present approach enables the analysis of Hg2+ with a minimum detectable concentration that corresponds to 10 nM. This is probably attributed to that Mn2+ binds strongly to the phosphate backbone of DNA, thereby accelerating Hg2+-induced aggregation of the T33-AuNPs. Because Mn2+ can stabilize the folded structure of the Hg2+–T33 complex, Hg2+ facilitates the removal of T33 from the NP surface in the presence of Mn2+. This probe was successfully applied to the determination of Hg2+ in pond water.  相似文献   

6.
The seawater cations, Na+, K+, Mg2+, and Ca2+, each stimulated MnO2-reductase activity of whole cells and cell extracts of Bacillus 29. Concentrations of Na+ and K+ which stimulated whole cells and cell extracts maximally were equivalent to those in two- to fivefold diluted seawater. Cell-extract activity was strongly stimulated by Ca2+ and Mg2+ up to a concentration of 0.01 M Mg2+ and 0.002 M Ca2+, with little additional stimulation above these concentrations. Whole-cell activity was stimulated biphasically with increasing concentrations of Ca2+ and Mg2+. Comparison of the effects of individual cations or mixtures of them at concentrations equivalent to their concentration in fivefold diluted seawater showed that more activity was obtained with 0.01 M Mg2+ or 0.002 M Ca2+ than with 0.1 M Na+, and more with 0.1 M Na+ than with 0.0022 M K+. Fivefold diluted seawater permitted as much or more activity as solutions of individual or synthetic mixtures of the cations. Pre-exposure experiments showed that the ionic history of whole cells was important to their ultimate activity. The MnO2-reductase activity of induced whole cells exhibited a temperature optimum near 40 C. Cell extracts had different temperature optima (Topt), depending on whether induced glucose-linked activity (Topt = 25 C), uninduced glucose-linked, ferricyanide-dependent activity (Topt = 30 C), or uninduced ferrocyanide-linked activity (Topt = 40 C) were being measured. Some of these optima are higher than previously reported.  相似文献   

7.
Multiple-equilibrium equations were solved to investigate the individual and separate effects of Mg2+, Mn2+, Ca2+, ATP4–, and their complexes on the kinetics of brain adenylate cyclase. The effects of divalent metals and/or ATP4– (in excess of their participation in complex formation) were determined and, from the corresponding apparent affinity values, the following kinetic constants were obtained:K m(MgATP)=1.0 mM,K i(ATP4–)=0.27 mM,K m(MnATP)=0.07 mM, andK i(CaATP)=0.015 mM. MgATP, MnATP, ATP4–, and CaATP were shown to compete for the active site of the enzyme. Hence, it is proposed that endogenous metabolites with a strong ligand activity for divalent metals, such as citrate and some amino acids, become integrated into a metabolite feedback control of the enzyme through the release of ATP4– from MgATP. Ca2+ fluxes may participate in the endogenous regulation of adenylate cyclase by modifying the level of CaATP. The free divalent metals show an order of affinityK 0.5(Ca2+)=0.02 mM,K 0.5(Mn2+)=3.8 mM,K 0.5(Mg2+)=4.7 mM, and an order of activity Mn2+>Mg2+>Ca2+. The data indicate that Mn2+ and Mg2+ ions may compete for a regulatory site distinct from the active site and increaseV m without changingK m(MgATP),K m(MnATP), orK i(ATP4–). The interactions of ATP4– and CaATP, which act as competitive inhibitors of the reaction of the enzyme with the substrates MgATP and MnATP, and Mg2+ and Mn2+, which act as activators of the enzyme in the absence of hormones, are shown to follow the random rapid equilibrium BiBi group-transfer mechanism of Cleland with the stipulation that neither Mg2+ nor Mn2+, in excess of their respective participation in substrate formation, are obligatorily required for basal activity. ATP4– and CaATP are involved in dead-end inhibition. For MgCl2 saturation curves at constant total ATP concentration, the computer-generated curves based on the RARE BiBi model predict a change in the Hill cooperativityh from a basal value of 2.6, when Mg2+ is not obligatorily required, to 4.0 when the addition of hormones or neurotransmitters induces an obligatory requirement for Mg2+.Abbreviations used: Me, divalent metal; MeT (MgT or MnT), total Me (Me2+ and its complexes); ATPT, total ATP (ATP4– and its complexes).  相似文献   

8.
The physical properties of the plasma membrane of the aquatic phycomycete Blastocladiella emersonii were investigated, in particular the effects of cations on membrane structure. Intact zoospores and lipid extracts were labelled with the spin-labels 5-nitroxystearate (5-NS), 12-nitroxystearate (12-NS), and 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo). Electron spin resonance spectroscopy indicated a total of three breaks in plots of the hyperfine splitting parameter, 2T|, order parameter, S, and the partition coefficient, f, vs. temperature. The first and third break points (TL and TH) were found to be independent of the external K+, Ca2+, or Mg2+ concentrations. They were similar to the break points found in aqueous dispersions of lipid extracts and correlate well with the temperature limits for zoospore viability. In contrast, the middle break point (TM) was markedly influenced by the external Ca2+ concentration. Ca2+ increased TM from 12°C (no Ca2+ added) to 22°C (10 mM Ca2+), i.e., growth temperature. K+ reversed this Ca2+ effect, downshifting TM from 22°C to 10°C. A comparison of the physico-chemical effects of these ions on the membrane, as revealed by the cation-induced shift in TM, is closely correlated with the temperature dependence and physiological effects of cations on zoospore differentiation. This suggests that cations may modify the physical state of the plasma membrane and be involved in regulating the initial changes during zoospore encystment.  相似文献   

9.
The physical properties of the plasma membrane of the aquatic phycomycete Blastocladiella emersonii were investigated, in particular the effects of cations on membrane structure. Intact zoospores and lipid extracts were labelled with the spin-labels 5-nitroxystearate (5-NS), 12-nitroxystearate (12-NS), and 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo). Electron spin resonance spectroscopy indicated a total of three breaks in plots of the hyperfine splitting parameter, 2T|, order parameter, S, and the partition coefficient, f, vs. temperature. The first and third break points (TL and TH) were found to be independent of the external K+, Ca2+, or Mg2+ concentrations. They were similar to the break points found in aqueous dispersions of lipid extracts and correlate well with the temperature limits for zoospore viability. In contrast, the middle break point (TM) was markedly influenced by the external Ca2+ concentration. Ca2+ increased TM from 12°C (no Ca2+ added) to 22°C (10 mM Ca2+), i.e., growth temperature. K+ reversed this Ca2+ effect, downshifting TM from 22°C to 10°C. A comparison of the physico-chemical effects of these ions on the membrane, as revealed by the cation-induced shift in TM, is closely correlated with the temperature dependence and physiological effects of cations on zoospore differentiation. This suggests that cations may modify the physical state of the plasma membrane and be involved in regulating the initial changes during zoospore encystment.  相似文献   

10.
Membrane Ca2+-ATPase activity was stimulated in vitro separately by T4 (10−10 M) and by epinephrine (10−6 M). In the presence of a fixed concentration of T4, additions of 10−8 and 10−6 M epinephrine reduced the T4 effect on the enzyme. β-Adrenergic blockade with propranolol (10−6 M) prevented stimulation by epinephrine of Ca2+-ATPase activity, but did not prevent the suppressive action of epinephrine on T4-stimulable Ca2+-ATPase. In contrast α1-adrenergic blockade with unlabelled prazosin restored the effect of T4 on Ca2+-ATPase activity in the presence of epinephrine. Like propranolol, prazosin prevented enhancement of enzyme activity by epinephrine in the absence of thyroid hormone. Neither prazosin nor propranolol had any effect on the stimulations by T4 of red cell Ca2+-ATPase in the absence of epinephrine. Analysis of radiolabelled prazosin binding to human red cell membranes revealed the presence of a single class of high-affinity binding sites (Kd, 1.2 × 10−8 M; Bmax, 847 fmol/mg membrane protein). Thus, the human erythrocyte membrane contains α1-radrenergic receptor sites that are capable of regulating Ca2+-ATPase activity.  相似文献   

11.
An intense green photostimulated luminescence in BaAl2O4:Eu2+ phosphor was prepared. The thermoluminescence results indicate that there are at least three types of traps (T1, T2, T3) with different trap depths in BaAl2O4:Eu2+ phosphor according to the bands located at 327, 361 and 555 K, respectively, which are closely associated with the phosphor's long persistent luminescence and photostimulated luminescence properties. In addition, as a novel optical read‐out form, a photostimulated persistent luminescence signal can be repeatedly obtained in BaAl2O4:Eu2+ phosphor. This shows that re‐trapping of the electron released from a deep trap plays an important role in photostimulated persistent luminescence. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

12.
The effect of Cr2+ ions on the Tm (melting temperature) of DNA has been investigated under appropriate conditions for the stabilization of DNA by Mg2+ ions. A significant lowering of Tm, analogous to that observed for Cu2+ under normal conditions, was found, for Cr2+ at pH = 4.2 and [Mg2+] = 5.3 mol per mole of DNA base pair. Cu2+ also lowers Tm under similar conditions. The similarity of the effects of Cr2+ and Cu2+ under comparable conditions may be related to similarities in their coordination properties. It is proposed that Cr2+ and Cu2+ ions facilitate denaturation by holding together groups on the DNA chains in such a manner that base pairing and base stacking are inhibited. Comparative results for Cr3+ and Co2+ are also given for these low pH/Mg2+ ion conditions.  相似文献   

13.
Zinc and calcium ions play important roles in the biosynthesis and storage of insulin. Insulin biosynthesis occurs within the β-cells of the pancreas via preproinsulin and proinsulin precursors. In the golgi apparatus, proinsulin is sequestered within Zn2+- and Ca2+-rich storage/secretory vesicles and assembled into a Zn2+ and Ca2+ containing hexameric species, (Zn2+)2(Ca2+)(Proin)6. In the vesicle, (Zn2+)2(Ca2+)(Proin)6 is converted to the insulin hexamer, (Zn2+)2(Ca2+)(In)6, by excision of the C-peptide through the action of proteolytic enzymes. The conversion of (Zn2+)2(Ca2+)(Proin)6to (Zn2+)2(Ca2+)(In)6 significantly lowers the solubility of the hexamer, causing crystallization within the vesicle. The (Zn2+)2(Ca2+)(In)6 hexamer is an allosteric protein that undergoes ligand-mediated interconversion among three global conformation states designated T6, T3R3 and R6. Two classes of allosteric sites have been identified; hydrophobic pockets (3 in T3R3 and 6 in R6) that bind phenolic ligands, and anion sites (1 in T3R3 and 2 in R6) that bind monovalent anions. The allosteric states differ widely with respect to the physical and chemical stability of the insulin subunits. Fusion of the vesicle with the plasma membrane results in the expulsion of the insulin crystals into the intercellular fluid. Dissolution of the crystals, dissociation of the hexamers to monomer and transport of monomers to the liver and other tissues then occurs via the blood stream. Insulin action then follows binding to the insulin receptors. The role of Zn2+ in the assembly, structure, allosteric properties, and dynamic behavior of the insulin hexamer will be discussed in relation to biological function.  相似文献   

14.
Two thermostable xylanase isoforms T60 and T80 were purified to homogeneity from the cladodes of the xerophytic Cereus pterogonus plant species. After three consecutive purification steps, the specific activity of T60 and T80 isoforms were found to be 178.6 and 216.2 U mg−1 respectively. The molecular mass of both isoforms was determined to be 80 kDa. The optimum temperature for T60 and T80 xylanase isoforms were 60 and 80 °C respectively. The pH was 5.0 for both isoforms. The presence of divalent metal ions (10 mM Co2+) showed stimulatory effects of both catalytic activities, where as in the presence of Hg2+, Cd2+, Cu2+ showed inhibitory effect on these activities at all concentrations studied. The thermodynamic analysis of xylanase activity using denaturation kinetics and the presence divalent cations at 30–100 °C, showed lower ΔH, ΔS, and ΔG values at all the temperatures investigated. The melting temperature of purified T80 xylanase isoform as determined by TG/DTA analysis and it showed the unfolding temperature was 80 °C. The g value and hyperfine (A) value purified xylanase T80 isoform was 2.017 and 10.80 respectively. Immunoblot analysis with antiserum raised against the purified T80 xylanase isoforms revealed single immunolgically related polypeptides of 80 kDa, identical with the polypeptide band produced on SDS-PAGE. The results of double immunodiffusion against the T80 isoforms showed a single precipitin line indicating that the serum used was specific to these xylanase isoforms. The kinetic and thermodynamic properties suggested that xylanase from C. pterogonus may have a potential usage in various industries.  相似文献   

15.
A tRNAPhe derivative carrying ethidium at position 37 in the anticodon loop has been used to study the effect of spermine on conformational transitions of the tRNA. As previously reported (Ehrenberg, M., Rigler, R. and Wintermeyer, W. (1979) Biochemistry 18, 4588–4599) in the tRNA derivative the ethidium is present in three states (T1–T3) characterized by different fluorescence decay rates. T-jump experiments show two transitions between the states, a fast one (relaxation time 10–100 ms) between T1 and T2, and a slow one (100–1000 ms) between T2 and T3. In the presence of spermine the fast transition shows a negative temperature coefficient indicating the existence of a preequilibrium with a negative reaction enthalpy. Spermine shifts the distribution of states towards T3, as does Mg2+, but the final ratio obtained with spermine is higher than with Mg2+, which we tentatively interpret to mean that spermine stabilizes one particular conformation of the anticodon loop.  相似文献   

16.
Although the biological effects of thyroid hormones are mediated by nuclear receptors (genomic mechanisms), interactions with receptors associated with the plasma membrane (non-genomic mechanisms) of target cells are not clear. In this study we investigated the rapid stimulatory effect of thyroxine (T4) on 45Ca2+ uptake as well as ionic currents and intracellular messengers involved in the stimulatory action of T4 in amino acid accumulation in immature rat testes. Results indicated that 10?9 M or 10?6 M T4 was able to increase immediately 45Ca2+ uptake after 60 s of hormone exposure. These results indicate for the first time that voltage-dependent Ca2+ channels and ATP-dependent K+ channels can be seen as a set-point in the stimulatory effect of T4 on amino acid accumulation. Apamin-sensitive small-conductance Ca2+-activated K+ channels (SKCa) and chloride channels were shown to be partially involved in this mechanism. The amino acid accumulation triggered by the PKC pathway suggests a functional link between different ion channel activities and the stimulatory effect of T4 on amino acid accumulation. In conclusion, we show in this study a rapid and stimulatory effect of T4 on calcium uptake and on amino acid accumulation, both events initiated at the plasma membrane, which strongly characterizes a non-genomic effect of T4 in immature rat testes.  相似文献   

17.
A tRNAPhe derivative carrying ethidium at position 37 in the anticodon loop has been used to study the effect of spermine on conformational transitions of the tRNA. As previously reported (Ehrenberg, M., Rigler, R. and Wintermeyer, W. (1979) Biochemistry 18, 4588–4599) in the tRNA derivative the ethidium is present in three states (T1–T3) characterized by different fluorescence decay rates. T-jump experiments show two transitions between the states, a fast one (relaxation time 10–100 ms) between T1 and T2, and a slow one (100–1000 ms) between T2 and T3. In the presence of spermine the fast transition shows a negative temperature coefficient indicating the existence of a preequilibrium with a negative reaction enthalpy. Spermine shifts the distribution of states towards T3, as does Mg2+, but the final ratio [T2][T1] obtained with spermine is higher than with Mg2+, which we tentatively interpret to mean that spermine stabilizes one particular conformation of the anticodon loop.  相似文献   

18.
The binding affinity of Fe(III) to methioninehydroxamate (MX) has been studied spectrophotometrically at I=0.15 M NaCl and T=25 °C. Equilibrium data have been assessed by the program SQUAD(II) in the wavelength range 400–550 nm and in the pH range 1.5–5.0. Five formation constants were determined for the species Fe(MX)(H)3+, Fe(MX)2+, Fe(MX)2(H)23+, Fe(MX)2(H)2+ and Fe2(MX)33+. The stopped-flow kinetic data studied at 470 nm and in the pH range 1.0–3.0 is collectively expressed by the following rate equation at a given pH Rate=(A + BTMX)TMX where TMX=the analytical connection of MX and the parameters A and B are both functions of pH in the range 1.7–3.0, but only A in the range 1.2– 1.7. A proposed mechanism was discussed, based on the equilibrium study, where the role of the chloro species of Fe(OH)2+ and Fe(OH)2+ in the complex formation of Fe(III) with MX has been emphasized. Correlation of the results with pertinent systems has also been discussed.  相似文献   

19.
L H Chang  A G Marshall 《Biopolymers》1986,25(7):1299-1313
The unfolding of B. subtilis 5S RNA is examined by direct calorimetric measurement in the presence of various concentrations of Na+ and Mg2+. The composite differential scanning calorimetry (DSC) curve is analyzed into 3–5 individual two-state melting transitions. In the absence of added Na+ or Mg2+, the 5S RNA segments melt together at Tm = 40°C. Addition of Na+ stabilizes the molecular structure (Tm = 56°C) and widens the melting temperature range, so that up to five component transitions are observed. Addition of Mg2+ alone produces a very stable structure (Tm = 75°C) with highly cooperative melting. Finally, addition of both Na+ and Mg2+ produces the highest stability (Tm = 76°C). The results are interpreted according to hypothetical secondary and tertiary base-pairing schemes. The conformational changes demonstrated here may facilitate the movement of the protein synthesis machinery during RNA translation.  相似文献   

20.
Iodothyronine monodeiodinase activities in homogenates of cultured monkey hepatocarcinoma cells were measured by the deiodination of [3,5-125I]triido-l-thyronine or 3-[3′5′-125I]triido-l-thyronine (phenolic ring-labeled ‘reverse’ triiodothyronine). The assay system utilized a small ion-exchange column (AG50W-X4, 0.9×~1 cm) to measure 125I?. Both deiodinases were destroyed by boiling for 1 min.Maximal nonphenolic ring deiodination was observed at pH 7.9 whereas maximal phenolic ring deiodination was at pH 6.3. Both reactions were enhanced strongly by dithiothreitol (0.1–5 mM), and slightly by 5 mM β-mercaptoethanol. Phenolic ring deiodination was strongly inhibited by 0.1 mM propylthiouracil. Nonphenolic ring deiodination was accelerated by EDTA (1.2 mM) and inhibited by Mg2+ (5 mM). Methylmercaptoimidazol and Mg2+, Ca2+ and Mn2+ (0.1–1.0 mM) had little or no effect on either reaction, but Zn2+ (0.1 mM) strongly inhibited both.Both reactions were inhibited by excess iodothyronine analogues at 10 mM to 10μM, and thyroxine was shown to be a competitive inhibitor in both cases. On the basis of relative affinities and inhibitory effects, it appears that the order of affinity for the phenolic ring deiodinase is 3,3′,5′-triiodo-l-thyronine-(rT3) > l-thyroxine(T4) > 3,4,3′-triido-l-thyronine(T3), whereas for the nonphenolic ring deiodinase the order is T3 > T4 > rT3. Diiodotyrosine did not affect their deiodination.  相似文献   

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