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Lei Chen

Publications and source records attributed to Lei Chen.

179 records · Page 10Linked to original sources
PubMed2002

Measurement of packing tortuosity and porosity in capillary electrochromatography.

The application of conductivity measurements for packing structure characterization has been extended to a column consisting of a packed section and an open section as typically used in capillary electrochromatography (CEC). Because of the difference in electric conductivity between the packed and open sections, the electric fields applied across the two sections vary, depending on the length of the packed section relative to that of the total column. On the basis of mass conservation law, it can be shown that the ratio of the electric current measured in such a duplex column to that without packing is a function of the length and the geometric structure of the packing bed. Thus, knowing the lengths of the packed section and the whole column, we can readily calculate the obstructive factors, such as the porosity and the tortuosity factor, from the measured conductivity ratio. An example is given to demonstrate the application of this method, with experimental data taken from published work.

Journal Article↗
PubMed2002

Lignin deposition and associated changes in anatomy, enzyme activity, gene expression, and ruminal degradability in stems of tall fescue at different developmental stages.

Stem tissues of tall fescue (Festuca arundinacea Schreb.) were sampled at three elongation stages and three reproductive stages. Anatomical analysis showed the deposition of guaiacyl (G) and syringyl (S) lignin during plant development and the formation of a lignified sclerenchyma ring. A dramatic increase in Klason lignin content was found from elongation stage to reproductive stage. Lignin composition analyzed by gas chromatography-mass spectrometry revealed that S lignin content and S/G ratio increased with stem development, but contents of p-hydroxyphenyl (H) and G lignins decreased during the same period. S lignin content and S/G ratio also increased from the younger upper internode down to the older basal internode of the stem, but G and H lignin decreased in parallel. Relative O-methyltransferase activities increased during stem development and in parallel with the lignification process of stem. The pattern of enzyme activity during development varied with the choice of substrate, with highest activities seen when substrates were caffeoylaldehyde and 5-hydroxyferulic acid, and lowest activities were seen when caffeic acid and 5-hydroxyconiferyl alcohol were used as substrates. The expression of caffeic acid O-methyltransferase and cinnamyl alcohol dehydrogenase genes increased during the stem elongation stage and remained at high levels during the reproductive stages. The changes at anatomical, metabolic, and molecular levels during plant development were closely associated with lignification and degradability. This study provides an integrated picture of the molecular and chemical events that accompany changes in lignin deposition and ruminal degradability.

Alcohol Oxidoreductases↗
PubMed2002

Reciprocal regulation of CD4/CD8 expression by SWI/SNF-like BAF complexes.

Thymic development produces two sub-lineages of T cells expressing either CD4 or CD8 co-receptors that assist antibody production and mediate cell killing, respectively. The mechanisms for mutually exclusive co-receptor expression remain poorly defined. We find that mutations in the high mobility group (HMG) domain of BAF57--a DNA-binding subunit of the mammalian SWI/SNF-like chromatin-remodelling BAF complexes--or in the BAF complex ATPase subunit Brg, impair both CD4 silencing and CD8 activation. Brg is haploinsufficient for CD8 activation, but not for CD4 silencing, whereas BAF57 mutations preferentially impair CD4 silencing, pointing to target- and subunit-specific mechanisms of chromatin remodelling. BAF complexes directly bind the CD4 silencer, but the BAF57 HMG domain is dispensable for tethering BAF complexes to the CD4 silencer or other chromatin loci in vivo, or for remodelling reconstituted templates in vitro, suggesting that chromatin remodelling in vivo requires HMG-dependent DNA bending. These results indicate that BAF complexes contribute to lineage bifurcation by reciprocally regulating lineage-specific genes, reminiscent of the role of the yeast SWI/SNF complex in mediating mating-type switching.

Animals↗
PubMed2002

A glutamate residue at the C terminus regulates activity of inward rectifier K+ channels: implication for Andersen's syndrome.

G protein-coupled inward rectifiers (GIRKs) are activated directly by G protein betagamma subunits, whereas classical inward rectifiers (IRKs) are constitutively active. We found that a glutamate residue of GIRK2 (E315), located on a hydrophobic domain of the C terminus, is crucial for the channel activation. This glutamate (or aspartate) residue is conserved in all members of the Kir family. Substitution of alanine for the glutamate on GIRK1, GIRK2, and IRK2, expressed in HEK293 cells, greatly reduced the whole-cell currents. The whole-cell current of GIRK channels with a constitutively active gate, GIRK2(V188A), [Yi, B. A., Lin, Y. F., Jan, Y. N. & Jan, L. Y. (2001) Neuron 29, 657-667] was also reduced by the same glutamate mutation. Mean open time and conductance of single channels in GIRK2 and IRK2 were not affected by the mutation, indicating that the reduced whole-cell current resulted from a lowered probability of channel activation. The mutated GIRK and IRK showed normal trafficking to the cell membrane. The mutated GIRK2 retained the ability to interact with G protein betagamma subunits, and it showed almost the same inwardly rectifying property as the wild type. The mutated GIRK1 and GIRK2 retained ion selectivity to K(+) ions. This glutamate residue corresponds to one of the residues causing Andersen's syndrome [Plaster, N. M., Tawil, R., Tristani-Firouzi, M., Canun, S., Bendahhou, S., Tsunoda, A., Donaldson, M. R., Iannaccone, S. T., Brunt, E., Barohn, R., et al. (2001) Cell 105, 511-519]. Our interpretation is that this region of the glutamate residue is crucial in relaying the activating message from the ligand sensor region to the gate.

Amino Acid Sequence↗
PubMed2002

Synthesis and chromatographic properties of a novel chiral stationary phase derived from heptakis(6-azido-6-deoxy-2,3-di-O-phenylcarbamoylated)-beta-cyclodextrin immobilized onto amino-functionalized silica gel via multiple urea linkages.

A novel chiral stationary phase (PPHCDN7) was prepared by immobilization of heptakis(6-azido-6-deoxy-2,3-di-O-phenylcarbamoylated)-beta-cyclodextrin (PPHCD) onto the surface of amino-functionalized silica gel via multiple urea linkages derived from an extended application of the Staudinger reaction. A wide range of structurally divergent racemic drugs and other compounds were successfully separated into their enantiomers under both normal and reversed-phase conditions. Beta-adrenergic blockers and racemic tertiary, secondary and primary amines were readily separated using a mixture of methanol and aqueous triethylammonium acetate buffer. The optimal pH value for the separation falls in the range of 4.65 to 6.30. With atropine and isoproterenol, good enantioseparations with separation factors of alpha>5 were easily attainable.

Chromatography, High Pressure Liquid↗
PubMed2002

Air pollution and birth weight in northern Nevada, 1991-1999.

This study examined the association between particulate matter < or =10 microm in aerodynamic size (PM(10)), carbon monoxide (CO), and ozone (O(3)), and birth weight in Washoe County, NV, from 1991 through 1999. In total, 39,338 single births were included in this study. The mean birth weight was 3383 +/- 460 g and prevalence of low birth weight (LBW) was 2.46% for single births with a gestational age of 37-44 wk. After controlling for cofactors including infant sex, maternal residential city, education, medical risk factors, active tobacco use, drug use, alcohol use, prenatal care, mother's age, race and ethnicity of mothers, and weight gain of mothers, we found PM(10) exposure in the third trimester of pregnancy to be a significant predictor of birth weight of newborns. A 10-microg/m(3) increase in the 24-h PM(10) level in the third trimester of pregnancy can be associated with a birth-weight reduction of 11 g (95% CI: 2.3-19.8 g) using multiple linear regression; however, PM(10) was not found to be related with the risk of LBW from logistic regression. CO and O(3) were not found to be associated with birth weight or risk of LBW of newborns by the same modeling procedure. The results suggest PM(10) could be a risk factor associated with birth-weight reduction of newborns in urban northern Nevada; however, the current level of PM(10) is not a risk factor to increase the chance of LBW newborns.

Air Pollution↗
PubMed2002

Neonatal hearing loss in the indigent.

OBJECTIVE: To compare the risk factor profile for neonatal hearing loss (HL), and the follow-up rate of those identified with HL in an indigent population with those in an insured population. STUDY DESIGN: Retrospective review. METHODS: We studied 4526 neonates from the high-risk nursery or neonatal intensive care unit from two adjacent hospitals in Houston, Texas. Ben Taub General Hospital (BTGH) is a county public hospital that serves mainly the indigent. Texas Children's Hospital (TCH) is a private tertiary care center that serves patients with private insurance and Medicaid. RESULTS: Overall, 133 infants failed the screening test. Follow-up diagnostic testing identified 48 patients with definite HL. Although nearly twice as many patients at BTGH failed screening compared with TCH (88 vs. 45), four times as many patients at BTGH did not return for diagnostic testing (43 vs. 10). When a hearing aid was needed, there was a delay in getting one at BTGH (P <.05). There was a higher prevalence of dysmorphic facial features and central nervous system disease and a lower prevalence of long-term ventilatory support at BTGH (P <.05). There were no differences between BTGH and TCH in the prevalence of low birth weight, neonatal asphyxia, syndromic stigmata, neonatal infection, family history of HL, or neonatal transfusion (P >.1). CONCLUSIONS: Significant differences in the risk factor profile for neonatal HL exist between the indigent and the general population. A worrisome problem exists with the timely intervention in hearing-impaired indigent neonates.

Audiometry↗
PubMed2002

[Evaluation of abdominal visceral obesity from anthropometric parameters using receiver operating characteristic curves].

OBJECTIVE: To evaluate the best cut-off point, sensitivity and specificity of body mass index (BMI), waist circumference (WC) and Waist-to-hip ratio (WHR) in predicting of abdominal visceral obesity. METHODS: Abdominal visceral fat area (VA) was measured with magnetic resonance image (MRI) in 690 subjects (men: 305, women: 385). Meanwhile, BMI, WC, WHR were assessed. Receiver operating characteristic (ROC) curve was used as index for analysis. RESULTS: 1) 61.7% of over-weight/obesity (OW/OB) and 14.2% of normal weight (NW) individuals were abdominal visceral obesity (VA >/= 100 cm(2)) by MRI diagnosis. 2) VA was significantly positively correlated with anthropometric variables (BMI, WC, WHR), in which WC was the best (r = 0.73 - 0.77, P < 0.001). 3) The best cut-off points of these anthropometric parameters in assessing abdominal visceral obesity were as follow: BMI: 26 kg/m(2), WC: 90 cm, WHR: 0.93. Among them WC showed most sensitive and specific. 4) 95% men and 90% women appeared abdominal visceral obesity in subjects with BMI >/= 28 kg/m(2) or WC >/= 95 cm. CONCLUSION: BMI, WC, WHR can all predict abdominal visceral obesity, with WC the best.

Adipose Tissue↗
PubMed2002

[Development of conductance measurement technique for permeability of monolayer endothelial cells].

The conductance measurement method of endothelial permeability has been established by the principle that there is direct ratio between conductance and filter area, and has been compared with the albumin filter method. The results show that there is close correlation between conductance and filter area, the conductance measurement can be used for the detection of the monolayer endothelial cells permeability. The combination of the conductance measurement and the albumin filter measurement method can accurately reflect the permeability change in severity and scope.

Cell Culture Techniques↗
PubMed2002

[Interspecific and intraspecific comparison of valepotrates contents in three Valeriana plants].

OBJECTIVE: To compare the contents of three valepotrates intraspecifically and intraspecifically in three medicinal Valeriana plants grown in different areas in China. METHODS: The method of RE-HPLC was adopted. RESULTS: The results showed that the contents of three valepotriates in Valeriana jatamansi Jones, V. officinalis L. and V. officinalis var. latifolia Miq. were different. Among them, that of Valeriana jatamansi Jones was the highest. The contents of three valepotrates in Valeriana jatamansi Jones varied significantly from different areas, samples collected from Longli of Guizhou province ranking the highest. The contents of three valepotriates in V. officnalis L. also varied significantly from different areas, samples from Liuba of Shaanxi province showing the highest. The results also showed that the contents of three valepotriates in underground part are all higher than that in aerial part. CONCLUSION: The interspecific and intraspecific differences of valepotriates in three Valeriana plans were obvious.

Chromatography, High Pressure Liquid↗
PubMed2002

[Analysis on the pharmic chemical component of cerebral cell active factor].

The contents of amino acids, vitamins and chemical elements were determined by LC-60A Shimadzu and P-E3030 made in USA. Results show that drug has 18 kinds of amino acids, 3 kinds of vitamins (not complete) and 20 kinds of chemical elements. The results can provide the referential data for improving the mixture method and clinical administration.

Amino Acids↗
PubMedDate not supplied

GABA-B receptor activation in the rat globus pallidus potently suppresses pentylenetetrazol-induced tonic seizures.

To determine the involvement of the globus pallidus in mediating epilepsy, the effects of microinjection of a GABA uptake blocker (tiagabine), a benzodiazepine agonist (zolpidem) and a GABA-B receptor agonist (baclofen) on pentylenetetrazol (PTZ)-induced tonic seizure were examined in adult rats. Administration of PTZ induced tonic seizures in all control animals, accompanied with a 100% mortality rate. Pretreatment with bilateral intrapallidal microinjection of tiagabine (1 mM) suppressed the incidence of tonic seizures to 67.7% and reduced the mortality rate to 16.7%. Furthermore, the latency to tonic seizures was 1,275 +/- 277 s, which was significantly longer than that of the corresponding control animals (319 +/- 225 s). On the other hand, administration of zolpidem (1 mM) was without significant effects on the mortality rate, the incidence and latency of the tonic seizure. In sharp contrast, microinjection of baclofen (1 mM) completely suppressed PTZ-induced tonic seizures and reduced the mortality rate to 0%. This effect was largely abolished by co-injection of the GABA-B receptor antagonist CGP55845. To elucidate the direct cellular action of baclofen, the excitability and membrane potential of globus pallidus neurons were studied by cell-attached and whole-cell patch-clamp recordings in the brain slice. Bath application of baclofen (50 microM) significantly reduced the firing of these neurons, and was often accompanied by a clear membrane hyperpolarization, in a CGP55845-sensitive manner. These data suggest that activation of GABA-B receptors in globus pallidus could significantly modulate PTZ-induced tonic seizures.

Action Potentials↗
PubMedDate not supplied

Synthesis of novel porous magnetic silica microspheres as adsorbents for isolation of genomic DNA.

An improved procedure is described for preparation of novel mesoporous microspheres consisting of magnetic nanoparticles homogeneously dispersed in a silica matrix. The method is based on a three-step process, involving (i) formation of hematite/silica composite microspheres by urea-formaldehyde polymerization, (ii) calcination of the composite particles to remove the organic constituents, and (iii) in situ transformation of the iron oxide in the composites by hydrogen reductive reaction. The as-synthesized magnetite/silica composite microspheres were nearly monodisperse, mesoporous, and magnetizable, with as typical values an average diameter of 3.5 microm, a surface area of 250 m(2)/g, a pore size of 6.03 nm, and a saturation magnetization of 9.82 emu/g. These magnetic particles were tested as adsorbents for isolation of genomic DNA from Saccharomyces cerevisiae cells and maize kernels. The results are quite encouraging as the magnetic particle based protocols lead to the extraction of genomic DNA with satisfactory integrity, yield, and purity. Being hydrophilic in nature, the porous magnetic silica microspheres are considered a good alternative to polystyrene-based magnetic particles for use in biomedical applications where nonspecific adsorption of biomolecules is to be minimized.

Adsorption↗
PubMedDate not supplied

Intracranial auditory pathways: anatomy correlated with evoked response data.

The intracranial auditory pathways are complex, and they have been incompletely mapped. The purpose of the communication is to summarize what is known about these fibers onto clinically relevant imaging sections. Anatomic information is correlated with the clinical auditory evoked responses currently used to localize lesions in these pathways. This linkage will improve communication between imagers and clinicians. In addition, it can be used for teaching.

Auditory Cortex↗