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1.
The observed equilibrium constants (Kobs) for the l-phosphoserine phosphatase reaction [EC 3.1.3.3] have been determined under physiological conditions of temperature (38 °C) and ionic strength (0.25 m) and physiological ranges of pH and free [Mg2+]. Using Σ and square brackets to indicate total concentrations Kobs = Σ L-serine][Σ Pi]Σ L-phosphoserine]H2O], K = L-H · serine±]HPO42?][L-H · phosphoserine2?]H2O]. The value of Kobs has been found to be relatively sensitive to pH. At 38 °C, K+] = 0.2 m and free [Mg2+] = 0; Kobs = 80.6 m at pH 6.5, 52.7 m at pH 7.0 [ΔGobs0 = ?10.2 kJ/mol (?2.45 kcal/mol)], and 44.0 m at pH 8.0 ([H2O] = 1). The effect of the free [Mg2+] on Kobs was relatively slight; at pH 7.0 ([K+] = 0.2 m) Kobs = 52.0 m at free [Mg2+] = 10?3, m and 47.8 m at free [Mg2+] = 10?2, m. Kobs was insignificantly affected by variations in ionic strength (0.12–1.0 m) or temperature (4–43 °C) at pH 7.0. The value of K at 38 °C and I = 0.25 m has been calculated to be 34.2 ± 0.5 m [ΔGobs0 = ?9.12 kJ/mol (?2.18 kcal/ mol)]([H2O] = 1). The K for the phosphoserine phosphatase reaction has been combined with the K for the reaction of inorganic pyrophosphatase [EC 3.6.1.1] previously estimated under the same physiological conditions to calculate a value of 2.04 × 104, m [ΔGobs0 = ?28.0 kJ/mol (?6.69 kcal/mol)] for the K of the pyrophosphate:l-serine phosphotransferase [EC 2.7.1.80] reaction. Kobs = [Σ L-serine][Σ Pi][Σ L-phosphoserine][H2O], K = [L-H · serine±]HPO42?][L-H · phosphoserine2?]H2O. Values of Kobs for this reaction at 38 °C, pH 7.0, and I = 0.25 m are very sensitive to the free [Mg2+], being calculated to be 668 [ΔGobs0 = ?16.8 kJ/mol (?4.02 kcal/mol)] at free [Mg2+] = 0; 111 [ΔGobs0 = ?12.2 kJ/mol (?2.91 kcal/mol)] at free [Mg2+] = 10?3, m; and 9.1 [ΔGobs0 = ?5.7 kJ/mol (?1.4 kcal/mol) at free [Mg2+] = 10?2, m). Kobs for this reaction is also sensitive to pH. At pH 8.0 the corresponding values of Kobs are 4000 [ΔGobs0 = ?21.4 kJ/mol (?5.12 kcal/mol)] at free [Mg2+] = 0; and 97.4 [ΔGobs0 = ?11.8 kJ/ mol (?2.83 kcal/mol)] at free [Mg2+] = 10?3, m. Combining Kobs for the l-phosphoserine phosphatase reaction with Kobs for the reactions of d-3-phosphoglycerate dehydrogenase [EC 1.1.1.95] and l-phosphoserine aminotransferase [EC 2.6.1.52] previously determined under the same physiological conditions has allowed the calculation of Kobs for the overall biosynthesis of l-serine from d-3-phosphoglycerate. Kobs = [Σ L-serine][Σ NADH][Σ Pi][Σ α-ketoglutarate][Σ d-3-phosphoglycerate][Σ NAD+][Σ L-glutamat0] The value of Kobs for these combined reactions at 38 °C, pH 7.0, and I = 0.25 m (K+ as the monovalent cation) is 1.34 × 10?2, m at free [Mg2+] = 0 and 1.27 × 10?2, m at free [Mg2+] = 10?3, m.  相似文献   

2.
The nonenzymatic reaction of ethanol-derived CH3CHO with tissue constituents continues to be of interest as a potential mechanism underlying the toxicity of alcohol. The current study has focused on the spontaneous condensation of CH3CHO with H4folate under physiological conditions (38 °C, pH 7.0, I = 0.25 M). Computer analysis of uv spectral changes with increasing CH3CHO concentrations demonstrated the presence of at least two different adducts. The observed equilibrium constant (Kobs) for the formation of the first adduct is 91 ± 2 m?1 (121 ± 2 m?1 at 25 °C), a value which is unaffected by variations in ionic strength (0.06–1.0 m) or by free [Mg2+] up to 5 mm. The NMR spectrum is compatible with the structure: 5,10-CH3CH-H4folate analogous to the naturally occurring 5,10-CH2-H4folate. The formation of the latter compound from HCHO and H4folate, however, is much more favorable under the same conditions [Kobs = 3.0 ± 0.2 × 104 M?1 (38 °C), 3.6 ± 0.1 × 104 M?1 (25 °C)]. At the levels of CH3CHO which accumulate during ethanol metabolism in vivo only a small fraction of the H4folate will exist as the CH3CHO derivative, yet it may ultimately be the ratio of free CH3CHO to free HCHO in tissue which determines the physiological importance of the CH3CHO adduct. Other adduct(s) of CH3CHO with H4folate are observed at very high levels of CH3CHO but are unlikely to be of physiological significance.  相似文献   

3.
A sensitive and rapid method to estimate concentrations of functional metallothionein in small biological samples, based upon the acid stability of 203Hg binding and solubility of this protein in trichloroacetic acid is described. Sephadex G-10 minicolumns supported in centrifuge tubes afforded separation and quantitation of isotope bound metallothionein from unbound metal. Elution of metallothionein bound 203Hg was achieved by short term-low speed centrifugation that segregated chelator-ligand complex into the eluate while unbound ligand remained in the gel. A well characterized standard of pure metallothionein protein was utilized to verify the specificity and sensitivity of the modified assay. Metallothionein levels were estimated by 203Hg binding in extracts of wild type and cadmium resistant Chinese hamster ovary cells treated with maximum tolerable concentrations of CdCl2. Similar separation methods demonstrated [35S]-cysteine incorporation into induced metallothionein. Additionally, induction of metallothionein was observed after treatment with particulate CdS but not crystalline NiS particles. These results demonstrate that the modified assay system is easily applied to serial measurement of metallothionein levels in multiple small biological samples.  相似文献   

4.
Apo-horseradish peroxidase was affinity-labeled with the monosulfuric anhydride derivative of mesoheme. The stoichiometry of heme anhydride binding was 1.1 moles of the anhydride per mole of apo-peroxidase.Tryptic digestion of the affinity-labeled peroxidase yielded a major lysine peptide which corresponded in composition to peptides T8 and T9a in the sequence of horseradish peroxidase (Welinder, K. G., Eur. J. Biochem. 96: 483–502, 1979) which contained one mole of histidine (histidine 170) per mole of peptide.  相似文献   

5.
Analyses of sedimentation equilibrium data   总被引:1,自引:0,他引:1  
A numerical procedure is presented which can quite adequately compute the molecular weight averages as a function of solute concentration from sedimentation equilibrium data for homogeneous systems and for monomer-dimer associating systems with a possible extension to heterogeneous systems where monotonic variation in the weight average molecular weight is observed such as in weakly associating or dissociating systems. The procedure utilizes the method of orthogonal polynomials for curve fitting which allows for a rapid determination of best fit with minimal round off error. The procedure is particularly applicable in cases where the concentration of solute at the meniscus can be considered to be neither appreciable and reasonably well determined as in low speed sedimentation equilibrium experiments, nor essentially zero as in high speed sedimentation equilibrium experiments where the calculations become somewhat more simplified. The use of moderate speed sedimentation equilibrium has the advantage of providing a more broad concentration distribution in the centrifuge cell which yields more extensive information concerning dissociating systems yet still provides results at low solute concentrations where most solutes can be considered to be behaving ideally.  相似文献   

6.
The activity of 3-hydroxy-3-methylglutaryl CoA reductase in homogenates of the corpora allata of the tobacco hornworm, Manduca sexta, was competitively inhibited by compactin. The KI for the sodium salt form of compactin was 0.9 nM for the reductase from both male and female corpora allata. In intact female corpora allata juvenile hormone biosynthesis was also inhibited by approximately 50 percent at 10 nM compactin. Following injection with compactin, darkening of the cuticle, an indication of juvenile hormone deficiency, was observed in larvae after ecdysis from third to fourth instar. Hence, compactin shows potential as an inhibitor of insect growth and development.  相似文献   

7.
Isoelectric focusing (IEF) on 110- and 440-ml columns can result in the loss of enzyme activity. Such losses can be reduced or eliminated by focusing on 20-ml columns. Artifacts which arise during the IEF procedure may result from protein-protein interaction or from the interaction of pH with other sources of artifacts. The reasons for greater loss of activity on large columns and the mechanism of artifacts due to pH and protein-protein interactions are discussed.  相似文献   

8.
Natural-abundance 13C NMR spectroscopy at 15.04 MHz has been used to examine the effects of pH, calcium, and lanthanide ions on the polypeptide cardiac stimulant Anthopleurin-A in aqueous solution. The carboxyl resonance from the aspartic acid residue not observed in a previous study (R. S. Norton and T. R. Norton, 1979, J. Biol. Chem.254, 10220–10226) has been identified and an apparent pKa of 3.4 obtained. More accurate estimates have been derived for the apparent pKa values of the two histidine residues. Binding of Ca2+ ions has been found by equilibrium dialysis and 13C NMR to be weak (Kd > 0.1 M). The interaction with lanthanide ions is slightly stronger, but binding occurs at the C terminus as well as at a site involving one or both of the aspartate carboxylate groups. These results suggest that possible Anthopleurin-A-induced calcium translocation in the myocardial cell is a secondary effect. The interaction of Anthopleurin-A with lipid monolayers has also been examined. Binding occurs to neutral and zwitterionic lipids, but is stronger with negatively charged lipids, particularly cardiolipin. This interaction is also influenced by the presence of Ca2+ ions. The implications of these results for the mechanism of action of this polypeptide are discussed.  相似文献   

9.
The question of nonthymus-derived lymphocyte-mediated cytotoxicity was investigated with T and B cell subpopulations separated from the blood of normal donors. Mononuclear cells, T cells (E-RFC), and cell preparations enriched for B cells (non-E-RFC) by depletion of E-RFC gave negligible cytotoxic responses when incubated with either human melanoma or lung fibroblast target cells. In contrast, EAC and ZC rosetting cells separated from this same B-rich population consistently gave cytotoxic responses which were not dependent on either antibody or phagocytic cells. The cytotoxic effector cells appeared to be nonthymus-derived lymphocytes as characterized by C3 receptor rosetting and presence of surface membrane immunoglobulin on the majority of cells. In addition, supernatants from EAC-RFC cultures contained lymphotoxin (LT) activities which were eightfold higher than those of control E-RFC cultures. These findings suggest the existence of a nonthymus-derived cell cytotoxic effector mechanism, induced by the binding of membrane C3 receptors, which is independent of antibody.  相似文献   

10.
Temperature and starvation were found to be factors which affected the PPP dehydrogenase activities in brook trout liver. Fish acclimated at 5 °C possessed greater levels of G6PD, H6PD, and 6PGD activity than those fish maintained at 10 or 15 °C. This phenomenon was probably associated with increased lipogenesis during cold acclimation.During starvation hepatic G6PD and 6PGD activities decreased, whereas H6PD activity increased slightly. Upon refeeding, the G6PD level gradually increased, but the “overshoot” in enzyme activity reported in mammalian studies was not observed.When both cold acclimation and starvation were studied simultaneously, regulation by temperature was initially the dominant control factor. After 6 wk at 5 °C, there was no difference in specific activities between starved and fed fish. However, fish maintained at 5 °C for longer than 2 mo did show the normal response to starvation and refeeding. Therefore, regulation of the PPP by temperature appears to be a transitory phenomenon and may be associated with temporary metabolic reorganization in the fish.  相似文献   

11.
Concentrations of relaxin, prolactin, unchromatographed estradiol 17β (E) and progesterone (P4) were measured in serial samples of inferior vena caval blood, in three pigs, during late pregnancy, and parturition. Maximal relaxin concentrations occurred 60 to 24h before parturition, and ranged from 60 to 286ng/ml. Prolactin concentrations increased from 12.5ng/ml, 48 to 36 hours before parturition, to between 79 to 184ng/ml. At the time of the relaxin surge, E levels were high, and P4 concentrations were decreasing, thus raising the EP4 ratio. A surge in prolactin concentrations followed upon a decline of P4 to less than 10ng/ml, coinciding with the increase in relaxin concentrations in 2 gilts, and following the surge in relaxin in the third. Udder development occurred near the time of increased relaxin concentrations. ‘Milk let down’ followed maximal relaxin and prolactin concentrations in two gilts, and the increase in prolactin, rather than the increase in relaxin concentration, in the third.  相似文献   

12.
Rats tolerant to human serum albumin (HSA) were injected with selected lymphocyte populations and challenged with HSA plus adjuvant to test for loss of tolerance. Thoracic duct lymphocytes (TDL) from normal or immune rats, either untreated or depleted of Ig-bearing cells or HSA-binding cells by affinity chromatography were all equally effective in restoring the HSA antibody response in previously tolerant recipients. In contrast, recirculating B cells (TDL from B rats) were not effective. The results indicated that unresponsiveness to HSA was a lesion of the T- but not the B-cell compartment. However, antibody affinity failed to mature to a high level in tolerant rats that were restored with T cells, and the response of transferred primed B cells into unresponsive recipients was inhibited, suggesting that the tolerant state was not merely due to a T-cell deletion.  相似文献   

13.
During commonly used saturation procedures of transferrin with iron compounds, both as ferri and ferrous, polynuclear iron compounds are easily formed, even when nitrilotriacetate (NTA) is used as a strong iron ligand. The presence of these nonspecific bound irons is demonstrated with Mossbauer spectroscopy and with electronic optical spectroscopy. But no evidence, however, has been found of two different iron binding sites. Because dialysis is not able to remove all polynuclear iron, an easy method with gel filtration has been developed that does remove the polynuclear iron. Some notes are made about the often used method, in transferrin biochemistry, of saturation determination, i.e. the quotient of the absorbances of 470 and 280 nm.  相似文献   

14.
Experimental autoimmune encephalomyelitis (EAE) in mice is dependent upon the use of Bordetella pertussis suspensions as an adjuvant. Intravenous administration of B. pertussis causes an increased vascular permeability in brain tissue and an increased vascular sensitivity to vasoactive amines which promotes the development of EAE. The efficacy of different batches and strains of B. pertussis in the expression of EAE closely correlates with the vasoactive amine sensitization activity of each material tested. Pertussigen, the histamine sensitizing factor (HSF), is responsible for these adjuvant properties whereas purified endotoxin is inactive. The effect of cimetidine, diphenhydramine, methysergide, reserpine, and cyproheptadine on B. pertussis induced histamine sensitivity and the expression of EAE are examined. Cyproheptadine, an agent with mixed histamine and serotonin blocking properties, blocks both B. pertussis-induced vasoactive amine sensitization and the expression of EAE.  相似文献   

15.
Development of radioimmunoassay for thromboxane B2   总被引:3,自引:0,他引:3  
A simple method for the preparation of rat liver urate oxidase is described. The enzyme was purified from rat liver homogenate by cell fractionation, detergent treatment, alkali treatment, and affinity chromatography on 8-aminoxanthine-bound Sepharose 4B. This enzyme preparation had a specific activity of 9.1 U/mg of protein and was purified about 1000-fold from the liver homogenate. After sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis followed by staining with Coomassie brilliant blue, this preparation yielded one protein band at a position corresponding to a molecular weight of 33,000.  相似文献   

16.
Both internal and external proteins in vesicular stomatitis virus were labeled when intact virions were iodinated with 50 μm iodide; however, only the surface proteins were labeled when the same procedure was carried out at low iodide concentrations (below 0.5 μm). This result together with similar observations reported earlier with another enveloped virus, Rous-associated virus-61 (RAV-61), suggest that viral envelopes provide a barrier to iodination by chloramine-T at low, but not at high, iodide concentrations. By monitoring the permeability of the RAV-61 envelope to successive iodinations and to iodination in the presence of chaotropic thiocyanate ions, it was shown that the permeability of the viral envelope was not altered at the higher concentrations of iodide. Further results support the hypothesis that iodination mediated by chloramine-T inolves two different iodinating species: (a) a membrane impermeable one, possibly “iodamine-T,” which predominates at low iodide concentrations, and (b) a membrane permeable species, possibly molecular iodine, which predominates at high concentrations of iodide. These results reinforce the proposal that the chloramine-T procedure is a useful method for specifically labeling surface proteins of lipid-enveloped structures.  相似文献   

17.
The expression of transferrin receptors on murine peritoneal macrophages has been shown to be down regulated during functional activation in vivo. This observation suggested that the level of transferrin receptor expression varies in response to discrete extracellular signals known to induce macrophage activation. We have tested this concept directly and have shown that decreased transferrin receptor expression can be reproduced in vitro by treatment of inflammatory macrophages with preparations of interferon gamma derived from a T cell hybridoma supernatant. The ability of this agent to down regulate the expression of the transferrin receptor exhibited dose and time dependencies similar to those required for development of other macrophage functions in vitro. The addition of LPS produced no further decrease in receptor expression. Furthermore, murine gamma interferon, produced by recombinant DNA technology also caused a downshift in transferrin receptor expression at doses similar to those which have been shown previously to induce activation. The changes in receptor activity were the result of altered numbers of binding sites and the receptor:ligand affinity remained unaffected. These results indicate that altered expression of the transferrin receptor is one element of the pleiotypic change which macrophages undergo in response to IFN gamma. This system may, therefore, provide a useful model in which to study the biochemical basis of IFN gamma action in mononuclear phagocytes.  相似文献   

18.
A hypothesis of the mechanistic mode of prenylation of tryptophan, an early phase of ergot alkaloid biosynthesis, occurring by way of 3-(α,α-dimethylally)indolenine and its ring-tautomer indoline intermediates and a Cope rearrangement of the latter has been tested in a model study. 3-(α,α-Dimethylallyl)indolines derived from tetrahydrocarbazole and its cyclopentano equivalent have been synthesized by a five-step procedure. The compounds are stable up to ca. 200°C and liberate no benz-prenylated products even on pyrolysis beyond this temperature, thus diminishing the viability of the above biosynthetic hypothesis. The chemistry of the thermolyses is described.  相似文献   

19.
Solid dipyridine hemes which are unreactive toward oxygen lose both pyridine ligands upon heating under vacuum to give a solid which takes up O2, reversibly, one O2 per heme. Replacement of 16O2 by 18O2 reduces only infrared bands near 1660 and 1590 cm?1, frequencies near the vibrational band for gaseous O2. No FeO bands are detected. EPR spectra reveal a free radical and ferric iron; Mössbauer, NMR and infrared spectra support an iron(III) oxidation state. Limited molecular weight data indicate a dimer. Possibly two dioxygen molecules are held sandwich fashion between two porphyrins via donor-acceptor interactions, which are facilitated by electron transfer from iron(II) into the porphyrin forming a π-anion. Such O2 bonding is not found in oxy Hb and Mb or in oxyhemerythrin but may occur with cytochrome c oxidase and other oxygen utilizing (or producing) heme and other proteins.  相似文献   

20.
Endogenous interfering substances can be detected by applying the techniques of parallel-line analysis of variance to the assay of superoxide dismutase (SOD) activities in crude tissue extracts. The technique also allows expression of specific activities in terms of units of activity as defined by commercially available standards as opposed to definition by the specific assay conditions utilized. This analysis may be broadly applied to many of the indirect SOD assays currently in use. In the current investigation, SOD was assayed by its ability to inhibit the rate of reduction of acetylated ferricytochrome c by superoxide anion (O2?) generated via the xanthine-xanthine oxidase reaction. The parallel-line analysis was effective in detecting the presence of interference by exogenous ascorbate in the reaction system, and by unidentified endogenous reactants within extracts of whole blood and ocular choroidal tissues of the rainbow trout, Salmo gairdneri.  相似文献   

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