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Biomedical subjects

C Chan

Publications and source records attributed to C Chan.

At least 91 records · Page 5Linked to original sources
PubMed1998

Protease resistance of syntaxin.SNAP-25.VAMP complexes. Implications for assembly and structure.

A stable ternary complex formed with vesicle-associated membrane protein 2 (VAMP2) and plasma membrane proteins syntaxin 1A and synaptosome-associated protein of 25 kDa (SNAP-25) is proposed to function in synaptic vesicle exocytosis. To analyze the structural characteristics of this synaptic protein complex, recombinant binary (syntaxin 1A.SNAP-25), recombinant ternary, and native ternary complexes were subjected to limited trypsin proteolysis. The protected fragments, defined by amino-terminal sequencing and mass spectrometry, included a carboxyl-terminal region of syntaxin 1A, the cytoplasmic domain of VAMP2, and amino- and carboxyl-terminal regions of SNAP-25. Furthermore, separate amino- and carboxyl-terminal fragments of SNAP-25, when combined with VAMP2 and syntaxin 1A, were sufficient for stable complex assembly. Analysis of ternary complexes formed with full-length proteins revealed that the carboxyl-terminal transmembrane anchors of both syntaxin 1A and VAMP2 were protected from trypsin digestion. Moreover, the stability of ternary complexes was increased by inclusion of these transmembrane domains. These results suggest that the transmembrane domains of VAMP2 and syntaxin 1A contribute to complex assembly and stability and that amino- and carboxyl-terminal regions of SNAP-25 may function as independent domains.

Animals↗
PubMed1998

Rational drug design approach for overcoming drug resistance: application to pyrimethamine resistance in malaria.

Pyrimethamine acts by selectively inhibiting malarial dihydrofolate reductase-thymidylate synthase (DHFR-TS). Resistance in the most important human parasite, Plasmodium falciparum, initially results from an S108N mutation in the DHFR domain, with additional mutation (most commonly C59R or N51I or both) imparting much greater resistance. From a homology model of the 3-D structure of DHFR-TS, rational drug design techniques have been used to design and subsequently synthesize inhibitors able to overcome malarial pyrimethamine resistance. Compared to pyrimethamine (Ki 1.5 nM) with purified recombinant DHFR fromP. falciparum, the Ki value of the m-methoxy analogue of pyrimethamine was 1.07 nM, but against the DHFR bearing the double mutation (C59R + S108N), the Ki values for pyrimethamine and the m-methoxy analogue were 71.7 and 14.0 nM, respectively. The m-chloro analogue of pyrimethamine was a stronger inhibitor of both wild-type DHFR (with Ki 0.30 nM) and the doubly mutant (C59R +S108N) purified enzyme (with Ki 2.40 nM). Growth of parasite cultures of P. falciparum in vitro was also strongly inhibited by these compounds with 50% inhibition of growth occurring at 3.7 microM for the m-methoxy and 0.6 microM for the m-chloro compounds with the K1 parasite line bearing the double mutation (S108N + C59R), compared to 10.2 microM for pyrimethamine. These inhibitors were also found in preliminary studies to retain antimalarial activity in vivo in P. berghei-infected mice.

Animals↗
PubMed1998

Catheter closure of moderate- to large-sized patent ductus arteriosus using the new Amplatzer duct occluder: immediate and short-term results.

OBJECTIVES: The aim of this study was to assess the immediate and short-term results of anterograde catheter closure of a moderate- to large-sized patent ductus arteriosus (PDA) using the new self-expandable, respositionable Amplatzer duct occluder (ADO) device. BACKGROUND: Transcatheter closure of a PDA using devices or coils is technically challenging and may be accompanied by a 38% incidence of residual shunts. METHODS: Twenty-four patients (6 male, 18 female) underwent attempted transcatheter closure of a PDA using the ADO at a median age of 3.8 years (range 0.4 to 48) and a median weight of 15.5 kg (range 6 to 70). The mean PDA diameter at its narrowest segment was 3.7+/-1.5 mm. A 6F long sheath was used for delivery of the ADO. Follow-up evaluation was performed with color flow mapping of the main pulmonary artery within 24 h and at 1 and 3 months after closure. RESULTS: Twenty three of the 24 patients had successful device placement. Angiography showed that 7 patients had complete immediate closure, 14 had a trace shunt (foaming through the device with no jet), and 2 had a small residual shunt (with a jet). Within 24 h, color Doppler revealed complete closure in all patients. The unsuccessful attempt was during an initial trial with a prototype that has been modified. The median fluoroscopy time was 13.5 min (range 6.3 to 47). All patients were discharged home the next day. There were no complications. Of the 23 patients, 21 completed the 1-month follow-up, all (95% confidence interval [CI] 86% to 100%) with complete closure, and 18 of 23 patients completed the 3-month follow-up, also all (95% CI 83% to 100%) with complete closure. CONCLUSIONS: Anterograde transcatheter closure using the new ADO is an effective therapy for patients with a PDA diameter up to 6 mm. Further clinical trials are underway.

Alloys↗
PubMed1998

Phenothiazine inhibitors of trypanothione reductase as potential antitrypanosomal and antileishmanial drugs.

Given the role of trypanothione in the redox defenses of pathogenic trypanosomal and leishmanial parasites, in contrast to glutathione for their mammalian hosts, selective inhibitors of trypanothione reductase are potential drug leads against trypanosomiasis and leishmaniasis. In the present study, the rational drug design approach was used to discover tricyclic neuroleptic molecular frameworks as lead structures for the development of inhibitors, selective for trypanothione reductase over host glutathione reductase. From a homology-modeled structure for trypanothione reductase, replaced in the later stages of the study by the X-ray coordinates for the enzyme from Crithidia fasciculata, a series of inhibitors based on phenothiazine was designed. These were shown to be reversible inhibitors of trypanothione reductase from Trypanosoma cruzi, linearly competitive with trypanothione as substrate and noncompetitive with NADPH, consistent with ping-pong bi bi kinetics. Analogues, synthesized to define structure-activity relationships for the active site, included N-acylpromazines, 2-substituted phenothiazines, and trisubstituted promazines. Analysis of Ki and I50 data, on the basis of calculated log P and molar refractivity values, provided evidence of a specially favored fit of small 2-substituents (especially 2-chloro and 2-trifluoromethyl), with a remote hydrophobic patch on the enzyme accessible for larger, hydrophobic 2-substituents. There was also evidence of an additional hydrophobic enzymic region available to suitable N-substituents of the promazine nucleus. Ki data also indicated that the phenothiazine nucleus can adopt more than one inhibitory orientation in its binding site. Selected compounds were tested for in vitro activity against Trypanosoma brucei, T. cruzi, and Leishmania donovani, with selective activities in the micromolar range being determined for a number of them.

Animals↗
PubMed1998

Is splenectomy necessary in devascularization procedures for treatment of bleeding portal hypertension?

OBJECTIVE: To investigate whether splenectomy as a part of devascularization procedures is necessary. DESIGN: Prospective, controlled, randomized trial. SETTING: University hospital, referral center. PATIENTS: A total of 55 patients (Child-Pugh class A and B) with a history of bleeding portal hypertension were treated by means of a modified Sugiura-Futagawa procedure. Twenty-three patients underwent splenectomy and 22 did not. METHODS: Postoperative outcome was recorded and comparison of the 2 groups was done with the Fisher exact test. Kaplan-Meier survival curves were constructed. Main outcome and postoperative differences between the patients who underwent splenectomy and those who did not were investigated. RESULTS: Both groups were comparable in the postoperative period. Significant differences were observed in transfusion requirements and postoperative portal vein thrombosis, both favoring the group without splenectomy. No differences in rebleeding, encephalopathy rate, operative time, or postoperative complications were observed. CONCLUSION: Splenectomy is not routinely necessary in devascularization procedures for bleeding portal hypertension.

Adolescent↗
PubMed1998

Primary stenting of de novo lesions in small coronary arteries: a prospective, pilot study.

Technical advancement and new anti-thrombotic regimens have recently shown so much improvement in the results of coronary stenting that the conventional contra-indication for stenting in small coronary arteries (<3 mm) needs to be revised. We undertook a prospective pilot study of elective Palmaz-Schatz stenting in de novo lesions located in coronary arteries of less than 3 mm diameter. Fifty consecutive patients (63 +/- 9 years) with stable (n = 38) and unstable angina (n = 12) were included. Philips-DCI quantitative coronary analysis was used to measure reference diameter, minimal lumen diameter and percent diameter stenosis before PTCA, after stenting and at 6-month angiographic follow-up study. All measurements were performed after intracoronary injection of nitroglycerin (300 microg). All patients received ticlopidine (250 mg/day) and aspirin (100 mg/day). The mean lesion length was 9 +/- 3 mm. The balloon size used for stent delivery was 2.75 mm in 30 patients and 2.5 mm in 20 patients and the mean balloon inflation pressure used for stent deployment was 12 +/- 2 atm. All stents were deployed successfully. In-hospital complications occurred in two patients, diagonal branch occlusion at day 2 requiring emergency PTCA in one and a hematoma at the femoral puncture site requiring surgery in the other. Major adverse cardiac event (MACE) rate remained 2% (nonfatal infarct in one). Follow-up angiography (n = 46, 92%) at 6 +/- 3 months showed a 30% restenosis rate. Target vessel revascularization (TVR) rate was 13%. We conclude that elective stenting in small coronary arteries is feasible and involves an acceptable risk of restenosis.

Angina Pectoris↗
PubMed1998

A new molecular variant of luteinizing hormone associated with female infertility.

OBJECTIVE: To investigate whether the newly described G1502 to A1502 mutation in exon 3 of the LH beta-subunit gene, causing the amino acid substitution of Ser102 for Gly102, is related to female infertility. DESIGN: Screening of fertile and infertile women for the G1502 to A1502 mutation in the LH beta-subunit gene. SETTING: Clinics and laboratories of the National University Hospital obstetrics and gynecology department, Singapore. PATIENT(S): Two hundred twelve healthy fertile women; 40 infertile women with menstrual disorders, polycystic ovary syndrome, and endometriosis; and 12 women with idiopathic infertility. INTERVENTION(S): Exon 3 of the LH beta-subunit gene was analyzed using polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP), and PCR-mediated direct DNA sequencing. MAIN OUTCOME MEASURE(S): The PCR products of patients were analyzed by RFLP, and the results were compared with those of fertile controls. DNA sequencing radiographs were compared between two mutation-bearing patients and four controls. RESULT(S): The mutation was identified in only two infertile women with endometriosis; other women studied were found to be negative for this mutation. CONCLUSION(S): The missense mutation in the LH beta-subunit gene may be implicated in female infertility, possibly endometriosis-associated infertility in some women.

Adult↗
PubMed1998

Community-based trauma systems in the United States: an examination of structural development.

OBJECTIVE: To examine the organizational, political, and community characteristics that facilitate or impede community progress in developing a coordinative network of health services for trauma delivery. STUDY SETTING/DESIGN: A comparative case study design was used to examine trauma network development in 6 U.S. cities with a population of 1,000,000 or more. Five key coordinative activities were selected for study. Each study site varied in the set of activities that had been implemented. DATA SOURCES: Information on the structure and composition of local trauma coordinating councils; interviews with a common set of informants in each site using a semi-structured interview protocol. STUDY METHODS: The literature on interorganizational community structures and local policy development was drawn upon to create a conceptual framework for assessing the development of a coordinative service network. Analytical techniques included network analysis to understand the linkages across organizations in overseeing trauma network operations, assessment of leadership structures to identify central actors and organizations, and pattern matching techniques of case study analysis to identify factors that affected trauma network development. PRINCIPAL FINDINGS: Leaders capitalized on local events and were instrumental in keeping network development on the top of the political agenda. Successful leaders spent substantial time and energy documenting problems, assessing the needs and understanding of stakeholders, educating stakeholders and politicians, and creating trust and shared understanding of values. CONCLUSIONS: Prior research has documented the importance of central actors and organizations in developing coordinative networks. The unique contribution of our research is its insights on how central actors and organizations are more likely to motivate collaboration in situations where they lack control over the allocation of payments across involved organizations. Our research suggests that under these circumstances central players should focus their time and energy educating stakeholders and developing a shared understanding rather than using their centrality to impose a particular coordinative structure. To date, U.S. trauma networks have served as models for other industrialized countries, and thus, lessons learned in the U.S. about implementing interorganizational networks of trauma care can assist other countries achieve more effective coordination and avoid mistakes that impede progress.

Community Health Planning↗
PubMed1998

The synaptic SNARE complex is a parallel four-stranded helical bundle.

The heterotrimeric synaptic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, consisting of the synaptic vesicle-associated membrane protein 2 (VAMP2) and presynaptic plasma membrane proteins SNAP-25 (synaptosome-associated protein of 25,000 Mr) and syntaxin 1A, is a critical component of the exocytotic machinery. We have used spin labeling electron paramagnetic resonance spectroscopy to investigate the structural organization of this complex, particularly the two predicted helical domains contributed by SNAP-25. Our results indicate that the N- and C-terminal domains of SNAP-25 are parallel to each other and to the C-terminal domain of syntaxin 1A. Based on these findings, we propose a parallel four-stranded coiled coil model for the structure of the synaptic SNARE complex.

Animals↗
PubMed1998

The occurrence of retinol and carotenoids in human subretinal fluid.

PURPOSE: To investigate if retinol and carotenoids are present in the subretinal space following rhegmatogenous retinal detachment. METHODS: Blood and subretinal fluid were collected from patients at the time of surgical repair of retinal detachment. After removal of cellular contents in a specimen by centrifugation, the supernatant fraction was analyzed by liquid chromatography using a silica column eluted by 16% dioxane in hexane. Retinol and carotenoids were identified in the chromatograms based on their retention time and absorption spectrum. RESULTS: The retinol concentrations (mean+/-SD) in the serum and subretinal fluid were 305+/-144 and 166+/-96 ng/ml respectively. The 450 nm chromatogram had 7 peaks with the characteristic absorption spectrum of carotenoids. Peak 1 and 7 coincided with the retention time of beta-carotene (1.8 min) and lutein (10.8 min) respectively. The concentrations of beta-carotene and lutein in serum were 161+/-63 and 142+/-98 ng/ml respectively. There was very little beta-carotene in subretinal fluid (4.7+/-2.4 ng/ml). Lutein was the major carotenoid peak in subretinal fluid (41.4+/-14.1 ng/ml). The minor carotenoid peaks of serum were not observed in subretinal fluid. CONCLUSION: There is a substantial amount of retinol and lutein in subretinal fluid. The high proportion of lutein and very low amount of beta-carotene in the subretinal fluid support the occurrence of a highly selection transport mechanism of lutein from the blood to the retina.

Adult↗
PubMed1998

Use of a live attenuated varicella vaccine to boost varicella-specific immune responses in seropositive people 55 years of age and older: duration of booster effect.

Varicella-zoster virus (VZV)-specific T cell immunity was measured in 130 persons > or = 55 years of age 6 years after they received a live attenuated VZV vaccine. Circulating T cells, which proliferated in vitro in response to VZV antigen, were enumerated (VZV responder cell frequency assay). Six years after the booster vaccination, the VZV-responding cell frequency (1/61,000 circulating cells) was still significantly (P < .05) improved over the baseline measurements (1/70,000) and appears to have diminished the expected decline in frequency as these vaccinees aged (to 1/86,000). Ten herpes-zoster--like clinical events were recorded. Although the frequency of these events, approximately 1/100 patient-years, is within the expected range of such events for this age cohort, the number of lesions was small, there was very little pain, and there was no postherpetic neuralgia. These results support the development of a vaccine to prevent or attenuate herpes zoster.

Aged↗
PubMed1998

Clock-face drawing, reading and setting tests in the screening of dementia in Chinese elderly adults.

Comprehensive neuropsychological batteries focus on the subtle cognitive deficits in dementia, but a brief screening instrument is also of immense practical value. As the clock-drawing test encompasses a number of cognitive domains frequently disturbed by the dementing process, it is considered to be a suitable screening instrument for the disorder. We documented the usefulness of a new scoring method of the clock-drawing test for screening of dementia in the elderly Chinese in Hong Kong. Fifty-three demented individuals and 53 healthy elderly controls were assessed. At a cutoff score of 3/4, the sensitivity and specificity of the clock-drawing test in screening of dementia was 83% and 79%. With a composite test of clock reading and clock setting, the positive predictive value of the clock face test was 98%. This new scoring method of clock-drawing proved to be a valid measure for screening of dementia. It is applicable in non-English speaking populations and should be a useful adjunct for quick screening assessment of dementia.

Aged↗
PubMed1998

Occlusion of Fontan fenestrations using the Amplatzer septal occluder.

OBJECTIVE: To evaluate the efficacy and safety of the Amplatzer septal occluder device for occlusion of Fontan fenestrations. SUBJECTS: Five children aged 5-10 years who had undergone a fenestrated Fontan operation. SETTING: Tertiary paediatric cardiology centre. METHODS: Each patient had right and left heart catheterisation to assess haemodynamic suitability for fenestration closure. Sizing of the defect was achieved with a balloon wedge catheter and transoesphageal echocardiography. Transcatheter occlusion of the fenestration was accomplished using a 4 mm device in three patients, and 5 mm or 9 mm devices in the other two patients. Residual shunting following occlusion was assessed using angiography and echocardiography. RESULTS: 100% occlusion rate of the fenestration was achieved in all patients. No complications or device failures were seen during the three month follow up period. CONCLUSION: The Amplatzer septal occluder device is safe, and effectively occludes the Fontan fenestration.

Angiography↗
PubMed1998

EGF-Receptor phosphorylation and signaling are targeted by H2O2 redox stress.

Inflammation of the respiratory tract is associated with the production of reactive oxygen species, such as hydrogen peroxide (H2O2) and superoxide (O2-), which contribute extensively to lung injury in diseases of the respiratory tract. The mechanisms and target molecules of these oxidants are mainly unknown but may involve modifications of growth-factor receptors. We have shown that H2O2 induces epidermal growth factor (EGF)-receptor tyrosine phosphorylation in intact cells as well as in membranes of A549 lung epithelial cells. On the whole, total phosphorylation of the EGF receptor induced by H2O2 was lower than that induced by the ligand EGF. Phosphorylation was confined to tyrosine residues and was inhibited by addition of genistein, indicating that it was due to the activation of protein tyrosine kinase (PTK). Phosphoamino acid analysis revealed that although the ligand, EGF, enhanced the phosphorylation of serine, threonine, and tyrosine residues, H2O2 preferentially enhanced tyrosine phosphorylation of the EGF receptor. Serine and threonine phosphorylation did not occur, and the turnover rate of the EGF receptor was slower after H2O2 exposure. Selective H2O2-mediated phosphorylation of tyrosine residues on the EGF receptor was sufficient to activate phosphorylation of an SH2-group-bearing substrate, phospholipase C-gamma (PLC-gamma), but did not increase mitogen-activated protein (MAP) kinase activity. Moreover, H2O2 exposure decreased protein kinase C (PKC)-alpha activity by causing translocation of PKC-alpha from the membrane to the cytoplasm. These studies provide novel insights into the capacity of a reactive oxidant, such as H2O2, to modulate EGF-receptor function and its downstream signaling. The H2O2-induced increase in tyrosine phosphorylation of the EGF receptor, and the receptor's slower rate of turnover and altered downstream phosphorylation signals may represent a mechanism by which EGF-receptor signaling can be modulated during inflammatory processes, thereby affecting cell proliferation and thus having implications in wound repair or tumor formation.

Calcium-Calmodulin-Dependent Protein Kinases↗
PubMed1998

H2O2 acts on cellular membranes to generate ceramide signaling and initiate apoptosis in tracheobronchial epithelial cells.

Hydrogen peroxide (H2O2) is an inflammatory oxidant which contributes to the pathogenesis of chronic diseases such as lung injury of the respiratory tract, atherosclerosis and cancer. The mechanisms and target sites of this reactive oxidant are mainly unknown. So far there are opposing reports as to whether reactive oxidants inhibit or promote apoptosis. We activated the death pathway in primary tracheobronchial epithelial (TBE) cells with H2O2 (20-200 microM) and observed the morphological changes, DNA laddering patterns, and DNA fragmentation associated with apoptosis. Elevation of ceramide with exogenous ceramide analogs was sufficient for apoptosis induction with the same characteristics and in the same time frame. H2O2 induced rapid sphingomyelin hydrolysis to ceramide, the elevation of which paralleled the induction of apoptosis. Furthermore, H2O2 acted directly on TBE cells membrane preparations devoid of nuclei, stimulating sphingomyelin hydrolysis through a neutral Mg2+ dependent sphingomyelinase (SMase). These data suggest that the formation of ceramide from sphingomyelin in the plasma membrane is a key event in H2O2-induced apoptosis in tracheobronchial epithelial cells.

Animals↗
PubMed1998

The clinical impact of platelet glycoprotein IIb/IIIa receptor blockade in cardiovascular medicine.

Several of the adverse events that occur in acute coronary syndromes and after percutaneous coronary revascularization procedures are believed to be mediated by platelets. Recently, using molecular biology techniques, the platelet glycoprotein IIb/IIIa receptor was identified as the final common pathway for platelet aggregation. Thus, blocking the action of this receptor would seem to be an attractive proposition for reducing ischemic complications. A monoclonal antibody was the first agent in this new pharmacological family to be designed, but several peptide and peptide-like substances have subsequently been developed. This paper reviews the development of this class of agents and the various preclinical and clinical trials that have been undertaken. Early studies evaluated such agents during percutaneous coronary revascularization procedures. Because of the overwhelming benefits observed in such patients, together with the current limitations of treatments for acute coronary syndromes, the scope of investigations has been extended. Preliminary reports have been encouraging.

Blood Platelets↗