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Biomedical subjects

J Choi

Publications and source records attributed to J Choi.

At least 19 recordsLinked to original sources
PubMed2003

Suppressed pi(0) production at large transverse momentum in central Au+Au collisions at sqrt[s(NN)]=200 GeV.

Transverse momentum spectra of neutral pions in the range 1<p(T)<10 GeV/c have been measured at midrapidity by the PHENIX experiment at BNL RHIC in Au+Au collisions at sqrt[s(NN)]=200 GeV. The pi(0) multiplicity in central reactions is significantly below the yields measured at the same sqrt[s(NN)] in peripheral Au+Au and p+p reactions scaled by the number of nucleon-nucleon collisions. For the most central bin, the suppression factor is approximately 2.5 at p(T)=2 GeV/c and increases to approximately 4-5 at p(T) approximately 4 GeV/c. At larger p(T), the suppression remains constant within errors. The deficit is already apparent in semiperipheral reactions and increases smoothly with centrality.

Journal Article↗
PubMed2003

Reperfusion differentially induces caspase-3 activation in ischemic core and penumbra after stroke in immature brain.

BACKGROUND AND PURPOSE: Different strategies for neuroprotection of neonatal stroke may be required because the developing brain responds differently to hypoxia-ischemia than the mature brain. This study was designed to determine the role of caspase-dependent injury in the pathophysiology of pure focal cerebral ischemia in the immature brain. METHODS: Postnatal day 7 rats were subjected to permanent or transient middle cerebral artery (MCA) occlusion. Diffusion-weighted MRI was used during occlusion to noninvasively map the evolving ischemic core. The time course of caspase-3 activation in ischemic brain tissue was determined with the use of an Asp-Glu-Val-Asp-aminomethylcoumarin cleavage assay. The anatomy of caspase-3 activation in the ischemic core and penumbra was mapped immunohistochemically with an anti-activated caspase-3 antibody in coronal sections that matched the imaging planes on diffusion-weighted MRI. RESULTS: A marked increase in caspase-3 activity occurred within 24 hours of reperfusion after transient MCA occlusion. In contrast, caspase-3 activity remained significantly lower within 24 hours of permanent MCA occlusion. Cells with activated caspase-3 were prominent in the penumbra beginning at 3 hours after reperfusion, while a more delayed but marked caspase-3 activation was observed in the ischemic core by 24 hours after reperfusion. CONCLUSIONS: In the neonate, caspase-3 activation is likely to contribute substantially to cell death not only in the penumbra but also in the core after ischemia with reperfusion. Furthermore, persistent perfusion deficits result in less caspase-3 activation and appear to favor caspase-independent injury.

Animals↗
PubMed2003

AnyHeart: a single-piece heart-saving implantable artificial heart (BVAD)--monitoring and estimation.

AnyHeart is a single-piece, implantable biventricular assist device. This electromechanical BVAD has a moving-actuator mechanism. To monitor the status of AnyHeart from anywhere at any time, a portable personal digital assistant (PDA) monitor and web-based remote monitoring system were developed. The PDA local monitoring system has replaced bulky personal computer monitoring systems. The web-based remote monitoring system has several functions such as data collecting, storing, and posting through the internet. Basically, interventricular pressure (IVP) is a parameter indicating the filling level of the blood chambers of AnyHeart. The pump output can be estimated using IVP, which is acquired noninvasively from AnyHeart. With the proposed method, we can estimate the pump output with a small margin of error.

Computers, Handheld↗
PubMed2003

The vortex concentrator for suspended solids treatment.

The use of vortex concentrators is becoming increasingly popular for suspended solids reduction in combined sewer overflows and stormwater. This study is a laboratory investigation of the use of vortex concentrators to reduce the solids concentration of synthesized stormwater. The synthesized stormwater was made with water and addition of particles; sand, granular activated carbon, and sewer sediments. The vortex concentrator was made of acryl resin 300 mm in diameter. To determine the efficiency for various influent suspended solids (SS) concentrations, tests were performed with different SS concentrations. The samples were taken simultaneously at the influent storage tank and effluent tank, and measured SS concentrations. The range of surface loading rates were 120 to 850 m3/m2/day, and influent SS concentrations were varied from 300 to 5,000 mg/L. To determine the optimum coagulant dosage, jar tests were conducted with coagulants such as PAM and PAC. It was found that optimum coagulant and its dosage were PAM and 2 mg/L. The overall SS removal efficiency of the vortex concentrator for typical stormwater was estimated at about 65%. With an increase of SS concentration, the removal efficiency was increased. Since the SS concentration of stormwater was higher than 1,000 mg/L, the removal efficiency of the vortex concentrator for stormwater could be estimated to be 65-70%. The SS removal efficiency was increased with an increase of retention time, and the optimum retention time was 0.15-1.0 minutes. With an increase of the foul to overflow Q(F)/Q(o), a key parameter for vortex concentrator operation, the removal efficiency was increased. An alternative solution to improve treatment efficiency might be to set a follow-up retention basin. Based on a series of settling tests on the treated overflow water from the vortex concentrator, 5 to 10 minutes hydraulic retention time in a follow-up retention basin would substantially improve the results.

Carbon↗
PubMed2002

[Rapid analysis of biocompatibility with graded test samples exemplified by Ni-NiTi-Ti].

The biocompatibility of nickel-titanium alloys was investigated by single-culture experiments on functionally graded samples with a stepwise change in composition from nickel to titanium, including NiTi shape memory alloy of a 50:50 mixture. This approach permitted a considerable decrease of experimental resources by simultaneously studying a full variation of composition. The results indicate a good biocompatibility for a nickel content up to about 50%. The cells used in the biocompatibility studies comprised human osteoblast-like osteosarcoma cells (SAOS-2, MG-63), primary human osteoblasts (HOB), and murine fibroblasts (3T3).

Animals↗
PubMed2002

A kinetic model of N-nitrosodimethylamine (NDMA) formation during water chlorination/chloramination.

Experiments were conducted to investigate the hypothesis that N-nitrosodimethylamine (NDMA) is a potential disinfection by-product. NDMA was formed by the reaction of dimethylamine (DMA) with monochloramine and also with free chlorine in the presence of ammonia. We proposed a mechanism for NDMA formation which does not require the presence of nitrite as in N-nitrosation. The critical NDMA formation reactions consist of i) the formation of monochloramine by combination of free chlorine with ammonia, ii) the formation of 1,1-dimethylhydrazine (UDMH) intermediate from the reaction of DMA with monochloramine followed by, iii) the oxidation of UDMH by monochloramine to NDMA, and iv) the reversible chlorine transfer reaction between free chlorine/monochloramine and DMA which is parallel with i) and ii). A kinetic model was developed to validate the proposed mechanism.

Carcinogens↗
PubMed2001

Low-density cDNA array-coupled to PCR differential display identifies new estrogen-responsive genes during the postnatal differentiation of the rat hypothalamus.

To identify estrogen (E)-responsive genes that may play important roles in the sexual differentiation and maturation of the neuroendocrine hypothalamus, we used mRNA differential display PCR to analyze hypothalamic RNA derived from estrogen-sterilized rats (ESRs). Neonatal rats were s.c.-injected with 100 microg of 17 beta-estradiol-benzoate (EB) for 5 days. Approximately 300 out of more than 2000 RNAs examined displayed a differential expression pattern between hypothalami of the ESR females compared to their 60-day-old controls. EB-dependent expression of these genes was further analyzed by low-density cDNA array using cDNA probe sets reverse-transcribed from the same groups; 98 genes were confirmed to be differentially expressed. We selected 41 clones that showed higher density differences between the two probe sets than mean density difference in control cyclophilin cDNA blots in the cDNA array. After being cloned into pGEM-T vectors, their sequences were analyzed. Homology searches identified four genes as a protein kinase C (PKC)-binding protein, NELL2 (clone 6-1), a thyroid nuclear factor, TTF-1 (9-1), Munc18-1 (17-6), and leuserpin-2 (18-5). The other 22 genes were similar to reported genes or cDNAs such as mouse kinesin-associated protein 3 (KAP3, 8b), mouse IgE binding lectin (15-1), normalized rat brain cDNA (5-1), rat cDNA (8-1) and rat embryonic cDNA (17-1). Fifteen clones such as clone 7-3 showed no match in the GenBank Database. Further characterization of eight clones (17-1, 7-3, 8-1, 5-1, NELL2, KAP3 homolog, IgE binding lectin homolog, and TTF-1) showed that their expression in the adult female rat hypothalamus is sensitive to neonatal treatment with EB. They showed brain-specific expression and moreover, showed an increase in their mRNA level before the initiation of puberty. Some of them showed gender differences in their different postnatal expression pattern. We speculate that further study will demonstrate that many of the E-regulated genes identified in the present study play important roles in the regulation of the sexual differentiation and E-dependent maturation of the hypothalamus.

Animals↗
PubMed2001

Organic/inorganic hybrid composites from cubic silsesquioxanes.

A new class of epoxy nanocomposites with completely defined organic/inorganic phases was prepared by reacting octakis(glycidyldimethylsiloxy)octasilsesquioxane [(glydicylMe(2)SiOSiO(1.5))(8)] (OG) with diaminodiphenylmethane (DDM) at various compositional ratios. The effects of reaction curing conditions on nanostructural organization and mechanical properties were explored. A commercial epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) was used as a reference material throughout these studies. FTIR was used to follow the curing process and to demonstrate that the silsesquioxane structure is preserved during processing. OG/DDM composites possess comparable tensile moduli (E) and fracture toughness (K(IC)) to, and better thermal stabilities than, DGEBA/DDM cured under similar conditions. Dynamic mechanical analysis and model reaction studies suggest that the maximum cross-link density is obtained at N = 0.5 (NH(2):epoxy groups = 0.5) whereas the mechanical properties are maximized at N = 1.0. Digestion of the inorganic core with HF followed by GPC analysis of the resulting organic tether fragments when combined with the model reaction studies confirms that, at N = 0.5, each organic tether connects four cubes, while, at N = 1.0, linear tethers connecting two cubes dominate the network structure. Thus, well-defined nanocomposites with controlled variation of the organic tether architecture can be made and their properties assessed.

Journal Article↗
PubMed2001

Mutational effects on thermostable superoxide dismutase from Aquifex pyrophilus: understanding the molecular basis of protein thermostability.

We designed two mutants of superoxide dismutase (SOD), one is thermostable and the other is thermolabile, which provide valuable insight to identify amino acid residues essential for the thermostability of the SOD from Aquifex pyrophilus (ApSOD). The mutant K12A, in which Lys12 was replaced by Ala, had increased thermostability compared to that of the wild type. The T(1/2) value of K12A was 210 min and that of the wild type was 175 min at 95 degrees C. However, the thermostability of the mutant E41A, which has a T(1/2) value of 25 min at 95 degrees C, was significantly decreased compared to the wild type of ApSOD. To explain the enhanced thermostability of K12A and thermolabile E41A on the structural basis, the crystal structures of the two SOD mutants have been determined. The results have clearly shown the general significance of hydrogen bonds and ion-pair network in the thermostability of proteins.

Bacterial Proteins↗
PubMed2001

Synthesis of a novel hepatitis C virus protein by ribosomal frameshift.

Hepatitis C virus (HCV) is an important human pathogen that affects approximately 100 million people worldwide. Its RNA genome codes for a polyprotein, which is cleaved by viral and cellular proteases to produce at least 10 mature viral protein products. We report here the discovery of a novel HCV protein synthesized by ribosomal frameshift. This protein, which we named the F protein, is synthesized from the initiation codon of the polyprotein sequence followed by ribosomal frameshift into the -2/+1 reading frame. This ribosomal frameshift requires only codons 8-14 of the core protein-coding sequence, and the shift junction is located at or near codon 11. An F protein analog synthesized in vitro reacted with the sera of HCV patients but not with the sera of hepatitis B patients, indicating the expression of the F protein during natural HCV infection. This unexpected finding may open new avenues for the development of anti-HCV drugs.

Amino Acid Sequence↗
PubMed2001

Haplotype variation and linkage disequilibrium in 313 human genes.

Variation within genes has important implications for all biological traits. We identified 3899 single nucleotide polymorphisms (SNPs) that were present within 313 genes from 82 unrelated individuals of diverse ancestry, and we organized the SNPs into 4304 different haplotypes. Each gene had several variable SNPs and haplotypes that were present in all populations, as well as a number that were population-specific. Pairs of SNPs exhibited variability in the degree of linkage disequilibrium that was a function of their location within a gene, distance from each other, population distribution, and population frequency. Haplotypes generally had more information content (heterozygosity) than did individual SNPs. Our analysis of the pattern of variation strongly supports the recent expansion of the human population.

Alleles↗
PubMed2001

Salt-thermal zeolitization of fly ash.

The molten-salt method has been recently proposed as a new approach to zeolitization of fly ash. Unlike the hydrothermal method, this method employs salt mixtures as the reaction medium without any addition of water. In this study, systematic investigation has been conducted on zeolitization of fly ash in a NaOH-NaNO3 system in order to elucidate the mechanism of zeolite formation and to achieve its optimization. Zeolitization of fly ash was conducted by thermally treating a powder mixture of fly ash, NaOH, and NaNO3. Zeolitization of fly ash took place above 200 degrees C, a temperature lower than the melting points of salt and base in the NaOH-NaNO3 system. However, it was uncertain whether the reactions took place in a local molten state or in a solid state. Therefore, the proposed method is renamed the "salt-thermal" method rather than the "molten-salt" method. Mainly because of difficulty in mobility of components in salt mixtures, zeolitization seems to occur within a local reaction system. In situ rearrangement of activated components seems to lead to zeolite formation. Particle growth, rather than crystal growth through agglomeration, resulted in no distinct morphologies of zeolite phases. Following are the optimal zeolitization conditions of the salt-thermal method: temperature, 250-350 degrees C; time, 3-12 h; weight ratio of NaOH/NaNO3, 0.3-0.5; weight ratio of NaNO3/fly ash, 0.7-1.4. Therefore, it is clear from this work that the salt-thermal method could be applied to massive zeolitization of fly ash as a new alternative method for recycling this waste.

Carcinogens↗
PubMed2001

Hydrogen peroxide is a novel inducer of connective tissue growth factor.

Connective tissue growth factor (CTGF) has recently been described as a fibrogenic factor and is greatly induced by various extracellular stimuli, such as transforming growth factor-beta (TGF-beta), dexamethasone, and serotonin. CTGF induces collagen type I and fibronectin, and the deposition of such molecules leads to fibrotic disease in many tissues. Intracellular reactive oxygen species (ROS) are generated by extracellular stress conditions and are produced as by-products of cellular metabolism. Imbalanced cellular redox status is a potent pathogenic factor that leads to various degenerative diseases, including tissue fibrosis. Since CTGF is believed to play a crucial role in fibrotic disease formation in many tissues, we examined the role of ROS in CTGF gene expression in human lens epithelial cell line B3. The results showed that CTGF was induced by reactive oxygen species such as hydrogen peroxide and hydroxyl radicals. Next, we examined whether CTGF induction by ROS is via newly synthesized TGF-beta. The results showed that ROS directly induced CTGF mRNA not via the increased TGF-beta synthesis or activation. Next, we treated AG490, which is the well-known inhibitor of Janus kinase (JAK), with hydrogen peroxide. AG490 abrogated the CTGF induction by ROS in a dose-dependent manner. The results suggest that JAK-2/-3 seems to be involved in the enhanced CTGF mRNA expression by hydrogen peroxide. In this report, we present that hydrogen peroxide is a novel inducer of CTGF gene expression and that JAK-2/-3 activation seems to play a role in CTGF induction.

Amino Acid Sequence↗