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101.
Rolf Andersson Olle Larm Elisabeth Scholander Olof Theander 《Carbohydrate research》1980,78(2):257-265
Sucrose and (1) were oxidised with bromine in aqueous solution at pH 7 and room temperature. The resulting keto derivatives were converted into their more-stable O-methyloximes, which were characterised by spectroscopic and chromatographic methods. Oxidation of 1 occurred at C-3 and C-5, with a preference for C-5. In the sucrose derivatives isolated after oxidation, those having a keto group in the glucopyranosyl moiety preponderated. The axial fructofuranosyl aglycon protects position 3 in the glucopyranosyl group and oxidation occurs only at C-2 and C-4. Small amounts of sucrose oxidised at C-3 in the fructofuranosyl moiety were also found. 相似文献
102.
Relationship between nitroglycerin, cyclic GMP and relaxation of vascular smooth muscle. 总被引:5,自引:0,他引:5
Nitroglycerin (NG) caused a dose dependent-relaxation of the bovine mesenteric artery with an ED50-value of 2.7 × 10?8M. The relaxant effect of NG was significantly correlated to an increase in the cGMP content of the artery. There was a significant non-linear component in the data. At moderate cGMP levels relaxation and cGMP changes were correlated. At high levels of cGMP, however, the mechanism responsible for the nitroglycerin-mediated relaxation seemed to be completely activated and a further increase in cGMP did not induce additional relaxation. The cGMP content of the preparation was not significantly changed by nitroglycerin. The cGMP increase induced by nitroglycerin preceded the relaxation. A maximal increase of cGMP was observed after 2 min and the levels subsequently declined. This decline was not accompanied by an increase in the tissue tension. It is suggested from these experiments that cGMP might cause a relaxation of the vascular smooth muscle. Furthermore, if this suggestion is true, there seems to exist a “receptor reserve” for NG with respect to its relaxing action, since an over-capacity for cGMP production is present. 相似文献
103.
104.
Hepatic tyrosine aminotransferase of the frog Rana temporaria was partially purified by (NH4)2SO4 fractionation and successive chromatography on DEAE-cellulose DE-52, Ultrogel AcA-34, DEAE-cellulose DE-52 again and, finally, hydroxyapatite. During the last step, the enzyme activity separated into two fractions; traces of a third fraction were also found. The major form was purified 6000-fold to a specific activity of 200 units/mg of protein; it was about 50% pure by electrophoretic criteria. It had mol.wt. about 85 000 as determined by gel filtration on a Sephadex G-100 column. It was not activated by added pyridoxal 5'-phosphate. The enzyme was, however, inactivated by the pyridoxal phosphate reactants canaline and amino-oxyacetate. The enzyme was specific for 2-oxoglutarate as the amino group acceptor. Homogentisate inhibited the enzyme and adrenaline was an activator; both effects were seen at low concentrations of the effectors. The relationship between initial rate and tyrosine or 2-oxoglutarate concentration was abnormal and complex. Form-2 enzyme had similar or identical molecular weight, cofactor requirements, oxo acid specificity and kinetics. 相似文献
105.
106.
107.
Isolation and characterization of the dut gene of Escherichia coli. II. Restriction enzyme mapping and analysis of polypeptide products 总被引:5,自引:0,他引:5
Restriction endonuclease mapping of previously constructed dut plasmids has been carried out using the enzymes PvuI, PvuII and SacI. Various dut plasmids were also tested in the "maxicell" protein-synthesizing system. They all show two protein bands in common, one of Mr 16000 in agreement with the size previously reported for the purified dUTPase subunit (Shlomai and Kornberg, 1978). With the information obtained the structural gene for dUTPase can be assigned to a 950-bp SacI-PvuII fragment of the E. coli genome. Studies, described in the preceding paper, on the overproduction of dUTPase by bacterial strains carrying different dut plasmids strongly suggest that the dut gene is transcribed in the direction from the SacI site towards the PvuII site and that the SacI site is located within the dut control region. The second protein band observed in the "maxicell" experiments has an Mr of 23500. Its identity is unknown but it may represent a precursor of dUTPase or the product of a separate gene located between dut and pyrE. 相似文献
108.
109.
C Andersson E Mosialou A E Adang G J Mulder A van der Gen R Morgenstern 《The Journal of biological chemistry》1991,266(4):2076-2079
The substrate specificity of rat liver microsomal glutathione transferase toward glutathione has been examined in a systematic manner. Out of a glycyl-modified and eight gamma-glutamyl-modified glutathione analogues, it was found that four (glutaryl-L-Cys-Gly, alpha-L-Glu-L-Cys-Gly, alpha-D-Glu-L-Cys-Gly, and gamma-L-Glu-L-Cys-beta-Ala) function as substrates. The kinetic parameters for three of these substrates (the alpha-D-Glu-L-Cys-Gly analogue gave very low activity) were compared with those of GSH with both unactivated and the N-ethylmaleimide-activated microsomal glutathione transferase. The alpha-L-Glu-L-Cys-Gly analogue is similar to GSH in that it has a higher kcat (6.9 versus 0.6 s-1) value with the activated enzyme compared with the unactivated enzyme but displays a high Km (6 versus 11 mM) with both forms. Glutaryl-L-Cys-Gly, in contrast, exhibited a similar kcat (8.9 versus 6.7 s-1) with the N-ethylmaleimide-treated enzyme but retains a higher Km value (50 versus 15 mM). Thus, the alpha-amino group of the glutamyl residue in GSH is important for the activity of the activated microsomal glutathione transferase. These observations were quantitated by analyzing the changes in the Gibbs free energy of binding calculated from the changes in kcat/Km values, comparing the analogues to GSH and each other. It is estimated that the binding energy of the alpha-amino group of the glutamyl residue in GSH contributes 9.7 kJ/mol to catalysis by the activated enzyme, whereas the corresponding value for the unactivated enzyme is 3.2 kJ/mol. The importance of the acidic functions in glutathione is also evident as shown by the lack of activity with 4-aminobutyric acid-L-Cys-Gly and the low kcat/Km values with gamma-L-Glu-L-Cys-beta-Ala (0.03 and 0.01 mM-1s-1 for unactivated and activated enzyme, respectively). Utilization of binding energy from a correctly positioned carboxyl group in the glycine residue (10 and 17 kJ/mol for unactivated and activated enzyme, respectively) therefore also appears to be required for optimal activity and activation. A conformational change in the microsomal glutathione transferase upon treatment with N-ethylmaleimide or trypsin, which allows utilization of binding energy from the alpha-amino group of GSH as well as the glycine carboxyl in catalysis, is suggested to account for at least part of the activation of the enzyme. 相似文献
110.
Interleukin-4 downregulates the p70 chain of the IL-2 receptor on peripheral blood mononuclear cells
Data that support a differential regulation of the interleukin-2 receptor (IL-2R) alpha-(p55 or Tac) and beta-chain (p70) expression by IL-4 are presented. Cytofluorometric analysis performed on peripheral blood mononuclear cells, some of which had been nylon wool passed (enriched for T-cells), in the presence or absence of phytohemagglutinin (PHA) or OKT3, demonstrated that IL-4 has a dose-dependent capacity to inhibit beta-chain IL-2R expression, whereas the alpha-chain is nearly unaffected. We could also, as a consequence of the decreased p70 expression, detect a slight increase in the amounts of IL-2 obtained from PHA-stimulated cultures, when IL-4 was present. Further, the proliferative response, especially to IL-2, but also to PHA alone, was depressed in the presence of IL-4. These data thus give further support to the idea that not only the IL-2R complex as such, but also the two individual IL-2R chains, can be independently regulated. 相似文献