Biomedical subjects

X Chen

Publications and source records attributed to X Chen.

At least 703 records · Page 39Linked to original sources

PubMed1999

Protein import into chloroplasts.

Chloroplasts have evolved an elaborate system of membrane and soluble subcompartments to organize and regulate photosynthesis and essential aspects of amino acid and lipid metabolism. The biogenesis and maintenance of organellar architecture rely on protein subunits encoded by both nuclear and plastid genomes. Import of nuclear-encoded proteins is mediated by interactions between the intrinsic N-terminal transit sequence of the nuclear-encoded preprotein and a common import machinery at the chloroplast envelope. Recent investigations have shown that there are two unique membrane-bound translocation systems, in the outer and inner envelope membranes, which physically associate during import to transport preproteins from the cytoplasm to the internal stromal compartment. This review discusses current understanding of these translocation systems and models for the way in which they might function.

Biological Transport↗

PubMed1999

Bacteria targeted by human natural antibodies using alpha-Gal conjugated receptor-specific glycopolymers.

Synthesis of polymerizable beta-lactosyl, Galalpha1-->3Gal and alpha-mannosyl acrylamide derivatives with either a hydrophobic aromatic spacer or a hydrophilic biocompatible oligoethoxyl spacer was accomplished. Radical terpolymerizations of beta-lactosyl monomer. alpha-mannosyl monomer, and acrylamide were conducted in aqueous media with ammonium persulfate and N,N,N',N'-tetramethylethylenediamine as initiators. The resulting water soluble glycopolymers were further transformed efficiently by a recombinant alpha1-->3 galactosyltransferase to afford mediators bearing Galalpha1-->3Gal termini as xenoactive antigens and alpha-mannosyl termini as specific ligands for bacterial cells. The binding of the resulting multivalent glycopolymer to bacteria was tested by its ability to inhibit agglutination of yeast to E. coli. The binding of human natural anti-Gal antibodies to the alpha-Gal containing glycopolymers and a monovalent alpha-Gal-Man glycoconjugate was demonstrated by an ELISA inhibition assay.

Antibodies↗

PubMed1999

Crystal structure of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase, a potential target for the development of novel antimicrobial agents.

BACKGROUND: Folate cofactors are essential for life. Mammals derive folates from their diet, whereas most microorganisms must synthesize folates de novo. Enzymes of the folate pathway therefore provide ideal targets for the development of antimicrobial agents. 6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) catalyzes the transfer of pyrophosphate from ATP to 6-hydroxymethyl-7,8-dihydropterin (HP), the first reaction in the folate biosynthetic pathway. RESULTS: The crystal structure of HPPK from Escherichia coli has been determined at 1.5 A resolution with a crystallographic R factor of 0.182. The HPPK molecule has a novel three-layered alpha beta alpha fold that creates a valley approximately 26 A long, 10 A wide and 10 A deep. The active center of HPPK is located in the valley and the substrate-binding sites have been identified with the aid of NMR spectroscopy. The HP-binding site is located at one end of the valley, near Asn55, and is sandwiched between two aromatic sidechains. The ATP-binding site is located at the other end of the valley. The adenine base of ATP is positioned near Leu111 and the ribose and the triphosphate extend across and reach the vicinity of HP. CONCLUSIONS: The HPPK structure provides a framework to elucidate structure/function relationships of the enzyme and to analyze mechanisms of pyrophosphoryl transfer. Furthermore, this work may prove useful in the structure-based design of new antimicrobial agents.

Adenosine Triphosphate↗

PubMed1999

Isotope edited product ion assignment by alpha-N labeling of peptides with [2H3(50%)]2,4-dinitrofluorobenzene.

An isotopic modification of Sanger's method for identifying peptide N-termini has been developed to assist peptide sequencing by tandem mass spectrometry. Tryptic peptides, such as Val-His-Leu-Thr-Pro-Val-Glu-Lys, are derivatized with an equimolar mixture of 2,4-dinitrofluorobenzene and [2H3]2,4-dinitrofluorobenzene. Under optimized derivatization conditions, the alpha-amino group could be derivatized while the epsilon-amine of the lysine side chain and the imidazole of histidine remained underivatized. The alpha-dinitrophenyl modified peptides were characterized by electrospray ionization-tandem mass spectrometry (ESI-MS/MS) and liquid chromatography (LC)-ESI-MS. The [M + H]+ ions showed a doublet pattern with a delta m/z of 3 and the [M + 2H]2+ ions were recognized as doublets with a delta m/z of 1.5. MS/MS was employed where both isotopic [M + 2H]2+ ions were alternately subjected to collision-induced dissociation in the second quadrupole. Fragmentation in the ionization source generated identical product ion patterns that were observed during fragmentation in the second quadrupole. In the product ion mass spectra, the N-terminal a and b ions (no c ion observed) are doublets with a delta m/z of 3 or 1.5, while the C-terminal y and z ions (no x ion observed) are singlets appearing at identical masses. Thus, the product ions containing the N-terminus derivatized with a dinitrophenyl group are unequivocally distinguished from the product ions containing the C-terminus. The dinitrophenyl modification generally enhanced the production of a and b ions without diminishing y and z ion yields.

Amino Acid Sequence↗

PubMed1999

Fingerprint patterns from laser-induced azido photochemistry of spin-labeled photoaffinity ATP analogs in matrix-assisted laser desorption/ionization mass spectrometry.

The photochemical reaction of azide derivatives induced by ultraviolet (UV) laser in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) is reported. A novel synthesized class of azide aromatic derivatives, spin-labeled photoaffinity non-nucleoside adenosine triphosphate (ATP) analogs which are useful probes in study of muscle contraction mechanism, is used in this investigation. In the negative ion MALDI spectra of these ATP analogs, "fingerprint" peaks corresponding to [M - 10 - 1]-, [M - 12 - 1]-, [M - 16 - 1]-, [M - 26 - 1]-, [M - 28 - 1]-, [M - 41 - 1]-, and [M - 42 - 1]- were observed with relative intensities depending on the MALDI matrix. Only the [M - 16 - 1]- is present in the similar mass spectra of the analog in which the azido group is replaced by a hydrogen. A model is suggested for the photochemical reactions of azide derivatives under UV laser irradiation. The photoreaction fingerprint information is diagnostically useful in characterization of azido compounds, especially for spin-labeled photoaffinity non-nucleoside ATP analogs.

Adenosine Triphosphate↗

PubMed1999

Homogeneous genotyping assays for single nucleotide polymorphisms with fluorescence resonance energy transfer detection.

A homogeneous detection mechanism based on fluorescence resonance energy transfer (FRET) has been developed for two DNA diagnostic tests. In the template-directed dye-terminator incorporation (TDI) assay, a donor dye-labeled primer is extended by DNA polymerase using allele-specific, acceptor dye-labeled ddNTPs. In the dye-labeled oligonucleotide ligation (DOL) assay, a donor dye-labeled common probe is joined to an allele-specific, acceptor dye-labeled probe by DNA ligase. Once the donor and acceptor dyes become part of a new molecule, intramolecular FRET is observed over background intermolecular FRET. The rise in FRET, therefore, can be used as an index for allele-specific ddNTP incorporation or probe ligation. Real time monitoring of FRET greatly increases the sensitivity and reliability of these assays. Change in FRET can also be measured by end-point reading when appropriate controls are included in the experiment. FRET detection proves to be a robust method in homogeneous DNA diagnostic assays.

Animals↗

PubMed1999

Smoking patterns of Asian-American youth in California and their relationship with acculturation.

PURPOSE: To examine patterns of smoking behavior among subgroups of Asian-American youth in California and their relationship with acculturation status. METHODS: Data were from the 1990-1996 California Tobacco Survey and the California Youth Tobacco Survey, which assessed smoking-related attitudes and behaviors among California youth in grades 7-12. Among the 20,482 respondents, 1,810 were Asian-Americans. Variables assessed included lifetime smoking prevalence, 30-day smoking prevalence, and age of smoking onset. Acculturation status was assessed with measures of English usage, language spoken at home, and age at immigration to the United States. RESULTS: Of the 1,810 Asian-Americans (52% male), 19% were Chinese, 33% Filipino, 8% Japanese, 13% Korean, and 26% other Asian-Americans. Lifetime smoking prevalence was 16.1% for Asians and 26.1% for non-Asians. The 30-day smoking rate was 6.9% for Asians and 14.2% for non-Asians. Subgroup-specific analyses revealed differences in lifetime smoking prevalence (18.9% for Filipinos, 17.3% for Japanese, 16.3 % for Koreans, 11.0% for Chinese, and 13.7% for other Asian-Americans) and 30-day smoking rate (8.6% for Filipinos, 8.3% for Koreans, 7.4% for Japanese, 2.8% for Chinese, and 7.2% for other Asian-Americans). Gender differences in lifetime smoking prevalence varied by Asian subgroup; smoking prevalence was higher among males than among females for Chinese and Koreans, but smoking prevalence was higher among females than among males for Japanese and other Asians. The average age of smoking onset was 12.9 years for Asians and 12.8 years for non-Asians. Age of smoking onset differed among Asian-American subgroups but did not differ by gender. In addition, smoking behaviors of the Asian respondents were significantly associated with their acculturation status. CONCLUSIONS: Asian-American youth have relatively lower smoking rates and later age of smoking onset than non-Asian youth in California. In addition, subgroups of Asian-American youth vary widely in their smoking behavior. High levels of acculturation among Asian-American youth are associated with higher smoking prevalence rates and earlier age of smoking onset.

Acculturation↗

PubMed1999

Inhibition of FEN-1 processing by DNA secondary structure at trinucleotide repeats.

The mechanism by which trinucleotide expansion occurs in human genes is not understood. However, it has been hypothesized that DNA secondary structure may actively participate by preventing FEN-1 cleavage of displaced Okazaki fragments. We show here that secondary structure can, indeed, play a role in expansion by a FEN-1-dependent mechanism. Secondary structure inhibits flap processing at CAG, CGG, or CTG repeats in a length-dependent manner by concealing the 5' end of the flap that is necessary for both binding and cleavage by FEN-1. Thus, secondary structure can defeat the protective function of FEN-1, leading to site-specific expansions. However, when FEN-1 is absent from the cell, alternative pathways to simple inhibition of flap processing contribute to expansion.

DNA↗

PubMed1999

HUA1 and HUA2 are two members of the floral homeotic AGAMOUS pathway.

The identities of the four floral organ types in an Arabidopsis flower are specified by the combinatorial activities of the floral homeotic A, B, and C function genes; AGAMOUS is the only known C function gene. We have identified two genes that interact with AG in the specification of floral structure, HUA1 and HUA2, from a screen for enhancers of a weak ag allele, ag-4. HUA1 and HUA2 are involved in all aspects of AG function. HUA2 encodes a novel protein that contains nuclear localization signals and signature motifs that suggest HUA2, like AG, may be a transcription factor. Molecular analyses suggest that HUA2 (and possibly HUA1) acts to facilitate AG action at the same hierarchical level as AG.

AGAMOUS Protein, Arabidopsis↗

PubMed1999

The p53 family: same response, different signals?

TP53, the gene that encodes p53, is a well-defined tumor suppressor gene that is frequently mutated in human cancers. Recently, two proteins homologous to p53, termed p73 and p63, were identified. Current data indicate that both p73 and p63, like p53, can induce cell-cycle arrest and apoptosis, suggesting that they might also be tumor suppressors. However, the physiological signals that can regulate p53, for example, DNA damage, have no effect on p73, as tested in several cell lines. Furthermore, the signaling pathways by which p73 (and possibly p63) induces cell-cycle arrest and apoptosis appear to be similar to those of p53, but also have important differences. Thus, the p53 family proteins are closely related but might have distinct physiological functions.

Animals↗

PubMed1999

Carbohydrates in transplantation.

Carbohydrate materials have become increasingly utilized in transplantation and cell/tissue engineering within the past year. This has been well documented in recent applications of immobilized or soluble alpha-galactosyl epitopes (i.e. oligosaccharides with a terminal Galalpha1-3Gal sequence) in preventing hyperacute rejection in pig-to-primate xenotransplantation. In addition, alpha-galactosyl polymers have been shown to exhibit much greater activity (up to 10(4) times) than alpha-galactosyl monomers in inhibiting the binding of anti-galactosyl antibodies to pig kidney epithelial cells and assisting in the prevention of cytotoxicity in human serum.

Carbohydrate Sequence↗

PubMed1999

Enzymatic digestion-high-pressure homogenization prior to slurry introduction graphite furnace atomic absorption spectrometry for selenium determination in plant and animal tissues.

Homogenization using a new flat valve homogenizer in combination with enzymatic digestion with a crude protease was investigated as a means of releasing Se compounds from zoological and botanical matrixes prior to slurry introduction GFAAS. Timed trials with four zoological certified reference materials (CRMs), three botanical reference materials (RMs), and a food crop indicated that Se release into 5% (v/v) ethanol-0.03 M TRIS containing 20 mg of protease was quantitative after homogenization or became quantitative within 1 h of digestion at 60 degrees C. For each of the zoological RMs (whole egg powder, dogfish muscle, and dogfish liver), three passes through the homogenizer in the presence of protease provided a quantitative release of selenium, and incubation with the enzyme was not necessary. No separation of the Se between the liquid phase and the particulate phase was evident even after several days of subsequent storage at 4 degrees C. The botanical matrixes (three milled wheat RMs and a rapeseed sample) were more resistant to selenium release and required up to 1 h of digestion with protease at 60 degrees C. Alternatively, 10 passes through the homogenizing valve (in the presence of the enzyme) resulted in the quantitative release of analyte.

Animals↗

PubMed1999

DNA repeats in the human genome.

Repetitive DNA sequences, interspersed throughout the human genome, are capable of forming a wide variety of unusual DNA structures with simple and complex loopfolding patterns. The hairpin formed by the fragile X repeat, (CCG)n, and the bipartite triplex formed by the Friedreich's ataxia repeat, (GAA)n/(TTC)n, show simple loopfolding. On the other hand, the doubly folded hairpin formed by the human centromeric repeat, (AATGG)n, the hairpin G-quartet formed by (TTAGGG)n at the 3' telomere overhang, and the hairpin G-quartet, and hairpin C+.C paired i-motif formed by the insulin minisatellite, [formula: see text] show multiple and complex loopfolding. We have performed high resolution nuclear magnetic resonance (NMR) spectroscopy and in vitro replication to show that unique base-pairing and loopfolding render stability to these unusual structures under physiological conditions. The formation of such stable structures offers a mechanism of unwinding which is advantageous during transcription. For example, the formation of the hairpin G-quartet, and hairpin C+.C paired i-motif upstream of the insulin gene may facilitate transcription. These unusual DNA structures also provide unique 'protein recognition motifs' quite different from a Watson-Crick double helix. For example, the hairpin G-quartet formed by (TTAGGG)n at the 3' telomere overhang is specifically recognized and stabilized by the human repair protein, Ku70/Ku80 hetero-dimer, which may be important in the stability of the telomere. However, the formation of the same unusual DNA structures during replication is likely to cause instability in the lengths of the DNA repeats. If the altered (generally expanded) length enhances the probability of the unusual structure during the next cycle of replication, it further increases the instability of the repeat causing a 'dynamic mutation'. In fact, NMR and in vitro replication studies show that the longer the repeat length the higher is the probability of hairpin formation by the fragile X repeat, (CCG)n. In addition, the hairpin of the fragile X repeat, upstream of the FMR-1 gene, is more susceptible to CpG methylation than its duplex thereby leading to methyl-directed suppression of transcription. Thus, the selective advantage of the unusual structures formed by the DNA repeats in the regulation of gene expression may be offset by the genomic instability caused by the same structures during replication. The repeat number is a critical parameter that helps maintain a balance between the advantage gained from an unusual structure during gene expression and the disadvantage posed by the same structure during replication.

Antigens, Nuclear↗

PubMed1999

Molecular characterization of catalytic-subunit cDNA sequences encoding protein phosphatases 1 and 2A and study of their roles in the gibberellin-dependent Osamy-c expression in rice.

To understand the molecular mechanism of gibberellin-dependent gene regulation, the effect of three phosphatase inhibitors on the germination of rice seeds and the expression of a target gene, the alpha-amylase gene, Osamy-c, were measured. We found that okadaic acid, microcystin-LR, and calyculin A, which are known to specifically inhibit Ser/Thr phosphatases 1 and 2A, strongly inhibit the expression of the Osamy-c and may be involved in the germination of rice seeds. The protein phosphatase enzyme activity assays showed that there is no obvious effect of GA3 on total PP1/PP2A activities. To further understand the possible role of protein phosphatases 1 and 2A in the GA-dependent expression of Osamy-c, we isolated cDNA clones encoding protein phosphatase 1 and protein phosphatase 2A from a rice aleurone cDNA library. These were designated OsPP1c and OsPP2Ac, respectively. Comparison of the deduced amino acid sequences of OsPP1c and OsPP2Ac with the catalytic subunits of PP1 or PP2A of rabbit skeletal muscle, Arabidopsis thaliana, maize and Brassica napus showed that the catalytic subunit sequences of PP1 or PP2A among these organisms are highly conserved (73% to 90% similarity). Genomic Southern blot analysis indicated that there are only one or two copies of OsPP1c genes and more than two copies of OsPP2Ac genes in the rice genome. Northern blot analysis showed that OsPP1c and OsPP2Ac genes are expressed in several organs of rice, including seed, shoot and root. We also showed by using 3' gene-specific probes of OsPP1c and OsPP2Ac cDNA, that the expression of neither gene is regulated by GA. Taken together, our results suggest that protein phosphatases PP1 or PP2A are involved in the GA-dependent expression of the rice Osamy-c gene, though the PP1 or/and PP2A enzymatic activities as well as mRNA levels do not increase upon GA3 treatment.

Amino Acid Sequence↗

PubMed1999

Temperature-controlled radiofrequency ablation of cardiac tissue: an in vitro study of the impact of electrode orientation, electrode tissue contact pressure and external convective cooling.

BACKGROUND: A variety of basic factors such as electrode tip pressure, flow around the electrode and electrode orientation influence lesion size during radiofrequency ablation, but importantly is dependent on the chosen mode of ablation. However, only little information is available for the frequently used temperature-controlled mode. The purpose of the present experimental study was to evaluate the impact during temperature-controlled radiofrequency ablation of three basic factors regarding electrode-tissue contact and convective cooling on lesion size. METHODS AND RESULTS: In vitro strips of porcine left ventricular myocardium were ablated in a tissue bath. Temperature-controlled ablation at 80 degrees C for 60 s was performed using a 7F 4 mm tip electrode in either perpendicular or parallel contact with the endocardium at a pressure of 10 or 20 g. Increased flow around the electrode was induced by circulating the saline in the tissue bath at a flow-velocity of 0.1 m/s. Lesion volume was determined by cutting lesions in 1 mm thick slices, staining with nitroblue tetrazolium and planimetering. A total of 107 lesions was created. Lesion size was significantly larger for perpendicular electrode orientation compared to parallel for both pressure-settings and both levels of flow around the electrode (p < 0.05). Increased flow around the electrode enlarged lesion size (p < 0.005). Electrode-tissue contact pressure had no significant impact on lesion size. CONCLUSIONS: During temperature-controlled radiofrequency ablation increased external cooling of the electrode tip due to either flow of the surrounding liquid or poor electrode tissue contact, as exemplified by perpendicular versus parallel electrode orientation, increases lesion size significantly. This is in contrast to the impact of these factors during power-controlled ablation due to the lack of increased power-delivery in the latter situation.

Animals↗

PubMed1999

Phenotype-genotype correlation in familial Mediterranean fever: evidence for an association between Met694Val and amyloidosis.

Familial Mediterranean fever (FMF) is an autosomal recessive disease characterised by recurrent attacks of inflammation of serosal membranes. Amyloidosis is the most severe complication of the disease. The aim of this study was to investigate the genotype-phenotype correlation and specifically the association between amyloidosis and the four common mutations in exon 10 of the gene causing FMF (MEFV) in a total of 83 FMF families from three ethnic groups: North African Jews, Armenians and Turks. A significant association was found between amyloidosis and the specific mutation at the MEFV gene: Met694Val (RR = 1.41, P = 0.02). Amyloidosis was present in 18 out of 87 homozygous FMF patients (20.7%) and in only two out of the 41 compound heterozygous FMF patients (4.9%). No patients carrying other mutations had amyloidosis. There was no significant association between the various mutations and the type or severity of the FMF symptoms. This finding underscores the importance of performing molecular studies on all suspect FMF patients. In addition to providing accurate diagnosis, these tests allow identification of presymptomatic genetically affected individuals, detection of carriers and assessment of the risk for amyloidosis in later life.

Adolescent↗

PubMed1999

The use of acupuncture in the treatment of erectile dysfunction.

The efficacy of acupuncture as a mono-therapy was evaluated in a pilot study of 16 patients suffering from erectile dysfunction (ED). In nine patients no organic co-morbidity was encountered. In a period of four weeks, acupuncture treatment was performed twice a week for a total of eight sessions. Each treatment session consisted of puncture of the same eight acupoints, four of which were connected to a Swiss made constant current Doltron ESA 600 stimulator. Low frequency electrical stimulation (5 Hz and 10 mA) was applied to these four acupoints, whereas no stimulation was applied to the other four points. After 30min, the electrical stimulation was terminated and all needles removed. Blood samples were drawn according to a fixed time schedule, to study the profile of a number of stress hormones, for example, adrenocorticotropic hormone, antidiuretic hormone and cortisol, the gonadotrophines follicle stimulating hormone and leutinizing hormone, and the sex steroid testosterone and its binding globulin, within the treatment period. Based on a diary of both patient and partner, and an interview one month after the end of treatment, the changes of sexual activity were evaluated over a period of 12 weeks, starting from the four weeks prior to the treatment, the four weeks during the treatment period and the four weeks after the treatment. An improvement of the quality of erection was experienced by 15% of patients, while 31% reported an increase in their sexual activity. No changes in the profiles of hormones were detected. The use of acupuncture as a mono-therapeutic modality in ED, did not influence the profile of the stress and sex hormones, but did improve the quality of erection and restored the sexual activity with an overall effect of 39%. No definite conclusions can be drawn from this pilot study. A controlled and blinded study including more patients will be needed before any definitive conclusion can be reached.

Acupuncture Therapy↗

PubMed1999

Baroreflex sensitivity and the blood pressure response to beta-blockade.

The objective of this analysis was to determine whether changes in baroreflex sensitivity (BRS) within 35 hypertensive patients (25 M, 10 F, mean age 47 years) treated with beta-blockade as monotherapy relate to reductions in ambulatory blood pressure (BP) or its variability. BP was recorded intra-arterially directly from the brachial artery before and during submaximal exercise. BRS was determined by the phenylephrine injection technique. MAP and its variability were determined for the awake period of 24-h BP monitoring. Subjects were randomised to one of atenolol, metoprolol, pindolol, or propranolol, and restudied after a mean of 5 months. Beta-blockade increased BRS in 24 patients and decreased BRS in 11. BRS increased from 6.53+/-4.94 to 9.40+/-8.62 ms/mm Hg (mean +/- s.d.) (P<0.01). Waking ambulatory MAP decreased from 125.8+/-15.8 to 106.4+/-16.2 mm Hg (P<0.0001), but its variability did not change. Higher BRS after chronic beta-blockade was associated with a decrease in waking ambulatory MAP (r = -0.55, P<0.001), but not with its variability (r = -0.08). Beta-blockade attenuated the pressor response to exercise, but there was a positive relationship between the effect of beta-blockade on BRS, and on the rise in systolic BP during bicycling (r = 0.63; P<0.001). Any dampening effect of beta-blockade on BP variability at rest in hypertensive patients with the greatest increase in BRS may be offset by increased pressor responses to physical activity such as exercise. Consequently, BP variability is unaffected, even though reductions in ambulatory BP during chronic beta-blockade are inversely related to changes in BRS. BP responses to beta-blockade may be a function of the action of this class of drugs on BRS. However, there is considerable variation, between subjects, in their effect on BRS. This may have implications for other conditions, such as dilated cardiomyopathy, or following myocardial infarction, in which improvement in BRS is one mechanism by which beta-adrenoceptor blockade could improve survival.

Adolescent↗