Biomedical subjects

D Tan

Publications and source records attributed to D Tan.

At least 55 records · Page 3Linked to original sources

PubMed1999

High physiological levels of melatonin in the bile of mammals.

Bile is an important physiological bodily fluid which functions in the regulation of cholesterol metabolism, promotes the absorption of lipid and fat-soluble vitamins by the gut and serves in the excretion of toxic substances from the liver. Conversely, due to autooxidative processes bile is highly toxic to the hepatocyte and gastrointestinal epithelium. In this investigation, extremely high day time physiological levels of the endogenous antioxidant, melatonin, were measured in the bile of several mammals including rat, guinea pig, rabbit, pig, monkey and humans. Melatonin concentrations in the bile samples ranged from 2,000 to 11,000 pg/ml when measured by radioimmunoassay (RIA). These melatonin levels in bile are 2 to 3 orders of magnitude higher than those in day time serum. The presence of melatonin in bile was confirmed by HPLC with an electrochemical detector. This method, like the RIA, also documented very high levels of melatonin in bile. The presence of high levels of melatonin in bile may be essential to prevent oxidative damage to biliary and small intestinal epithelium induced by bile acids and oxidized cholesterol derivatives.

Adult↗

PubMed1999

Augmentation of indices of oxidative damage in life-long melatonin-deficient rats.

The chief pineal secretory product, melatonin, is an efficient free radical scavenger and antioxidant. The current study tested whether the life-long reduction of endogenous melatonin levels due to pinealectomy would influence the accumulation of oxidatively damaged products as the animals aged. Rats were either pinealectomized or sham operated when they were 2-months-old. At 25 months of age these animals were killed along with 2-month-old controls. Aging in the pineal-intact animals was associated with increased levels of lipid peroxidation products (malondialdehyde and 4-hydroxyalkenals in the lung, kidney and skin), rises in an oxidatively damaged DNA product (8-hydroxy-deoxyguanosine in liver, kidney and pancreas), and in the levels of protein carbonyls (in the liver). Likewise, advanced age was associated with a significant decrease in membrane fluidity (increased membrane rigidity) of hepatic microsomes in pineal-intact rats. For all of these parameters and in a number of organs, pinealectomy caused further increases in the indices of oxidative damage. Consistent with previous suggestions, the implications of these findings is that aging is associated with the augmented accumulation of oxidatively damaged macromolecules and that these increases are exaggerated when a relative melatonin deficiency is induced by pinealectomy. The findings are consistent with the idea that the accelerated accumulation of oxidatively damaged products after pinealectomy was due to reduction in melatonin since it functions as a free radical scavenger and antioxidant. On the other hand, other pineal secretory products that were reduced as a consequence of pineal removal may have also been responsible for some of the observed changes.

Aging↗

PubMed1999

Pterygium: prevalence, demography and risk factors.

Pterygium is not just a degenerative disease, but may be a proliferative disorder of the ocular surface. The etiology of pterygia has intrigued researchers for centuries. Several surveys have consistently shown that countries nearer the equator have higher rates of pterygia. A possible reason for this geographic variation is that (ultraviolet) UV B light may be a risk factor for the development of pterygia. UV B radiation may induce cellular changes in the medial limbus of the cornea. Several case-control and cross-sectional studies have attempted to accurately quantify UV light exposure and document its relationship with pterygia. Genetic attributes and other lifestyle behaviors may also contribute to the development of pterygia. However, further research efforts are needed to enable us to better understand the relative contribution of the different risk factors and how each risk factor may be linked to pterygium formation. In addition, the underlying mechanism of the effects of UV radiation needs to be further evaluated. By readdressing these unresolved issues in a newly proposed epidemiological study, new measures might be taken to reduce incidences and improve clinical managements of diseases, in addition to preventing UV exposure by eliminating other contributory factors. Meanwhile, preventive measures such as protection of the eyes by the wearing of sunglasses with UV B protective lenses and brimmed hats outdoors are recommended.

Demography↗

PubMed1999

Analysis of hepatitis G virus (HGV) RNA, antibody to HGV envelope protein, and risk factors for blood donors coinfected with HGV and hepatitis C virus.

Serologic, biochemical, and molecular analyses were used to study hepatitis G virus (HGV), antibody to the HGV envelope protein (anti-E2), risk factors, clinical significance, and the impact of HGV on coexistent hepatitis C virus (HCV). Among 329 donors with confirmed HCV infection, 12% were HGV RNA-positive and 44% were anti-E2-positive (total exposure, 56%). HGV RNA and anti-E2 were mutually exclusive except in 9 donors (1.5%); 8 of 9 subsequently lost HGV RNA but anti-E2 persisted. HGV had little impact on alanine aminotransferase, aspartate aminotransferase, or gamma-glutamyl transpeptidase in donors with HGV infection alone or those coinfected with HCV. A multivariate analysis showed that intravenous drug abuse was the leading risk factor for HGV transmission, followed by blood transfusion, snorting cocaine, imprisonment, and a history of sexually transmitted diseases. In summary, HGV and HCV infections were frequently associated and shared common parenteral risk factors; HGV did not appear to cause hepatitis or to worsen the course of coexistent hepatitis C.

Adult↗

PubMed1999

Conjunctival grafting for ocular surface disease.

The procedure of transplanting healthy conjunctiva onto eyes with damaged ocular surface epithelium was first described by Thoft in 1977. Today, conjunctival autografting is widely used in pterygium surgery and has been shown to be both safe and effective in reducing pterygium recurrence. Several randomized, controlled, clinical trials have been reported in the literature, and conjunctival autografting is now recognized to have an efficacy equal to that of such adjunct therapies as beta-irradiation and mitomycin C, and yet it is safer than these treatments. Recent modifications of the conjunctival autograft include the conjunctival rotation autograft and the inferior conjunctival transposition flap, both of which have been reported to be associated with low rates of recurrence. With the discovery of the role of limbal stem cells in the maintenance and stability of the corneal surface, recent forms of ocular surface transplantation that have supplanted Thoft's conjunctival transplantation procedure include limbal autograft transplantation and limbal allograft transplantation. In addition, the adjunct use of human amniotic membrane in these ocular surface transplantation procedures has improved success and survival rates in the management of ocular surface disease, which still presents significant challenges to clinicians today.

Amnion↗

PubMed1999

Excimer laser photorefractive keratectomy for the correction of myopia.

AIM OF STUDY: To ascertain the efficacy, predictability, safety and stability of photorefractive keratectomy (PRK) for the correction of myopia. METHODS: Nine-hundred and seventy-one eyes of 628 patients had PRK performed for the correction of myopia. The mean pre-operative myopia was -5.7 D +/- 2.21 D (range -1.0 D to -15.25 D). The mean attempted correction was -5.0 D +/- 1.9 D (range -1.0 D to -9.90 D). RESULTS: Four-hundred and sixty-seven eyes had a follow-up of 6 months or more. Ninety-three percent of low myopic eyes (pre-operative myopia of less than or equal to 6.0 D) and 75% of high myopic eyes (pre-operative myopia of more than 6.0 D) attained an unaided visual acuity of 6/12 or better. Seventy-four percent of low myopic and 50% of high myopic eyes were within 1 D of the intended correction. The mean post-operative refraction at 6 months was 0.60 D +/- 1.46 D. Eight eyes in the high myopia group and 1 eye in the low myope group lost 2 or more lines or best-corrected visual acuity. CONCLUSION: PRK was safer and produced better results for low myopes.

Adult↗

PubMed1999

Changing trends in cataract surgery in Singapore.

AIM OF STUDY: To determine the incidence of cataract surgery in Singapore, both island-wide as well as at the Singapore National Eye Centre (SNEC). We also evaluated the changing techniques of cataract surgery performed locally. METHODS: We analysed all cataract operations performed in Singapore over a 10-year period from 1985 to 1996 using data obtained from Medisave claims. We also analysed data of all cataract operations in SNEC between 1992 and 1996 retrospectively, with emphasis on patient demographics and the changing surgical techniques. RESULTS: Data from Medisave claims revealed a significant increase in cataract operations from 5,679 in 1986 to 12,177 in 1995. There was a similar increase in SNEC with a rise from 3,714 in 1992 to 7,516 in 1996. This increase was disproportionately greater than the increase in the number of local surgeons over the same period. The incidence of cataract surgery rose from 2.25/1,000 in 1986 to 4.02/1,000 in 1995. Patients older than 60 years accounted for 75% of cases, with a rise in patients older than 80 years from 2.73% in 1992 to 11.43% in 1996. SNEC data showed that the incidence of phacoemulsification rose from 1% in 1992 to 25% in 1996. CONCLUSION: There is a rising incidence of cataract surgery in Singapore, particularly in the elderly population. There is also a significant change in the pattern of cataract surgery with a sharp rise in the incidence of phacoemulsification over the last 5 years. These findings have an impact on future planning considerations regarding ophthalmic manpower and resources in Singapore.

Adolescent↗

PubMed1999

Diagnosis of tuberculous peritonitis.

The diagnosis of tuberculous peritonitis may be difficult and elusive. The patient may present with non-specific symptoms of fever, general ill-health or vague abdominal pains. There may be no pulmonary symptoms and the chest X-ray may be normal. The CT scan of the abdomen is sometimes helpful in suggesting the diagnosis. We have found that laparoscopic examination of the abdominal contents and the peritoneum is an effective way to obtain a conclusive diagnosis.

Abdominal Pain↗

PubMed1999

[Effects of MMP-1 expressing plasmid on rat liver fibrosis].

OBJECTIVE: To observe the effects of MMP-1(Matrix Metalloproteinase-1) expressing plasmid on rat liver fibrosis. METHODS: We constructed the rat's MMP-1 recombinant plasmid which can be expressed in eucaryotic cells. After encapsulating the MMP-1 recombinant plasmid by lipofectin, we transferred it to the rat's fibrotic liver in vivo intraperitoneally, then observed the effects of the plasmid on the fibrotic liver by RT-PCR, immunohistochemistry and the observation of pathology. RESULTS: The recombinant plasmid of MMP-1 could increase the degradation of type I and type III collagen in the rat's fibrotic liver according to immunohistochemistry compared with fibrotic modal group(P < 0.01) or compared with colchicine group(P < 0.05). According to the observation of pathology, recombinant plasmid of MMP-1 has some reverse effects on the rat's fibrotic liver compared with fibrotic model group(P < 0.05), but no obvious difference was found between the plasmid and colchicine(P > 0.05). CONCLUSION: The recombinant plasmid of MMP-1 has some reverse effects on liver fibrosis, but the effects is limited.

Animals↗

PubMed1999

[PCR for the detection of 16S rRNA gene bacteria].

According to the high conservative region of 16S rRNA gene in bacteria, PCR primers of the broad-range bacteria, gram-positive bacteria and gram-negative bacteria were synthesized to detect 13 bacterium species and (40) clinical specimens. All the tested bacterium species were positive. The lowest concentration of Escherichia Coli detected by serial dilution was 4 CFU.ml-1. The positive rate of PCR (27/40) was higher than that of bacterium culture(18/40). The results indicate that these PCR primers possess high specificity and sensitivity in identifying 16S rRNA gene of bacteria.

Bacteriuria↗

PubMed1999

[Study on cytotoxicity to leukemia cell and enhancement of immunologic functions of jiexinkang].

Cytotoxicity to leukemia cell and enhancement of immunologic functions of jiexinkang (JXK, traditional Chinese medicine) were observed in 34 cases of leukemia in vitro. The results showed that the leukemia cytotoxicity in the group of vincristine combined with JXK was significantly higher than that in vincristine alone. It was also found among all the observed cases that 20 refractory or recurred patients resistant to vincristine showed an obvious intention of the anti-toxicity efficiency when JXK was added. Jiexinkang alone had anti-leukemia cell effect, too. We explored JXK effects on immunologic functions in mice, and found that IFNr and NK activities of normal and experimental model mice were significantly higher than those of control group(P < 0.01). The results indicate that JXK has an effect of anti-leukemia cell and may improve the cell-immunologic functions.

Adjuvants, Immunologic↗

PubMed1999

[Specific cellular and humoral immune responses induced by intramuscular injection of DNA vaccine containing HBV HBsAg gene in mice].

To know the influences on the function of cellular and humoral immunity in mice by DNA vaccine containing HBV HBsAg gene, the DNA vaccine, pcDNA3-HBsAg, in which the HBsAg gene sequences of HBV were inserted into downstream of CMV promoter in pcDNA3, were injected intramuscularly into Balb/C mice. The results were that the serum anti-HBsIgG was detected in all of the pcDNA3-HBsAg immunized mice, but none in control mice; serum levels of Th1 cytokines, interleukine 2, and interferon gamma, were significantly higher in pcDNA3-HBsAg immunized mice than these in controls. It is indicated that the good responses of cellular and humoral immunity can be induced by injection of pcDNA3-HBsAg in Balb/C mice.

Animals↗

PubMed1999

[Experimental and clinical study to remove lead and to prevent lead damage of kidney with salt of high dose KI].

OBJECTIVE: To study and make a salt prevent lead poisoning. METHODS: SD rats were given 0.25% lead acetate by food intake and were given 10 mg.kg-1.d-1, 20 mg.kg-1.d-1 and 30 mg.kg-1.d-1 potassium iodide from drinking water in respective groups, in order to look for the dose of KI which prevents lead poisoning. The salt to prevent lead poisoning was made, 50 workers were given the salt as to flavouring of daily food intake. PbB, Pbu, ZPP, delta-ALA, NAG(N-acetyl-beta-D-glucosaminidase), RP(Retinol protein) were observed. RESULTS: The dose of KI to prevent experimental lead poisoning of SD rats was 10 mg.kg-1.d-1 by oral intake. People who contacted lead had an increase of urine lead in the third day, and had most urine lead on ninth day after taking the salt with KI. Normal urine lead on the fourth week. Pre- and post-oral intake of the salt with high dose KI, Hb, ZPP, delta-ALA, PbB, NAG and Rp the people mentioned above had the following indexes, (101.2 and 128) g.L-1 (P = 0.05), (33.5 and 7.7) micrograms.g-1 Hb(P < 0.01), (123.12 and 65.35) mumol.L-1 (P < 0.05), (2.123 and 0.85) mumol.L-1 (P < 0.01), (20.9 and 6.7) U.L-1 (P < 0.01), (1.5 and 0.34) mg.L-1 (P < 0.05) respectively. CONCLUSION: The salt with 1% KI may remove lead and prevent lead damage of the kidney.

Adolescent↗

PubMed1998

Role of arginine 439 in substrate binding of 5-aminolevulinate synthase.

5-Aminolevulinate synthase (EC 2.3.1.37) catalyzes the first reaction in the heme biosynthetic pathway in nonplant eukaryotes and some prokaryotes. Homology sequence modeling between 5-aminolevulinate synthase and some other alpha-family pyridoxal 5'-phosphate-dependent enzymes indicated that the residue corresponding to the Arg-439 of murine erythroid 5-aminolevulinate synthase is a conserved residue in this family of pyridoxal 5'-phosphate-dependent enzymes. Further, this conserved arginine residue in several enzymes, e.g., aspartate aminotransferase, for which the three-dimensional structure is known, has been shown to interact with the substrate carboxyl group. To test whether Arg-439 is involved in substrate binding in murine erythroid 5-aminolevulinate synthase, Arg-439 and Arg-433 of murine erythroid 5-aminolevulinate synthase were each replaced by Lys and Leu using site-directed mutagenesis. The R439K mutant retained 77% of the wild-type activity; its K(m) values for both substrates increased 9-13-fold, while the activity of R433K increased 2-fold and the K(m) values for both substrates remained unchanged. R439L had no measurable activity as determined using a standard 5-aminolevulinate synthase enzyme-coupled activity assay. In contrast, the kinetic parameters for R433L were comparable to those of the wild-type. Dissociation constants (Kd) for glycine increased 5-fold for R439K and at least 30-fold for R439L, while Kd values for glycine for both R433K and R433L mutants were similar to those of the wild-type. However, there was not much difference in methylamine binding among the mutants and the wild-type, excepting of a 10-fold increase in K(d)methylamine for R439L. R439K proved much less thermostable than the wild-type enzyme, with the thermotransition temperature, T1/2, determined to be 8.3 degrees C lower than that of the wild-type enzyme. In addition, in vivo complementation analysis demonstrated that in the active site of murine erythroid 5-aminolevulinate synthase, R439 is contributed from the same subunit as K313 (which is involved in the Schiff base linkage of the pyridoxal 5'-phosphate cofactor) and D279 (which interacts electrostatically with the ring nitrogen of the cofactor), while another subunit provides R149. Taken together, these findings suggest that Arg-439 plays an important role in substrate binding of murine erythroid 5-aminolevulinate synthase.

5-Aminolevulinate Synthetase↗

PubMed1998

The role of tyrosine 121 in cofactor binding of 5-aminolevulinate synthase.

5-Aminolevulinate synthase (EC 2.3.1.37) is the first enzyme in the heme biosynthesis in nonplant eukaryotes and some prokaryotes. It functions as a homodimer and requires pyridoxal 5'-phosphate as an essential cofactor. Tyr-121 is a conserved residue in all known sequences of 5-aminolevulinate synthases. Further, it corresponds to Tyr-70 of Escherichia coli aspartate aminotransferase, which has been shown to interact with the cofactor and prevent the dissociation of the cofactor from the enzyme. To test whether Tyr-121 is involved in cofactor binding in murine erythroid 5-aminolevulinate synthase, Tyr-121 of murine erythroid 5-aminolevulinate synthase was substituted by Phe and His using site-directed mutagenesis. The Y121F mutant retained 36% of the wild-type activity and the Km value for substrate glycine increased 34-fold, while the activity of the Y121H mutant decreased to 5% of the wild-type activity and the Km value for glycine increased fivefold. The pKa1 values in the pH-activity profiles of the wild-type and mutant enzymes were 6.41, 6.54, and 6.65 for wild-type, Y121F, and Y121H, respectively. The UV-visible and CD spectra of Y121F and Y121H mutants were similar to those of the wild-type with the exception of an absorption maximum shift (420 --> 395 nm) for the Y121F mutant in the visible spectrum region, suggesting that the cofactor binds the Y121F mutant enzyme in a more unrestrained manner. Y121F and Y121H mutant enzymes also exhibited lower affinity than the wild-type for the cofactor, reflected in the Kd values for pyridoxal 5'-phosphate (26.5, 6.75, and 1.78 microM for Y121F, Y121H, and the wild-type, respectively). Further, Y121F and Y121H proved less thermostable than the wild type. Taken together, these findings indicate that Tyr-121 plays a critical role in cofactor binding of murine erythroid 5-aminolevulinate synthase.

5-Aminolevulinate Synthetase↗

PubMed1998

Outdoor work and the risk of pterygia: a case-control study.

AIM: To investigate the relationship between outdoor work and pterygium formation in a hospital-based case-control study in Singapore. METHOD: Sixty-one patients with pterygium (49 males, mean age 54.2 years) and 125 controls (41 males, mean age 50.2 years), excluding patients with cataract and age-related macular degeneration presenting at outpatient clinics of the Singapore National Eye Centre were interviewed face-to-face with a standard questionnaire. Demographic data and information on outdoor work and eye protection, were collected. RESULTS: Thirty-eight subjects (20.4%) were outdoor workers and 148 (79.6%) were indoor workers. Outdoor workers were more likely to be male smokers and alcohol drinkers with lower family income and higher sunlight exposure than indoor workers. The crude odds ratio for pterygium in outdoor workers was 7.0 (95% confidence interval 3.2, 15.3). The adjusted odds ratio was 4.2 (95% confidence interval 1.7, 10.1). CONCLUSION: There is a statistically significant association between outdoor work and pterygium formation, which may be related to the higher exposure to sunlight and dust in outdoor workers. Efforts may be made to educate outdoor workers to wear proper equipment to protect their eyes against excessive sunlight.

Adult↗

PubMed1998

Hospitalization for pneumonia in children in Auckland, New Zealand.

OBJECTIVE: To describe the epidemiology of hospitalizations for pneumonia in children in Auckland, New Zealand. METHODS: A consecutive sample of children hospitalised with pneumonia at the Starship Childrens Hospital from 1 July 1993 to 30 June 1996. Subjects were Pacific Island, Maori, and European/other children aged 0-14 years resident in north, west and central Auckland who were hospitalized with pneumonia. Comparisons were made of the number of hospitalisations by year, ethnicity, age and season; and of the hospitalisation rates by year, ethnicity and age. RESULTS: There were 681 children who were hospitalized with pneumonia during 1993-94, 731 during 1994-95 and 630 during 1995-96. The average annual hospitalization rate was 5.0 per 1000 children aged 0-14 years (95% CI 4.8-5.2). The average annual hospitalisation rate for Pacific Island children was 14.0 per 1000 (95% CI 13.0-14.9), for Maori children 6.7 per 1000 (95% CI 6.0-7.4) and for European/other children was 2.7 per 1000 (95% CI 2.6-2.9). Fifty-three per cent of the hospitalised children were less than 2 years of age. A larger percentage of Pacific Island (61%) and Maori (60%) children were aged less than 2 years compared to European/other (42%) children (P < 0.001). There was marked seasonal variability in the number of hospitalizations, with peaks in hospitalizations corresponding to peaks in positive respiratory viral isolates. CONCLUSIONS: Pneumonia was a consistent cause of hospitalisation for a large number of Auckland children during this 3-year period. Hospitalisation rates and age distribution varied with ethnicity. Hospitalization rates were highest for Pacific Island. intermediate for Maori and lowest for European/other children. Based on these hospitalisation data, pneumonia is a significant cause of morbidity for children in Auckland, New Zealand.

Adolescent↗

PubMed1998

Role of nitric oxide-related mechanisms in renal function in ageing rats.

BACKGROUND: The impaired renal function and vasodilatation that accompany age need to be re-addressed based upon the new knowledge concerning vascular nitric oxide (NO)-dependent systems. The present study examined the effects of age on the NO-related renal response. METHODS: The study was performed in euvolaemic, conscious Wistar rats, aged 5 and 18 months. Renal function and haemodynamic measurements with fluorescent microspheres were employed to assess differences between groups. RESULTS: A first set of experiments showed that ageing rats had a reduced natriuretic and diuretic response to acetylcholine, whereas the response to sodium nitroprusside was preserved. In the same regard, a reduction of the renal functional effects of L-arginine (L-Arg) and L-glycine (L-Gly) was found in the older rats. In the ageing rats, these responses were accompanied by an enhanced effect of the L-Arg competitive analogue, NwNLA, which provoked a marked reduction of renal function. This effect of NwNLA was blocked by the simultaneous administration of a small dose of L-Arg in the ageing but not in the young rats. Systemic haemodynamic studies revealed that in ageing rats, NwNLA reduced renal blood flow and increased renal vascular resistances in a significantly higher proportion than in younger animals. However, flow to other organs, namely, brain, spleen or liver, was affected in a similar manner in both young and old rats. Ultrastructural alterations were found in endothelial cells, which might constitute the anatomical basis for the observed functional derangements. CONCLUSIONS: The present experiments reveal that ageing is accompanied by significant differences in NO-related responses in the kidney which do not appear to affect blood flow to other organs. The response to L-Arg and L-Arg competitive analogues supports the existence of a marked dependency on NO-related mechanisms in the ageing rats, but not of a decreased baseline activity of the NO-dependent pathways.

Acetylcholine↗