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21.
R Dante M Arnaud A Niveleau 《Biochemical and biophysical research communications》1983,114(1):214-221
The treatment of transformed rat cells with micromolar amounts of 5'deoxy 5'methyl thioadenosine induces rapid effects on the rate of methylation of DNA concomitantly with alterations of intracellular pools of S-adenosyl methionine and S-adenosyl homocysteine. Pulse chase labelling experiments indicate that 5'deoxy 5'methylthioadenosine does not inhibit the degradation of S-adenosyl homocysteine but inhibits the consumption of S-adenosyl methionine. In vitro transmethylation assays performed with heterologous DNA show that low doses of the thioethernucleoside do not significantly affect the DNA methyltransferase activity of cellular extracts. The biological role of 5'deoxy 5'methylthioadenosine, a natural molecule formed during the synthesis of polyamines is discussed. 相似文献
22.
Adenosine deaminase normalizes cyclic AMP responses of hypothyroid rat fat cells to forskolin, but not beta-adrenergic agonists 总被引:2,自引:0,他引:2
Lipolysis and cyclic AMP accumulation in response to beta-adrenergic agonists or forskolin are severely impaired in fat cells from the hypothyroid rat. Incubating hypothyroid rat fat cells with adenosine deaminase normalizes the cyclic AMP response to forskolin, but not to beta-adrenergic agonists. Increased sensitivity to adenosine action in the hypothyroid state appears to be the basis for the impaired cyclic AMP response to forskolin, but does not appear to be the underlying defect responsible for the impaired response to beta-adrenergic agonists. 相似文献
23.
Calcium-dependent activation and deactivation of rod outer segment phosphodiesterase is calmodulin-independent 总被引:2,自引:0,他引:2
ATP-dependent activation and deactivation of retinal rod outer segment phosphodiesterase is affected by calcium [Kawamura, S. and Bownds, M. D., J. Gen. Physiol. 77:571-591(1981)]. Our data demonstrate that although calmodulin has been found in rod outer segments [Liu, Y. P. and Schwartz, H., Biochim. Biophys. Acta 526:186-193(1978); Kohnken, R. E. et al, J. Biol. Chem. 256:12517-12522(1981)], this protein is not involved in calcium-dependent phosphodiesterase activation at light levels at which calcium clearly affects this enzyme's activity. Furthermore, calmodulin does not mediate the calcium-dependent deactivation of phosphodiesterase. 相似文献
24.
A highly sensitive enzymatic assay for diadenosine 5′,5?-P1,P3-triphosphate (Ap3A) has been established on the basis of the coupled luminescence assay for diadenosine 5′,5?-P1,P3-tetraphosphate (A. Ogilvie (1981)Anal. Biochem.115, 302–307). Snake venom phosphodiesterase splits Ap3A into AMP plus ADP which can be measured in a luminescence reaction containing pyruvate kinase, phosphoenolpyruvate and luciferin-luciferase. The procedure is linear with Ap3A levels ranging from 0.1 to 2 pmol. The assay has been used to measure Ap3A in various eukaryotic cells after ion-exchange chromatography and high-performance liquid chromatography of acidic extracts of the cells. The level of diadenosine triphosphate was higher in all instances than the level of diadenosine tetraphosphate. When growing in the abdominal cavity of mice, Ehrlich ascites tumor cells contained high amounts of Ap3A (), allowing direct optical determination in the HPLC chromatography. The quantitative measurement of Ap3A with the luminescence assay gave identical results. Ap3A extracted from Ehrlich cells was also chromatographed with authentic nucleotide in two thin-layer systems providing additional proof for the existence of Ap3A in biological material. 相似文献
25.
Abstract: The mechanisms by which deoxycytidine enters and leaves brain, choroid plexus, and CSF were investigated by injecting [3H]deoxycytidine intraarterially, intravenously, and intraventricularly. After intracarotid injection of deoxycytidine (1.0 μM) into rats, deoxycytidine did not pass through the blood-brain barrier at a faster rate than sucrose. [3H]Deoxycytidine, either alone or together with unlabeled deoxycytidine, was infused at a constant rate into conscious adult rabbits. At 130 min, [3H]deoxycytidine readily entered CSF, choroid plexus, and brain. In brain, approx. 60% of the nonvolatile radioactivity was attributable to [3H]deoxycytidine phosphates. The addition of 0.22 mmol/kg unlabeled deoxycytidine to the infusion syringe decreased the phosphorylation of [3H]deoxycytidine in brain by approx. 50%; the addition of 2.2 mmol/kg of unlabeled deoxycytidine to the infusion syringe decreased the relative entry of [3H]deoxycytidine into CSF and brain by approx. 50 and 75%, respectively. Two hours after the intraventricular injection of [3H]deoxycytidine, [3H]deoxycytidine was rapidly cleared from CSF, in part, to brain, where approx. 65% of the [3H]deoxycytidine was converted to [3H]deoxycytidine phosphates. The intraventricular injection of unlabeled deoxycytidine with the [3H]deoxycytidine decreased the phosphorylation of [3H]deoxycytidine in the brain significantly and also decreased the clearance of [3H]deoxycytidine from the CSF. These results were interpreted as showing that the entry of deoxycytidine from blood into CSF occurs by a saturable transport system within the choroid plexus. Once within the CSF, the deoxycytidine can enter brain, undergo phosphorylation to deoxycytidine phosphates, and subsequently be incorporated into DNA. 相似文献
26.
Antimutagenic effects of cinnamaldehyde on mutagenesis by chemical agents were investigated in Escherichia coli WP2 uvrA- trpE-. Cinnamaldehyde, when added to agar medium, greatly reduced the number of Trp+ revertants induced by 4-nitroquinoline 1-oxide (4-NQO) without any decrease of cell viability. This antimutagenic effect could not be explained by inactivation of 4-NQO caused by direct interaction with cinnamaldehyde. Mutagenesis by furylfuramide (AF-2) was also suppressed significantly. Mutations induced by methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS) were slightly inhibited. However, cinnamaldehyde was not at all effective on the mutagenesis of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Two derivatives of cinnamaldehyde, cinnamyl alcohol and trans-cinnamic acid, did not have as strong antimutagenic effects on 4-NQO mutagenesis as cinnamaldehyde had. Because cinnamaldehyde showed marked antimutagenic effects against mutations induced by UV-mimic mutagens but not those induced by MNNG or EMS, it seems that cinnamaldehyde might act by interfering with an inducible error-prone DNA repair pathway. 相似文献
27.
Michael Ready Sandra Bird Gail Rothe J.D. Robertus 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,740(1):19-28
It has been known for some time that pokeweed antiviral protein acts by enzymatically inhibiting protein synthesis on eucaryotic ribosome systems. The site of this action is known to be the ribosome itself. In this paper we show that the pokeweed antiviral protein reaction against ribosomes is a strong function of salt concentrations, where 160 mM K+ and 3 mM Mg2+ retards the reaction, while 20 mM K+ and 2 mM Mg2+ allows maximum reaction rate. It is also shown, however, that an unidentified protein in the postribosomal supernatant solution, together with ATP, allows the ribosome to be attacked even in the presence of high salt. Kinetic analysis of the antiviral protein reaction has been carried out under both sets of conditions, and reveals that the turnover number for the enzyme is about 300–400 mol/mol per min. in each case. The Km for ribosomes is 1 μM in the presence of low salt and 0.2 μM at higher salt in the presence of postribosomal supernatant factors plus ATP. The antiviral protein reaction is also shown to be pH dependent and is controlled by a residue with pKa value of approx. 7.0, apparently a histidine. Stoichiometric reaction of the enzyme with iodoacetamide results in a significant loss of antiribosomal activity. 相似文献
28.
A chemiosmotic molecular mechanism for proton-translocating adenosine triphosphatases 总被引:16,自引:0,他引:16
P Mitchell 《FEBS letters》1974,43(2):189-194
29.
W D McCubbin G E Willick C M Kay 《Biochemical and biophysical research communications》1973,50(3):926-933
The fluorescent analog of adenosine triphosphate (ATP)1 1,N6-ethenoadenosine triphosphate, (εATP), has been utilized as a substitute for ATP in the myosin and heavy meromyosin ATPase systems. For myosin, the analog εATP replaced ATP with a somewhat larger Km (2.6 × 10?4 mole ??1 for εATP as opposed to 8.8 × 10?5 mole ??1 for ATP), indicating that the apparent affinity of the enzyme for εATP is less than for ATP. Perhaps of more interest, further comparison yielded a Vmax for εATP about two and one half times the value for ATP (20 μmole PO4 sec?1 g protein?1 as opposed to 8.1 μmole sec?1 g protein?1). Results for the HMM-εATPase system were similar, yielding a Km value of 1.47 × 10?4 mole ??1 and a Vmax of 54.2 μmole PO4 sec?1 g protein?1, as opposed to corresponding Km and Vmax values of 1.23 × 10?4 mole ??1 and 20.4 μmole PO4 sec?1 g protein?1, respectively for the HMM-ATP interaction. The pH dependence of εATPase for both systems was comparable to ATP, suggesting a similarity in the mechanism of hydrolysis of the two nucleotides. Activation of εATPase by Ca2+ in the presence of 0.5 M KCl was comparable to ATPase for both systems, but inhibition by Mg2+ seemed to be more effective for εATPase. These results indicate that εATP is an excellent substitute for ATP in the myosin and heavy meromyosin systems and because of its insertion into the active site of these muscle proteins, it promises to be a very useful probe for conformation studies at this level. 相似文献
30.
Analysis of Adenosine Immunoreactivity, Uptake, and Release in Purified Cultures of Developing Chick Embryo Retinal Neurons and Photoreceptors 总被引:2,自引:1,他引:1
Roberto Paes de Carvalho Karen M. Braas† Solomon H. Snyder† Ruben Adler† 《Journal of neurochemistry》1990,55(5):1603-1611
We have investigated the presence of endogenous adenosine and of mechanisms for adenosine uptake and release in chick embryo retinal neurons and photoreceptors grown in purified cultures in the absence of glial cells. Simultaneous autoradiographic and immunocytochemical analysis showed that endogenous adenosine and the uptake mechanism for this nucleoside colocalize in practically all the photoreceptors, but only in approximately 20% of the neurons. Approximately 25% of the neurons showed either immunocytochemical labeling or autoradiographic labeling, while greater than 50% of the neurons were unlabeled with both techniques. [3H]Adenosine uptake was saturable and could be inhibited by nitrobenzylthioinosine and dipyridamole and by pretreatment of the [3H]adenosine with adenosine deaminase. Although these observations indicate that the uptake is specific for adenosine, only 35% of accumulated radioactivity was associated with adenosine, with the remaining 65% representing inosine, hypoxanthine, and nucleotides plus uric acid. Adenosine as well as several of its metabolites were released by the cells under basal as well as K(+)-stimulated conditions. Potassium-enhanced release was blocked by 10 mM CoCl2 or in Ca2(+)-free, Mg2(+)-rich solutions. The results indicate that retinal cells that synthesize, store, and release adenosine differentiate early during embryogenesis and are therefore consistent with a hypothetical role for adenosine in retinal development. 相似文献