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Biomedical subjects

C H Tan

Publications and source records attributed to C H Tan.

At least 37 records · Page 2Linked to original sources
PubMed2001

Toward the creation of NMR databases in chiral solvents for assignments of relative and absolute stereochemistry: proof of concept.

[structure: see text] An NMR database approach in a chiral solvent allows us to predict both relative and absolute stereochemistry of an unknown compound without degradation and/or derivatization. N,alpha-Dimethylbenzylamine (DMBA) is a suitable solvent for this purpose. Using the C.5-C.10 portion of oasomycin A, the feasibility and reliability of this approach is demonstrated.

Anti-Bacterial Agents↗
PubMed2001

The synthesis of (+)-allopumiliotoxin 323B'.

This paper describes the synthesis of (+)-allopumiliotoxin 323B' (1) using the intramolecular [3 + 2]-cycloaddition reaction of the (Z)-N-alkenylnitrone 4. This synthesis began with (R)-tert-butyl-3-hydroxy-pent-4-enoate [(R)-13] which was obtained by enzymatic resolution with Amano PS lipase. A series of manipulations gave intermediate 17 and in situ coupling with 4-benzoyloxybutanal lead to the (Z)-N-alkenylnitrone 4 which underwent an intramolecular [3 + 2]-cycloaddition reaction to give the isoxazolidine 3 as the major cycloadduct. Isoxazolidine 3 provided the piperidinone 24 which upon diastereofacial selective addition of MeMgBr gave the required tertiary alcohol 25. Formation of the indolizidine core 2 was achieved by an intramolecular S(N)2 reaction. The side chain was assembled from a Wittig reaction between the phosphorane 8 and the enantiomerically pure aldehyde 9. Further modifications afforded the aldehyde 7 which underwent an aldol condensation with the potassium enolate of the indolizidone core 2. Dehydration gave the enone 37 which was converted into the anti-diol 38 by intramolecular hydride reduction. Finally, deprotection of the BOM protecting group gave (+)-allopumiliotoxin 323B' (1).

Alkaloids↗
PubMed2001

Acoustical diffraction tomography in a finite form and its computer simulations.

Until now, all acoustical diffraction reconstruction algorithms are in infinite forms. All of these algorithms have disadvantages: severe limitations on scatterers or tedious calculations. In this paper, we present a new reconstruction algorithm in a finite form using the method of formal parameter, which is very simple. This new algorithm gives an exact reconstruction when the amplitude of the scattered wave is smaller than that of the incident wave everywhere. This assumption is much less restrictive than that for the first- and second-order Born approximations. Although this new algorithm is in a finite form, it is still an approximate one when the amplitude of the scattered wave is not smaller than that of the incident wave everywhere. However, it still gives a good reconstruction when the amplitude of the scattered wave is a little greater than that of the incident wave in some area. Some numerical examples have confirmed these conclusions.

Journal Article↗
PubMed2001

Ruptured abdominal aortic aneurysms: a personal experience.

Despite advances in surgical technique and peri-operative care, mortality from ruptured abdominal aortic aneurysm presenting to a hospital remains around 50%. This is in contrast to the mortality rate of < 5% for elective repair. In a two and a half year period,the principal surgeon operated on 10 patients with ruptured AAA, with a peri-operative and overall mortality of 30%. One of the ten patients had a ruptured mycotic aneurysm. We present our experience with these patients and also correlate this with recent publications.

Abdominal Pain↗
PubMed2001

Problem-based learning in medical education: the Singapore hybrid.

INTRODUCTION: Problem-based learning (PBL) is an innovative educational strategy to enhance student-centred, interactive and integrated learning through intensive small group tutorials. PBL is problem-first learning in which a commonly encountered medical problem serves as the primary focus of and stimulus for learning. Several adaptations have evolved since its implementation at McMaster University in 1969. In 1999, the Singapore medical school implemented PBL as a key feature within its new hybrid curriculum. METHODS: The review will focus on the educational implications and impact of the present curriculum reform with emphasis on the implementation of PBL within the new hybrid medical curriculum. RESULTS: Our early experience in implementing PBL resembles those of several other "traditional" medical schools, namely, a generally favourable and positive response from staff and students mixed with varying degree of anxiety to change. CONCLUSION: Appropriate training and changing the mindsets of staff and students, strong leadership from the Dean and a deep commitment by all concerned are essential to ensure the successful implementation of PBL within a "traditional" medical school. PBL will enhance the quality of medical education and the educational preparation of our students to meet the challenges of future medical practice.

Education, Medical↗
PubMed2001

Laparoscopic fenestration of a simple hepatic cyst.

Hepatic cysts are usually asymptomatic, most being detected incidentally during imaging. Distinction has to be made between a simple hepatic cyst from more sinister types such as a parasitic cyst and a cyst occurring as part of a neoplastic process. Intervention in a simple hepatic cyst is only indicated when they become symptomatic or when they grow at a rapid rate. Surgery is the mainstay of treatment, with laparoscopic intervention becoming more and more widely accepted. We present here the first case of laparoscopic fenestration of liver cyst performed in our hospital and also review the literature on this modality of treatment.

Cysts↗
PubMed2001

[Determination of trace Sn in geosamples by flow injection on-line co-precipitation hydride generation atomic fluorescence spectrometry].

It is reported that a FI on-line co-precipitation technique for the pre-concentration of trace amount of Sn with the precipitate of Mg(OH)2 as carrier is hyphenated with AFS determination. The co-precipitation is carried out in NaOH medium and the precipitate is collected in a precipitation collector. The precipitate is then dissolved by HCl and reacted with NaBH4, the hydride is transferred directly into the atomizer. The relative standard deviation is 5.15% for 10 determinations of 30 ng.mL-1 Sn and the detection limit of 0.21 ng.g-1 is obtained. The developed method has been applied to the determination of trace Sn in geosamples with satisfactory results.

Chemical Precipitation↗
PubMed2000

Attempted suicide and polymorphism of the serotonin transporter gene in Chinese patients with schizophrenia.

Abnormalities of serotonin synthesis and metabolism may be associated with suicidality. The serotonin transporter gene (5-HTT) is one of the important genes involved in the regulation of serotonin neurotransmission. We examined the association of suicidal behavior in Chinese schizophrenic patients with a functional polymorphism of the promoter region of the 5-HTT gene (5-HTTLPR). The 5-HTTLPR genotype was determined by polymerase chain reaction for 76 suicidal and 262 non-suicidal patients with a diagnosis of schizophrenia (DSM-IV criteria). All subjects were unrelated to each other, and all were Chinese. There was no significant genotypic or allelic association of the 5-HTTLPR polymorphism with history of attempted suicide. From our results, this 5-HTTLPR polymorphism is unlikely to have a major effect on suicidal behavior in Chinese patients with schizophrenia.

Adult↗
PubMed2000

Tardive dyskinesia is not associated with the serotonin gene polymorphism (5-HTTLPR) in Chinese.

Neuroleptics are the mainstay of treatment for schizophrenia, but one of the complications is the development of tardive dyskinesia (TD). The pathophysiology of TD may involve dopamine-serotonin interaction. The serotonin transporter participates in the reuptake and termination of serotonin neurotransmission, and the gene that codes for this protein is thus a candidate gene for the development of TD. There is a functional polymorphism in the transcriptional control region of the serotonin transporter gene, and we investigated the association between this polymorphism and TD in Chinese schizophrenic patients. The patients who did not differ in age and sex distribution did not show variation on the rates of TD and Abnormal Involuntary Movements Scale (AIMS) scores with genotypes. Our findings suggest that 5-HTTLPR polymorphism is not a risk factor for TD in Chinese. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:712-715, 2000.

Adult↗
PubMed2000

Corticosteroid biosynthesis in vitro by testes of the grouper (Epinephelus coioides) after 17alpha-methyltestosterone-induced sex inversion.

This study reports the unique compartmentalization of cortisol and 11-deoxycortisol biosynthesis in vitro from [(3)H]17alpha-hydroxyprogesterone (17P) in testicular tissues of groupers after sex inversion induced by 17alpha-methyltestosterone (MT). Before MT implantation, the ovarian tissues produced only nonpolar metabolites. Following sex inversion some 6 months later, synthesis of these nonpolar metabolites was not detectable. Instead, cortisol and 11-deoxycortisol, with yields of about 3% and 14%, respectively, were synthesized together with two other polar metabolites. The corticosteroids and polar metabolites were distinctly nondetectable in ovarian tissues of the control fish throughout the experiment. While the significance of this testicular synthesis of corticosteroids is presently unclear, it could be related to the increased energy demands arising from the reorganization of gonadal tissues during sex inversion.

Animals↗
PubMed2000

Adenosine modulates cell growth in baby hamster kidney (BHK) cells.

Adenosine is known to modulate cell growth in a variety of mammalian cells either via the activation of receptors or through metabolism. We investigated the effect of adenosine on Baby Hamster Kidney (BHK) cell growth and attempted to determine its mechanism of modulation. In wild-type BHK cells, adenosine evoked a biphasic response in which a low concentration of adenosine (1-5 microM) produced an inhibition of colony formation but at higher concentrations (up to 50 microM) this inhibition was progressively reversed. However, no biphasic response was observed in an "adenosine kinase" deficient BHK mutant, "5a", which suggests that adenosine kinase plays an important role in the modulation of growth response to adenosine. Adenosine receptors did not appear to have a role in regulating cell growth of BHK cells. Specific A1 and A2 receptor antagonists were unable to reverse the effect of adenosine on cell growth. Even though a specific A3 adenosine receptor antagonist MRS-1220 partly reversed the inhibition in colony formation at 1 microM adenosine, it also affected the transport of adenosine. Thus adenosine transport and metabolism appears to play the major role in this modulation of cell growth as 5'-amino-5'-deoxyadenosine, an adenosine kinase inhibitor, reversed the inhibition of cell growth observed at 1 microM adenosine. These results, taken together, would suggest that adenosine modulates cell growth in BHK mainly through its transport and metabolism to adenine nucleotides.

4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone↗
PubMed2000

Effects of Averrhoa bilimbi leaf extract on blood glucose and lipids in streptozotocin-diabetic rats.

The present study was designed to investigate the hypoglycemic and hypolipidemic activities of an ethanolic extract of Averrhoa bilimbi Linn. leaves (Oxalidaceae, Common name: Bilimbi) in streptozotocin (STZ)-diabetic rats. The optimal hypoglycemic dose (125 mg kg(-1)) was determined by performing the oral glucose tolerance test (OGTT) in both normal and STZ-diabetic rats. To investigate the effect of repeated administration of an ethanolic extract of Averrhoa bilimbi (ABe) leaves, diabetic rats were treated with vehicle (distilled water), ABe (125 mg kg(-1)) or metformin (500 mg kg(-1)) twice a day for 2 weeks. Like metformin, ABe significantly lowered blood glucose by 50% and blood triglyceride by 130% when compared with the vehicle. ABe also significantly increased the HDL-cholesterol concentrations by 60% compared with the vehicle. ABe thus significantly increased the anti-atherogenic index and HDL-cholesterol/total cholesterol ratio. However, like metformin, ABe did not affect total cholesterol and LDL-cholesterol concentrations, but significantly reduced the kidney lipid peroxidation level. These data show that ABe has hypoglycemic, hypotriglyceridemic, anti-lipid peroxidative and anti-atherogenic properties in STZ-diabetic rats.

Animals↗
PubMed2000

Severe acute graft-versus-host disease occurring after syngeneic BMT for AML in a patient not given prior cyclosporin A therapy.

A syndrome akin to graft-versus-host disease in the recipient of syngeneic stem cells is hitherto described as being milder, self-limiting and confined to the skin. It is enhanced by prior cyclosporin A therapy. We describe here a recipient of a syngeneic marrow transplant who did not receive priming with cyclosporin A and yet developed severe and progressive graft-versus-host disease which necessitated and responded to high-dose immunosuppressive therapy. We believe that this is because the conditioning regimen in stem cell transplant acts to reset the immune system enabling it to recognise 'self' antigens. Bone Marrow Transplantation (2000) 25, 205-207.

Acute Disease↗
PubMed1999

Evidence for A1 and A2B adenosine receptors in baby hamster kidney (BHK) cells.

Adenosine is known to produce biphasic effects in the renal tissues via adenosine receptors. However, the presence of more than one subtype of adenosine receptor on a type of kidney cell or tissue has not been conclusively demonstrated. To address this issue, we investigated the presence of A1 and A2 adenosine receptors in baby hamster kidney (BHK) cells by use of radioligand binding and the reverse transcription-polymerase chain reaction. Ligand binding studies with (3H)-DPCPX revealed a single class of binding site with a K(D) of 9.2 +/- 2.0 nM, a Bmax of 1.7 +/- 0.2 pmol/mg protein and a pharmacological profile characteristic of A1 adenosine receptor on the BHK cell membrane. As the presence of A2 adenosine receptors could not be conclusively determined by ligand binding studies, the more sensitive method of RT-PCR was employed. The presence of A1 and A2B adenosine receptors was detected by RT-PCR with specific primers and the subsequent sequencing of the resultant amplification product. The sequences obtained were 75-90% homologous to the respective adenosine receptor mRNA of rat, mouse and human.

Adenosine-5'-(N-ethylcarboxamide)↗
PubMed1999

Apple juice inhibits human low density lipoprotein oxidation.

Dietary phenolic compounds, ubiquitous in vegetables and fruits and their juices possess antioxidant activity that may have beneficial effects on human health. The phenolic composition of six commercial apple juices, and of the peel (RP), flesh (RF) and whole fresh Red Delicious apples (RW), was determined by high performance liquid chromatography (HPLC), and total phenols were determined by the Folin-Ciocalteau method. HPLC analysis identified and quantified several classes of phenolic compounds: cinnamates, anthocyanins, flavan-3-ols and flavonols. Phloridzin and hydroxy methyl furfural were also identified. The profile of phenolic compounds varied among the juices. The range of concentrations as a percentage of total phenolic concentration was: hydroxy methyl furfural, 4-30%; phloridzin, 22-36%; cinnamates, 25-36%; anthocyanins, n.d.; flavan-3-ols, 8-27%; flavonols, 2-10%. The phenolic profile of the Red Delicious apple extracts differed from those of the juices. The range of concentrations of phenolic classes in fresh apple extracts was: hydroxy methyl furfural, n.d.; phloridzin, 11-17%; cinnamates, 3-27%; anthocyanins, n.d.-42%; flavan-3-ols, 31-54%; flavonols, 1-10%. The ability of compounds in apple juices and extracts from fresh apple to protect LDL was assessed using an in vitro copper catalyzed human LDL oxidation system. The extent of LDL oxidation was determined as hexanal production using static headspace gas chromatography. The apple juices and extracts, tested at 5 microM gallic acid equivalents (GAE), all inhibited LDL oxidation. The inhibition by the juices ranged from 9 to 34%, and inhibition by RF, RW and RP was 21, 34 and 38%, respectively. Regression analyses revealed no significant correlation between antioxidant activity and either total phenolic concentration or any specific class of phenolics. Although the specific components in the apple juices and extracts that contributed to antioxidant activity have yet to be identified, this study found that both fresh apple and commercial apple juices inhibited copper-catalyzed LDL oxidation. The in vitro antioxidant activity of apples support the inclusion of this fruit and its juice in a healthy human diet.

Antioxidants↗