Biomedical subjects

X Zhang

Publications and source records attributed to X Zhang.

At least 325 records · Page 18Linked to original sources

PubMed2001

X-ray structure analysis and crystallographic refinement of lumazine synthase from the hyperthermophile Aquifex aeolicus at 1.6 A resolution: determinants of thermostability revealed from structural comparisons.

An open reading frame optimized for expression of 6,7-dimethyl-8-ribityl-lumazine synthase of the hyperthermophilic bacterium Aquifex aeolicus in Escherichia coli was synthesized and expressed in a recombinant E. coli strain to a level of around 15 %. The recombinant protein was purified by heat-treatment and gel-filtration. The protein was crystallized in the cubic space group I23 with the cell dimensions a = b = c = 180.8 A, and diffraction data were collected to 1.6 A resolution. The structure was solved by molecular replacement using lumazine synthase from Bacillus subtilis as search model. The structure of the A. aeolicus enzyme was refined to a resolution of 1.6 A. The spherical protein consists of 60 identical subunits with strict icosahedral 532 symmetry. The subunit fold is closely related to that of the B. subtilis enzyme (rmsd 0.80 A). The extremely thermostable lumazine synthase from A. aeolicus has a melting temperature of 119.9 degrees C. Compared to other icosahedral and pentameric lumazine synthases, the A. aeolicus enzyme has the largest accessible surface presented by charged residues and the smallest surface presented by hydrophobic residues. It also has the largest number of ion-pairs per subunit. Two ion-pair networks involving two, respectively three, stacking arginine residues assume a distinct role in linking adjacent subunits. The findings indicate the influence of the optimization of hydrophobic and ionic contacts in gaining thermostability.

Amino Acid Sequence↗

PubMed2001

Solution structure of an A-tract DNA bend.

The solution structure of a DNA dodecamer d(GGCAAAAAACGG)/d(CCGTTTTTTGCC) containing an A-tract has been determined by NMR spectroscopy with residual dipolar couplings. The structure shows an overall helix axis bend of 19 degrees in a geometry consistent with solution and gel electrophoresis experiments. Fourteen degrees of the bending occurs in the GC regions flanking the A-tract. The remaining 5 degrees is spread evenly over its six AT base-pairs. The A-tract is characterized by decreasing minor groove width from the 5' to the 3' direction along the A strand. This is a result of propeller twist in the AT pairs and the increasing negative inclination of the adenine bases at the 3' side of the run of adenine bases. The four central thymine bases all have negative inclination throughout the A-tract with an average value of -6.1 degrees. Although this negative inclination makes the geometry of the A-tract different from all X-ray structures, the proton on N6 of adenine and the O4 of thymine one step down the helix are within distance to form bifurcated hydrogen bonds. The 5' bend of 4 degrees occurs at the junction between the GC flank and the A-tract through a combination of tilt and roll. The larger 3' bend, 10 degrees, occurs in two base steps: the first composed of tilt, -4.1 degrees, and the second a combination of tilt, -4.2 degrees, and roll, 6.0 degrees. This second step is a direct consequence of the change in inclination between an adjacent cytosine base, which has an inclination of -12 degrees, and the next base, a guanine, which has 3 degrees inclination. This bend is a combination of tilt and roll. The large change in inclination allows the formation of a hydrogen bond between the protons of N4 of the 3' cytosine and the O6 of the next 3' base, a guanine, stabilizing the roll component in the bend. These structural features differ from existing models for A-tract bends.For comparison, we also determined the structure of the control sequence, d(GGCAAGAAACGG)/d(CCGTTTCTTGCC), with an AT to GC transition in the center of the A-tract. This structure has no negative inclination in most of the bases within the A-tract, resulting in a bend of only 9 degrees. When ligated in phase, the control sequence has nearly normal mobility in gel electrophoresis experiments.

Autoradiography↗

PubMed2001

Replacement of segmental bone defects using porous bioceramic cylinders: a biomechanical and X-ray diffraction study.

A porous ceramic material [hydroxyapatitetricalcium phosphate (HA-TCP)] was implanted in the femora of 30 dogs to investigate the possibility of using this material to repair segmental bone defects. A bone segment, 1.5 cm in length, was removed from the diaphysis of one femur in each dog to create the defect. Cylinders of corresponding size were inserted into the defects. The animals were divided into three groups with recovery times of 2 months, 4 months, and 6 months, respectively. The implants were harvested and subjected to biomechanic tests (bending strength) and X-ray diffraction analysis. The bending strengths of the implant construct increased gradually over time postoperatively. The values of strength for the three different time groups had significant variations (p < 0.05). The X-ray diffraction analysis indicated that the peaks of the TCP included in the cylinders decreased in intensity after implantation and tended to be similar to those of natural bone by 6 months after operation. Conversely, the peaks for the HA had fewer changes compared with preimplantation values. Based on the results of this experiment it was concluded that the porous HA-TCP ceramic cylinders have potential for repair of segmental bone defects if assisted by adequate stabilizing fixtures during the early postoperative period.

Animals↗

PubMed2001

Determination of salicylate, gentisic acid and salicyluric acid in human urine by capillary electrophoresis with laser-induced fluorescence detection.

Acetylsalicylic acid (Aspirin) is rapidly metabolized to salicylic acid (salicylate) and other compounds, including gentisic acid and salicyluric acid. Monitoring of salicylate and its metabolites is of toxicological, pharmacological and biomedical interest. Three capillary electrophoresis (CE) methods featuring alkaline aqueous buffers, laser-induced fluorescence (LIF) detection and no solute extraction or derivatization have been explored. A competitive binding, electrokinetic capillary-based immunoassay is developed that recognizes the presence of salicylate and gentisic acid in urine. Differentiation of the two compounds, however, is problematic. With appropriate ultraviolet excitation, many salicylate-related compounds are fluorescent so that CE with direct urine injection and LIF detection permits the determination of salicylate, gentisic acid and salicyluric acid. Using a HeCd laser with 325 nm produces interference-free monitoring of all three compounds. Using 257 nm excitation from a frequency doubled Ar ion laser, native fluorescence of an endogenous urinary compound that co-migrates with gentisic acid is observed. With wavelength-resolved fluorescence detection, however, the two substances are distinguished. Furthermore, this technique, with comparison to literature data, permits the putative assignment of several peaks to other salicylate metabolites, namely glucuronide conjugates of salicylate and salicyluric acid. All three CE-LIF techniques have been applied to toxicological patient urines and urines collected after ingestion of 500 mg acetylsalicylic acid. CE results compare favorably with those obtained by a commercial fluorescence polarization immunoassay and by a conventional photometric assay.

Aspirin↗

PubMed2001

U7C beamline and XAFS station of national synchrotron radiation laboratory.

The XAFS station on beamline U7C of National Synchrotron Radiation Laboratory (NSRL) was completely constructed in December 1998. The source for XAFS station is from a 3-pole superconducting wiggler with magnetic field of 6 T inserted in the straight section of the storage ring. Using a Si(111) double crystal monochromator with a fixed slit, the X-ray intensity at the sample position is about 3x10(9) photons/second at the energy of 8980 eV of Cu K-edge. The Keithley 65 17 electrometers are used to record the electron charges that are produced in the ionization chambers. A high ratio of signal to noise has been obtained for the XAFS spectra of Cu, Ni and Fe foils. Furthermore, the XAFS spectrum of Cu foil in NSRL is in good agreement with that obtained in BSRF and KEK.

Journal Article↗

PubMed2001

Annealed crystallization of ultrafine amorphous NiB alloy studied by XAFS.

XAFS has been used to investigate the local structure evolutions of ultrafine amorphous NiB alloy during the annealed crystallization process. A nanocrystalline Ni phase with the local structure of crystalline Ni-like and a crystalline Ni3B, have been produced for ultrafine amorphous NiB alloy under the annealed temperature of 573 K. The results rule out Rojo et al.'s devitrification mechanism of Ni80B20 amorphous alloy in which they considered that an amorphous pure Ni phase is formed in the first exothermic process. However, our results are almost identical with Riveiro et al.'s conclusion in which the intermediate state is interpreted as two metastable crystalline phases of Ni3B and Ni-rich NiB alloy. With the annealed temperature going onto 773 K, the ultrafine NiB sample is further decomposed and crystallized into crystalline Ni with long-range order.

Journal Article↗

PubMed2001

Local structures of nanocrystalline GaN studied by XAFS.

X-ray absorption fine structure (XAFS) was used to investigate the local structures around Ga atoms in the hexagonal nanocrystalline and crystalline GaN under 78 and 300 K. For the first nearest neighbor coordination shell of Ga-N. the average bond length R (0.194 nm), coordination number N (4.0), thermal disorder sigma (0.0052 nm) and static disorder sigmaS (0.0007 nm) are neatly independent of the measured temperature and crystalline state. This indicates that the Ga-N covalent bond is much stronger, and the 4 nitrogen atoms in first nearest neighbor around Ga atoms keep the tetrahedral structure (Td). For the second nearest neighbor coordination shell of Ga-Ga, their bond lengths are about 0.318 nm. However, the sigmaS (0.0057 nm) of nanocrystalline GaN is 0.0047 nm larger than that of crystalline GaN (0.001 nm), and the sigmaT of nanocrystalline is 0.0053 nm and 0.0085 nm at the temperature of 78 and 300 K, respectively. The result indicates that the difference of local structure around Ga atoms between nanocrystalline and crystalline GaN occurs mainly at the Ga-Ga second nearest-neighbor coordination shell. The reason is explained as the local lattice distortion and unsaturated surface atoms existing in nanocrystalline GaN.

Journal Article↗

PubMed2001

[Urodynamic characteristics of patients with tethered cord syndrome and its clinical significance].

OBJECTIVE: To evaluate the urodynamic characteristics of children with tethered cord syndrome (TCS) and its clinical significance. METHODS: We conducted urodynamic studies in 94 cases of TCS who underwent microsurgical release within one week preoperatively and 3 to 6 months postoperatively. RESULTS: 80 out of 94 (85.1%) children with TCS presented with lower urinary tract symptoms. Neuropathic injuries were found both in patients in group A (with lower urinary tract symptoms) and in 11 out of 14 patient in group B (without lower urinary tract symptoms). The maximum urethral closing pressure (MUCP) and maximum detrusor pressure (MDP) were significantly lower in group alpha (without upper urinary tract injury), which were 45 cm H(2)O +/- 18 cm H(2)O and 27 cm H(2)O +/- 15 cm H(2)O respectively, than in group beta (with upper urinary tract injury), which were 69 cm H(2)O +/- 24 cm H(2)O and 44 cm H(2)O +/- 17 cm H(2)O respectively, while bladder compliance (BC) in group alpha (5.6 cm H(2)O +/- 0.9 ml/cm H(2)O) was significantly higher than in group beta (2.5 ml/cm H(2)O +/- 1.2 ml/cm H(2)O). MUCP, MDP and BC were improved in group I (primary TCS) after cord untethering, 32 cm H(2)O +/- 14 cm H(2)O, 21 cm H(2)O +/- 12 cm H(2)O, 9.5 ml/cm H(2)O +/- 1.3 ml/cm H(2)O postoperatively and 63 cm H(2)O +/- 245 cm H(2)O, 37 cm H(2)O +/- 156 cm H(2)O, 5.1 ml/cm H(2)O +/- 0.8 ml/cm H(2)O preoperatively respectively, while in group II remained unchangeable. CONCLUSION: Urodynamic studies are effective methods to identify neuropathic characteristics of lower urinary tract symptom in children with TCS and to find neuropathic injury during subclinical period. According to urodynamic studies, increase of MUCP and DP and decrease of BC are high risk factors of vesicoureteral reflux and upper urinary tract injury. Microsurgical release can improve bladder function in primary TCS, while do no good to bladder function in secondary TCS.

Adolescent↗

PubMed2001

Single step on-column frit making for capillary high-performance liquid chromatography using sol-gel technology.

One step frit-making in packing fused-silica capillary column of high-performance liquid chromatography was developed using sol-gel technology. Frit fabrication procedure was quite simple without sintering. On-column frit was formed through gelling of sol solution with packing materials, silica gel, and jointing the particles together with capillary wall through bonded and immobilized networks. Solvent types and proportions in sol solution were selected. And the sol solution compositions as well as amount of silica gel particles were also optimized to achieve maximum strength. Such an on-column frit of 250 microm in diameter is capable of resistant packing pressure up to 500 bars in ultra-sonic bath action. Chemical resistance to solvents and extreme pHs were also tested. Scanning electron micrograms of the frit profile showed that the evolving sol-gel network joined particles to each other and onto the column wall. Routine runs in reversed-phase mode, the frits of several columns proved to be effective enough to resist pressures without collapse.

Chromatography, High Pressure Liquid↗

PubMed2001

Hyperoxia modified activation-induced blood oxygenation level-dependent response of human visual cortex (V1): an event-related functional magnetic resonance imaging study.

To investigate the effect that hyperoxia has on the blood oxygenation level-dependent (BOLD) response to visual stimulation of human V1, an event-related functional magnetic resonance imaging technique was applied. The event-related paradigm consisted of 2 s of stimulation by a checkerboard reversing at a frequency of 8 Hz, followed by 18 s of control scans. The peak height and peak time of the BOLD response curves were compared under normoxic and hyperoxic conditions. It was found that the peak height was larger and the peak time shorter for hyperoxia than for normoxia. These results suggest that hyperoxia modified the activation-induced hemodynamic response of human V1.

Adult↗

PubMed2001

Needle-type dual microsensor for the simultaneous monitoring of glucose and insulin.

A miniature needle-type sensor suitable for the simultaneous amperometric monitoring of glucose and insulin is described. The integrated microsensor consists of dual (biologically and chemically) modified carbon-paste working electrodes inserted into a 14-guage needle. The glucose probe is based on the biocatalytic action of glucose oxidase, and the insulin one relies on the electrocatalytic activity of ruthenium oxide. The analytical performance of the dual sensor is assessed under flow injection conditions. The needle dual detector exhibits a very rapid response to dynamic changes in the concentrations of glucose and insulin. No apparent cross reactivity is observed in mixtures containing millimolar glucose levels and nanomolar insulin concentrations. The response is highly linear (to at least 1000 nM insulin and 14 mM glucose) and reproducible (RSD = 2.6-4.1%). The combination microsensor holds great promise for real-time measurements of the insulin/glucose ratio and for improved management of diabetes.

Biosensing Techniques↗

PubMed2001

An important 2'-OH group for an RNA-protein interaction.

We have investigated the role of 2'-OH groups in the specific interaction between the acceptor stem of Escherichia coli tRNA(Cys) and cysteine-tRNA synthetase. This interaction provides for the high aminoacylation specificity observed for cysteine-tRNA synthetase. A synthetic RNA microhelix that recapitulates the sequence of the acceptor stem was used as a substrate and variants containing systematic replacement of the 2'-OH by 2'-deoxy or 2'-O-methyl groups were tested. Except for position U73, all substitutions had little effect on aminoacylation. Interestingly, the deoxy substitution at position U73 had no effect on aminoacylation, but the 2'-O-methyl substitution decreased aminoacylation by 10-fold and addition of the even bulkier 2'-O-propyl group decreased aminoacylation by another 2-fold. The lack of an effect by the deoxy substitution suggests that the hydrogen bonding potential of the 2'-OH at position U73 is unimportant for aminoacylation. The decrease in activity upon alkyl substitution suggests that the 2'-OH group instead provides a monitor of the steric environment during the RNA-synthetase interaction. The steric role was confirmed in the context of a reconstituted tRNA and is consistent with the observation that the U73 base is the single most important determinant for aminoacylation and therefore is a site that is likely to be in close contact with cysteine-tRNA synthetase. A steric role is supported by an NMR-based structural model of the acceptor stem, together with biochemical studies of a closely related microhelix. This role suggests that the U73 binding site for cysteine-tRNA synthetase is sterically optimized to accommodate a 2'-OH group in the backbone, but that the hydroxyl group itself is not involved in specific hydrogen bonding interactions.

Acylation↗

PubMed2001

Identification and characterization of prostein, a novel prostate-specific protein.

In this report, we describe the application of a systematic, genome-based approach to identify prostein, a novel prostate-specific protein expressed in normal and malignant prostate tissues. Characterization of the prostein gene shows that prostein cDNA encodes a 553-amino acid protein. The protein is predicted to be a type IIIa plasma membrane protein with a cleavable signal peptide and 11 transmembrane-spanning regions. The prostein gene is located on chromosome 1 at the WI-9641 locus between q32 and q42. Prostein mRNA is shown to be uniquely expressed in normal and cancerous prostate tissues using Northern blot, eDNA microarray, and real-time PCR analyses. Furthermore, prostein mRNA expression does not appear to be prostate tumor grade related and is restricted exclusively to prostate cell lines. Immunohistochemical staining using a mouse monoclonal antibody generated against prostein demonstrates that this protein is specifically detected in prostate tissues both at the plasma membrane and in the cytoplasm. Prostein expression is androgen responsive because treatment of LNCaP cells with androgen up-regulates prostein message and protein expression levels. These results validate prostein as a prostate-specific marker with potential utility in the diagnosis and treatment of prostate cancer.

Amino Acid Sequence↗

PubMed2001

[Fourier analysis of macular local electroretinogram in retinal detachment].

OBJECTIVE: To understand the sites and physiological characteristics of macular dysfunction in retinal detachment (RD) by evaluating fundamental harmonic (1f) and second harmonic (2f) components of flicker local electroretinogram (LERG). METHODS: LERG were recorded in 128 eyes of 120 patients to a flicker sinusoidally to 31Hz with a hand-held stimulator with direct visualization of the fundus through dilated pupils. RESULTS: Compared to controls, patients showed losses of both fundamental and second harmonic amplitudes. The abnormality rates of fundamental amplitude in groups with RD affected time < 1 weeks, 1 approximately weeks, 2 approximately weeks, 4 approximately weeks and > 8 weeks were 15%, 25%, 47%, 76% and 91% respectively; and the abnormality of rates fundamental-second harmonic amplitude ratios (1f/2f) were 5%, 14%, 28%, 64% and 87% respectively. On average, 2f decreased more quikly than 1f and 1f/2f tended to increase with the course of disease. The results show both 1f and 2f descend in the RD eyes with or no macular detachment (P < 0.05). 2f just descend obviously in the 48 macular detachment eyes (P < 0.01), so that 1f/2f ascend in these eyes(P < 0.05). There was a significant relation between amplitudes of LERG in the detached eyes before surgery and best postoperational correct vision in the attached eyes. CONCLUSION: In the eyes with retinal detachment, both receptoral and postreceptoral sites contribute to macular dysfunction. The level of macular dysfunction in the eyes with or no macular detachment is different. It would be of clinical value to obtain a fundamental-second harmonic ratio in order to assess the disturbance in the different site of macular. In our opinion: macular flicker local electroretinogram is an important indicator of prognosis to the functional recovery of the attached eye after corrective surgery.

Adolescent↗

PubMed2001

[Experimental study on the invasion of glioma in vivo].

OBJECTIVE: To investigate the invasion and metastasis of glioma cell in vivo by implanting C6 glioma cells with the enhanced green fluorescent protein (EGFP) gene into the brain of SD rats. METHODS: C6 glioma cells were transfected with a plasmid vector (pEGFP-N3) containing the EGFP gene. Stable EGFP-expressing clones were isolated and examined by flow cytometry and electron microscopy. EGFP-expressing cells were stereotactically injected into the brain parenchyma of SD rats to establish xenotransplanted tumor. Four weeks later, the rats were killed and consecutive brain sections were examined using fluorescence microscopy after adjacent sections were examined by immunohistochemistry or routine hematoxylin and eosin staining for the visualization and detection of tumor cell invasion. Xenograft was primarily cultured to determine the existence of the EGFP gene in vivo. RESULTS: There was no obvious difference in cell cycle and ultrastructure between transfected cells and untransfected cells. EGFP-transfected C6 glioma cells maintained stable high-level EGFP expression in the central nervous system during their growth in vivo. EGFP fluorescence clearly demarcated the primary tumor margins and readily allowed for the visualization of distant micrometastasis and local invasion on the single-cell level. Small locally invasive foci, including those immediately adjacent to the leading invasive edge of the tumor, were virtually undetectable by routine hematoxylin and eosin staining and immunohistochemistry. CONCLUSION: EGFP-transfected C6 glioma cells can be visualized by fluorescence microscopy after intracranial implantation. This model is an excellent experimental animal model in the research of glioma invasion and metastasis.

Animals↗

PubMed2001

Nonthermal production of weakly interacting massive particles and the subgalactic structure of the universe.

There is increasing evidence that conventional cold dark matter (CDM) models lead to conflicts between observations and numerical simulations of dark matter halos on subgalactic scales, which rules out the favored candidates for CDM, namely weakly interacting massive particles (WIMPs). We propose a mechanism of nonthermal production of WIMPs and study its implications on the power spectrum. Our results show that, in this context, WIMPs as candidates for dark matter can work well both on large scales and on subgalactic scales.

Journal Article↗

PubMed2001

A receptor presentation hypothesis for T cell help that recruits autoreactive B cells.

To uncover mechanisms that drive spontaneous expansions of autoreactive B cells in systemic lupus erythematosus, we analyzed somatic mutations in variable region genes expressed by a panel of (NZB x SWR)F(1) hybridomas representing a large, spontaneously arising clone with specificity for chromatin. A single mutation within the Jkappa intron that was shared by all members of the lineage indicated that the clone emanated from a single mutated precursor cell and led to the prediction that a somatic mutation producing a functionally decisive amino acid change in the coding region would also be universally shared. Upon cloning and sequencing the corresponding germline V(H) gene, we found that two replacement somatic mutations in FR1 and CDR2 were indeed shared by all seven clone members. Surprisingly, neither mutation influenced Ab binding to chromatin; however, one of them produced a nonconservative amino acid replacement in a mutationally "cold" region of FR1 and created an immunodominant epitope for class II MHC-restricted T cells. The epitope was restricted by IA(q) (SWR), and the SWR MHC locus is associated with systemic lupus erythematosus in (NZB x SWR)F(1) mice. These, and related findings, provoke the hypothesis that autoreactive B cells may be recruited by a "receptor presentation" mechanism involving cognate interactions between T cells and somatically generated V region peptides that are self-presented by B cells.

Amino Acid Sequence↗

PubMed2001

Differential regulation of the expression of CD95 ligand, receptor activator of nuclear factor-kappa B ligand (RANKL), TNF-related apoptosis-inducing ligand (TRAIL), and TNF-alpha during T cell activation.

Members of TNF superfamily are characterized by their ability to inflict apoptosis upon binding to their cognate receptors in a homotrimeric manner. These proteins are expressed on different cell types under various conditions. However, the mechanisms governing the expression of these molecules remain elusive. We have found that the TCR signal can elicit the expression of receptor activator of NF-kappaB ligand (RANKL), TNF-alpha, CD95L, and TNF-related apoptosis inducing ligand (TRAIL) in T cell hybridoma A1.1 cells, thus allowing us to examine the expression pattern of these molecules under precisely the same conditions. We have previously reported that CD95L expression requires both protein kinase C (PKC) translocation and Ca2+ mobilization and is inhibited by cyclosporin A, and dexamethasone. We demonstrate now that activation-induced expression of RANKL is mediated by Ca2+ mobilization. PKC activation does not induce RANKL expression nor does it synergize with the Ca2+ signal. Activation-induced RANKL expression is blocked by cyclosporin A, but not by dexamethasone. The expression of TNF, in contrast, is mediated by PKC, but not by Ca2+. TNF-alpha expression is not inhibited by cyclosporin A, but is sensitive to dexamethasone. A1.1 cells constitutively express TRAIL at low levels. Stimulation with anti-CD3 leads to an initial reduction and subsequent increase in TRAIL expression. TRAIL induction is not inhibited by cyclosporin A, but highly sensitive to dexamethasone. Therefore, expression of the TNF superfamily genes is regulated by distinct signals. Detailed understanding of the regulatory mechanisms could provide crucial information concerning the role of these molecules in the modulation of the immune system.

Animals↗