Biomedical subjects

V Q Nguyen

Publications and source records attributed to V Q Nguyen.

At least 19 recordsLinked to original sources

PubMed2003

Small-core As-Se fiber for Raman amplification.

We have demonstrated Raman small-core As-Se fiber. More than 20-dB of gain was observed in a 1.1-m length of fiber pumped by a nanosecond pulse of approximately 10.8-W peak power at 1.50 microm. The peak of the Raman gain occurred at a shift of approximately 240 cm(-1). The Raman gain coefficient is estimated to be approximately 2.3 x 10(-11) m/W, which is more than 300 times greater than that of silica. The large Raman gain coefficient coupled with the large IR transparency window of these fibers shows promise for development of As-Se Raman fiber lasers and amplifiers in the near-, mid-, and long-IR spectral regions.

Journal Article↗

PubMed2001

Cyclin-dependent kinases prevent DNA re-replication through multiple mechanisms.

The stable propagation of genetic information requires that the entire genome of an organism be faithfully replicated once and only once each cell cycle. In eukaryotes, this replication is initiated at hundreds to thousands of replication origins distributed over the genome, each of which must be prohibited from re-initiating DNA replication within every cell cycle. How cells prevent re-initiation has been a long-standing question in cell biology. In several eukaryotes, cyclin-dependent kinases (CDKs) have been implicated in promoting the block to re-initiation, but exactly how they perform this function is unclear. Here we show that B-type CDKs in Saccharomyces cerevisiae prevent re-initiation through multiple overlapping mechanisms, including phosphorylation of the origin recognition complex (ORC), downregulation of Cdc6 activity, and nuclear exclusion of the Mcm2-7 complex. Only when all three inhibitory pathways are disrupted do origins re-initiate DNA replication in G2/M cells. These studies show that each of these three independent mechanisms of regulation is functionally important.

CDC28 Protein Kinase, S cerevisiae↗

PubMed2001

A sperm cytoskeletal protein that signals oocyte meiotic maturation and ovulation.

Caenorhabditis elegans oocytes, like those of most animals, arrest during meiotic prophase. Sperm promote the resumption of meiosis (maturation) and contraction of smooth muscle-like gonadal sheath cells, which are required for ovulation. We show that the major sperm cytoskeletal protein (MSP) is a bipartite signal for oocyte maturation and sheath contraction. MSP also functions in sperm locomotion, playing a role analogous to actin. Thus, during evolution, MSP has acquired extracellular signaling and intracellular cytoskeletal functions for reproduction. Proteins with MSP-like domains are found in plants, fungi, and other animals, suggesting that related signaling functions may exist in other phyla.

Amino Acid Sequence↗

PubMed2001

Carboxy-terminal proteolytic processing of Helicobacter pylori vacuolating toxin.

The vacA gene of Helicobacter pylori strain 60190 encodes a 1, 287-amino-acid protoxin, which undergoes cleavage of a 33-amino-acid amino-terminal signal sequence and carboxy-terminal proteolytic processing to yield a mature secreted toxin. Several features of VacA suggest that it belongs to the autotransporter family of gram-negative bacterial secreted proteins. Based on matrix-assisted laser desorption ionization-time of flight mass spectrometric analysis, we calculate that the mature toxin has a mass of 88.2+/-0.2 kDa and consists of approximately 821 amino acids.

Bacterial Proteins↗

PubMed2000

Clb/Cdc28 kinases promote nuclear export of the replication initiator proteins Mcm2-7.

BACKGROUND: In the budding yeast Saccharomyces cerevisiae, the cyclin-dependent kinases of the Clb/Cdc28 family restrict the initiation of DNA replication to once per cell cycle by preventing the re-assembly of pre-replicative complexes (pre-RCs) at replication origins that have already initiated replication. This assembly involves the Cdc6-dependent loading of six minichromosome maintenance (Mcm) proteins, Mcm2-7, onto origins. How Clb/Cdc28 kinases prevent pre-RC assembly is not understood. RESULTS: In living cells, the Mcm proteins were found to colocalize in a cell-cycle-regulated manner. Mcm2-4, 6 and 7 were concentrated in the nucleus in G1 phase, gradually exported to the cytoplasm during S phase, and excluded from the nucleus by G2 and M phase. Tagging any single Mcm protein with the SV40 nuclear localization signal made all Mcm proteins constitutively nuclear. In the absence of functional Cdc6, Clb/Cdc28 kinases were necessary and sufficient for efficient net nuclear export of a fusion protein between Mcm7 and the green fluorescent protein (Mcm7-GFP), whereas inactivation of these kinases at the end of mitosis coincided with the net nuclear import of Mcm7-GFP. In contrast, in the presence of functional Cdc6, which loads Mcm proteins onto chromatin, S-phase progression as well as Clb/Cdc28 kinases was required for Mcm-GFP export. CONCLUSIONS: We propose that Clb/Cdc28 kinases prevent pre-RC reassembly in part by promoting the net nuclear export of Mcm proteins. We further propose that Mcm proteins become refractory to this regulation when they load onto chromatin and must be dislodged by DNA replication before they can be exported. Such an arrangement could ensure that Mcm proteins complete their replication function before they are removed from the nucleus.

Biological Transport↗

PubMed2000

Scanning by DOVAM-S detects all unique sequence changes in blinded analyses: evidence that the scanning conditions are generic.

The [detection of virtually all mutations]-SSCP (DOVAM-S) is a highly sensitive variant of single strand conformation polymorphism (SSCP). Mutations in the factor IX gene were used to find a set of five SSCP conditions that detects virtually all mutations. A blinded analysis of the factor IX gene in patients with hemophilia B detected 82 of 82 unique mutations. Since the method was developed and tested on the factor IX gene, it is possible that the conditions selected work more efficiently in the factor IX gene than in other genes. To test the general applicability of the conditions under which DOVAM-S detected all mutations in this gene, blinded analyses were performed in the human factor VIII and ataxia-telangiectasia (ATM) genes. Segments were amplified individually, combined into groups of 16 to 18 amplified segments and electrophoresed in five different nondenaturing conditions of varying matrices, buffers, temperatures and additives. Blinded analyses were performed in 92 samples from patients with hemophilia A (factor VIII gene) and 19 samples from A-T patients (ATM gene). Combined with an earlier blinded analysis in the factor IX gene, all of the 250 mutations and polymorphisms (180 of which are unique) were detected in both analyses. For two, three and four joint conditions, the average detection frequency ranged from 77%-97%, 91%-100% and 95%-100%, respectively. For each of the genes, one mutation may have been missed if only four conditions were used. With DOVAM-S, approximately 500 kb of autosomal sequence can be scanned in five gels with virtually 100% detection of mutations within the scanned region. The detection of 180 out of 180 unique sequence changes implies that DOVAM-S detects at least 96.5% (P = 0.03) of mutations. Blinded analyses that detect 400 unique sequence changes are required to determine that a scanning method detects at least 98.5% of mutations.

Ataxia Telangiectasia↗

PubMed1999

High-pressure liquid chromatography quantitation of cytochrome c using 393 nm detection.

The release of cytochrome c from the mitochondrial intermembrane space can induce apoptotic cell death. Previous methods to detect cytochrome c release from mitochondria have relied upon immunoblotting, a procedure that can be limited by nonlinearity of signal, epitope masking, and impracticality for large numbers of samples. In order to circumvent these limitations, we have developed a reverse-phase high-pressure liquid chromatography method for cytochrome c detection and quantitation by taking advantage of a novel acid-induced absorbance maximum at 393 nm for cytochrome c in buffer containing 0.1% trifluoroacetic acid. Using a C4 reverse-phase analytical column, this assay had a quantitation limit of 10 ng (0.8 pmol) of cytochrome c. We demonstrated the detection and quantitation of cytochrome c from isolated mitochondria. This method of cytochrome c analysis may be useful for the study of agents that cause mitochondrial dysfunction and apoptotic cell death.

Animals↗

PubMed1999

Mass spectrometric analysis of the electroeluates of fluorescent proteins after preparative electrophoresis in the automated HPGE-1000 apparatus.

Bands of green fluorescent protein (GFP) and R-phycoerythrin (PHYCO) in gel electrophoresis on the automated apparatus for gel electrophoresis with periodic fluorescence scanning (HPGE), the HPGE-1000 apparatus, were retrieved from the gel by electroelution. While PHYCO was recovered in a single volume of electroeluate buffer after the predicted migration time, GFP fluorescence was lost under the same conditions and could only be recovered using multiple changes of electroeluate buffer. The multiple volumes of buffer necessitated pooling, concentration, and storage, conditions under which a minor GFP component, GFP-II, formed artifactually. PHYCO after electroelution also exhibits a minor component present in the original preparation. The electroeluate of GFP, transferred into a mass spectrometer after pooling, concentration and storage, is indistinguishable in mass from the original preparation.

Automation↗

PubMed1997

Gas-phase protonation of pyridine. A variable-time neutralization-reionization and Ab initio study of pyridinium radicals.

Gas-phase protonation of pyridine with CH3NH3+, NH4+, t-C4H9+, H3O+ and CH5+ under thermal conditions was studied by variable-time neutralization-reionization mass spectrometry and ab initio calculations. N-Protonation was found to occur exclusively for CH3NH3+ through H3O+ and predominantly for CH5+. The calculated MP2/6-311G(2d,p) energies gave the proton affinities of N, C-2, C-3 and C-4 in pyridine as 924, 658, 686 and 637 kJ mol-1, respectively, which were in good agreement with previous experimental and theoretical results. Vertical neutralization of the N-protonated isomer (1H+) was accompanied by moderate Franck-Condon effects that deposited 20-21 kJ mol-1 in the 1H-pyridinium radicals (1H) formed. 1H was calculated by UMP2/6-311G(2d,p) and B3LYP/6-311G(2d,p) to be a bound species in its ground electronic state. A substantial fraction of stable 1H was detected in the spectra, which depended on the precursor ion internal energy. Deuterium labeling showed a specific loss of the N-bound hydrogen or deuterium in the radicals. The specificity increased with increasing internal energy in the radicals and decreasing contribution of ion dissociations following reionization. Variable-time measurements established specific loss of the N-bound deuterium also in dissociating low-energy 1D. Loss of hydrogen from 1H+ cations following reionization was highly endothermic and was accompanied by rearrangements that partially scrambled the ring hydrogens.

Chemical Phenomena↗

PubMed1997

Simultaneous assessment of conformation and aggregation of beta-amyloid peptide using electrospray ionization mass spectrometry.

Electrospray ionization mass spectrometry was used to study conformation and aggregation of the synthetic beta-amyloid peptide, residues 1-40 (betaA4), as a function of concentration and sample aging. All mass spectra showed a major envelope of peaks corresponding to charge states of 7-3 of the monomeric form of betaA4. In addition, weaker envelopes of peaks corresponding to charge states of dimeric, trimeric, and tetrameric betaA4 species were seen under gentle ionization conditions. The average charge state of the envelope associated with the monomeric form decreased by ca. 0.5 z as samples were aged, indicating that the relatively open form (likely random coil) of the peptide was modified into the more compact form (likely beta-sheet) as a function of sample aging. The aggregate forms became weaker and ultimately were absent both in the more dilute solutions and in aged aliquots of the concentrated sample. These aggregates were interpreted as assemblies of the random coil form. We interpret our inability to see an ion envelope that can be associated with aggregates of the beta-sheet form to be a consequence of the presumed very compact nature of this form. A model for the formation of betaA4 fibrils is proposed and discussed.

Amyloid beta-Peptides↗

PubMed1996

Energy effects in collisional neutralization with organic molecules.

Neutralization-reionization of CH3OH+., CH3NH2+. and (CH3)2NH+. ions was studied under conditions of exothermic and endothermic electron transfer and distributions of internal energy in the reionized ions were determined. The internal energy deposited on neutralization at kiloelectronvolt collision energies is governed by Franck-Condon effects for the systems under study, whereas the endothermic or exothermic energy balance in electron transfer from molecular targets has only a small effect on the high-energy fraction of the molecules formed. Electron transfer from low-lying molecular orbitals of the organic donor is suggested to occur during random orientations of the ion-molecule collision pair. The internal energy of the precursor ions to be neutralized has a large effect on the relative abundances of survivor ions in the spectra. Vertical recombination energies of CH3OH+. and CH3NH2+. are found by Gaussian-2 level calculations to differ substantially from vertical ionization energies of the corresponding neutral molecules. Franck-Condon effects are analyzed for the vibrational modes that are affected most by vertical electron transfer.

Chemical Phenomena↗

PubMed1996

Protonation sites in gaseous pyrrole and imidazole: a neutralization-reionization and ab initio study.

Mild gas-phase acids C4H9+ and NH4+ protonate pyrrole at C-2 and C-3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization-reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6-311G(2d,p) as 866, 845 and 786 kJ mol-1 for protonation at C-2, C-3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C-2- and C-3-protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N-3, C-4, C-2, C-5 and N-1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N-3-protonated imidazole is accompanied by large Franck-Condon effects which deposit on average 183 kJ mol-1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N-H bond cleavage and isomerization to the more stable C-2 H-isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.

Imidazoles↗

PubMed1993

Anti-TAPA-1 antibodies induce protein tyrosine phosphorylation that is prevented by increasing intracellular thiol levels.

We studied the signal induced by the anti-TAPA-1 antibody and compared it to the signal induced by anti-IgM antibodies in a human B cell line, OCl-LY8. We found that exposure of these cells to either antibody resulted in a rapid increase in protein tyrosine phosphorylation which was prevented by inhibitors of tyrosine kinases. Tyrosine phosphorylation was an early event in the cascase leading to the antiproliferative effect of the anti-TAPA-1 antibody. However, 2-ME, a reducing agent that is not an inhibitor of tyrosine kinases, prevented both tyrosine phosphorylation and the antiproliferative effect of the antibody. Cells grown in low concentrations of 2-ME did not exhibit an increase in tyrosine phosphorylation in response to the anti-TAPA-1 antibody and were insensitive to the antiproliferative effect of the antibody. In contrast, the same cells maintained in 2-ME were able to induce tyrosine phosphorylation in response to anti-IgM. The use of 2-ME resulted in an increase in intracellular thiols, mostly glutathione. Moreover, compounds that block glutathione synthesis rendered cells susceptible to the antibody, even in the presence of 2-ME. These experiments demonstrate that tyrosine kinases are involved in propagating the antiproliferative signal initiated by the anti-TAPA-1 antibody and suggest that this signal is dependent upon the level of intracellular thiols.

Antigens, CD↗

PubMed1992

Unilateral diaphragmatic paralysis: a matter for concern.

A dilemma often faces the clinician who is asked to evaluate unilateral elevation of a hemidiaphragm that is identified on routine chest roentgenogram. The possibilities include neoplasm, infection, neuromuscular disease, trauma, or benign etiologies. We present an asymptomatic patient with this finding to provide some guidelines for the nature and extent of further investigation of unilateral diaphragmatic paralysis.

Biopsy, Needle↗

PubMed1992

DaCosta's syndrome: chronic symptomatic hyperventilation.

In 1871, DaCosta published his observation of somatic symptoms preceded by significant hyperventilation. More than a century later, the hyperventilation syndrome remains a poorly defined but common clinical condition. Although familiar to most practitioners of medicine when it presents as an acute phenomenon, the diagnosis may go unrecognized in its chronic form. The ability of a chronic hyperventilatory state to mimic a life-threatening cardiopulmonary disease is not always appreciated.

Adult↗

PubMed1991

Structure and membrane topology of TAPA-1.

TAPA-1 (the target of an antiproliferative antibody) is a 26-kDa cell surface protein expressed on most human cell lines. TAPA-1 is a member of an evolutionarily related family of cell surface proteins all of which contain four transmembrane domains. A model is proposed for topology of TAPA-1 based on proteolysis studies in the in vitro translated protein embedded into microsomal membranes. This analysis predicts that the amino and the carboxyl termini of the molecule are cytoplasmic and that the two hydrophilic regions of the molecule are extracellular. The antigenic epitope of the human TAPA-1 is contained within a subregion of the second extracellular domain of the protein. This is the only region in the protein that has not been tightly conserved in mammalian evolution.

Amino Acid Sequence↗

PubMed1991

Interleukin-2 enhances the translocation of Escherichia coli from the intestines to other organs.

To determine if interleukin-2 (IL-2) would inhibit gastrointestinal bacterial translocation, mice were gut-decontaminated and recolonized with Escherichia coli C25; some groups were pretreated with 200 mg/kg cyclophosphamide. IL-2 (1.68 mg/kg) or sterile diluent was injected twice daily for 3 or 5 days, and mice were sacrificed the next day. High cecal levels of E. coli C25 were present in all mice. The overall incidence of E. coli C25 translocation to mesenteric lymph nodes was not reduced by IL-2. The median numbers of translocated E. coli C25/g of mesenteric lymph node were significantly (P less than .005) higher after both 3 days (659 vs. 117) and 5 days (550 vs. 50) of treatment with IL-2 with cyclophosphamide and after 5 days (1784 vs. 225) of IL-2 without cyclophosphamide. IL-2 prevented neutropenia and exacerbated lymphopenia caused by cyclophosphamide. The in vitro growth of E. coli C25 was not affected by up to 10(5) units/ml IL-2. Ileal and cecal structures assessed by light and electron microscopy were not altered by IL-2. Thus, IL-2 unexpectedly enhanced the translocation of E. coli C25 from the gastrointestinal tracts of both cyclophosphamide-pretreated and normal mice.

Animals↗

PubMed1991

Bronchogenic carcinoma and the acquired immunodeficiency syndrome.

The majority of patients with the acquired immunodeficiency syndrome will develop clinical and radiographic pulmonary involvement during the course of their disease. Although opportunistic pathogens account for the majority of these intrathoracic abnormalities, pulmonary neoplasms are being encountered more often than would be expected for the age group under consideration. Clinicians need to be vigilant for the possibility of the early appearance of bronchogenic carcinoma in this subset of patients.

Acquired Immunodeficiency Syndrome↗