Yuen, Shiu Yat Raymond
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BIOCHEMISTRY
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YONG LOO LIN SCH OF MEDICINE
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Publication Plasma urokinase activity and antigen levels in normal males, females and in late pregnancy(1992) Koh, S.C.L.; Yuen, R.; Viegas, O.A.C.; Ratnam, S.S.; BIOCHEMISTRY; OBSTETRICS & GYNAECOLOGYPublication A plasmin generation method for the determination of tissue plasminogen activator activity in tissues(1985) Koh, S.C.L.; Tsakok, F.H.M.; Yuen, R.; BIOCHEMISTRYPlasminogen activators are widely distributed in the body; they enzymatically convert the proenzyme plasminogen into plasmin. Plasminogen activators have been isolated either as urokinase-like or tissue type substances. Plasmin levels can serve as an index of fibrinolysis. Several methods have been used to quantitate plasminogen activator activity, including the fibrin plate method, euglobulin clot lysis time, immunoassays and, more recently, the use of a plasmin sensitive synthetic chromogenic substrate S-2251, a tripeptide H-D-Val-Leu-Lys-pNA. The present report describes a plasmin generation method for determining the tissue plasminogen activator (t-PA) activity of placental and uterine tissues. t-PA activity was also determined on the same treated tissue extracts by the fibrin plate method.Publication Plasminogen activators in the endometrium; a preliminary study in subfertile subjects(1987) Koh, S.C.L.; Wong, P.C.; Yuen, R.; BIOCHEMISTRY; OBSTETRICS & GYNAECOLOGYLow or absent fibrinolytic activity in the mid-cycle endometrium is suggested as a prerequisite for the adhesion of fertilized ova. Plasminogen activator (PA) activity was evaluated in acid-treated endometrial curettings of subfertile females with 12 cases each in the proliferative and late secretory and 16 cases in the early secretory phase. PA activity varied widely with a mean of 70 ± 67.3 iu in the proliferative, 50.8 ± 38.5 iu in the late secretory and 25.9 ± 29.3 urokinase iu/0.5g tissue in the early secretory endometrium. Compared with proliferative endometrium, early secretory endometrium showed significantly lower activity (p < 0.01); the reduction in late secretory endometrium was not statistically significant. Unlike the normal patient, 12 out of 16 subfertile patients showed PA activity in the early secretory endometrium (4 to 85 urokinase iu/0.5g tissue). The presence of PA activity in early secretory endometrium may therefore be a possible factor related to the patients' continued infertility.Publication A major lethal factor of the venom of Burmese Russel's viper (Daboia russelli siamensis); Isolation, N-terminal sequencing and biological activities of daboiatoxin(1995) Maung-Maung-Thwin; Gopalakrishnakone, P.; Yuen, R.; Tan, C.H.; ANATOMY; BIOCHEMISTRYA major lethal factor, daboiatoxin (DbTx), showing strong PLA2 activity (specific activity 91.7 nmoles/min/mg), was purified to homogeneity from the venom of Burmese Russell's viper (Daboia r. siamensis) by a combination of gel filtration on Sephadex G-75 and ion-exchange chromatography on CM-Sephadex C-25, followed by purification on high-performance gel filtration Shim-pack Diol-150 column. DbTx is a single-chain PLA2 toxin with approximate mol. wt 15,000 as determined by HPLC gel filtration and SDS-PAGE. It constitutes 12% of total venom-protein and is the main lethal component of Burmese Russell's viper venom with an LD50 i.p. (0.05 mg/kg) 12-fold greater than that of the whole venom (LD50 i.p. 0.6 mg/kg). DbTx produces neurotoxic symptoms in mice and exhibits potent oedema-inducing activity (minimum oedema dose 0.05 μg), indirect haemolytic activity and a strong myonecrotic activity, but no haemorrhagic activity. DbTx is cytotoxic to HeLa cells causing cytolysis of the cells 24 hr post-exposure to toxin (50 μg/ml). The first 20 N-terminal amino acid sequence (NFFQF AEMIV KMTGK EAVHS) shows a significant resemblance to those of the PLA2s from the venoms of Bulgarian viper (V. a. ammodytes) and Taiwan Russell's viper (V. r. formosensis).Publication Plasminogen activators t-PA, u-PA and its inhibitor (PAI) in normal males and females(1991) Koh, S.C.L.; Yuen, R.; Viegas, O.A.C.; Chua, S.E.; Ng, B.L.; Sen, D.K.; Ratnam, S.S.; BIOCHEMISTRY; OBSTETRICS & GYNAECOLOGYWe determined the plasma levels of tissue plasminogen activator (t-PA), plasminogen activator inhibitor (PAI) activity and their antigen levels including urokinase plasminogen activator (u-PA) in 33 male and 27 female normal subjects. Males had mean t-PA activity of 0.50 iu/ml which was significantly lower (p < 0.01) than the females 0.64 iu/ml. Males had higher (p < 0.001) mean PAI activity (15.5 AU/ml) as compared to females 10.3 AU/ml. The respective mean levels of t-PA and PAI antigen were significantly higher (p < 0.01) in males (8.1 ng/ml and 17.6 ng/ml) than in females (6.2 ng/ml and 12.1 ng/ml). The mean u-PA level in males was 1.54 ng/ml which was significantly higher (p < 0.01) than in females with 1.02 ng/ml. In post-venous occlusion studies, females had a greater mean response of 8.6 fold in t-PA activity as compared to males with a mean of 4.5 fold increase. The mean t-PA antigen response in males was 2.0 fold increase as compared to 2.6 fold increase in the females. No significant responses were seen in both sexes in either PAI activity or antigen levels when compared with the resting state. In zymography studies, free t-PA, its inhibitor complexes and u-PA were demonstrated in the euglobulin fractions of stored plasma. This study demonstrates that significant differences in t-PA, u-PA and PAI exist between male and female subjects which should be taken into account when determining their levels in clinical conditions.Publication Chemical composition of biliary calculi in relation to the pattern of biliary disease in Singapore(1985) Ti, T.K.; Yuen, R.; BIOCHEMISTRYThe chemical composition of biliary calculi from 25 Singapore patients operated upon over a three month period was determined by a microanalytical technique and equilibrium swelling tests. Twelve patients had biliary calculi confined to the gallbladder and the main component of six calculi was cholesterol. In the other 6 patients, the dark brown to black pigment calculi contained bilirubin and calcium and a small quantity of cholesterol, but the chief component was an insoluble pigment with the swelling characteristics of a bilirubin polymer. These cholesterol and pigment calculi have similarities in composition to gallstones of Western patients. In 13 patients with cholangitis, the brown, soft pigment calculi found in the dilated bile ducts (and also concurrently in the gallbladder in 5 patients) had bilirubin as the main component. Fatty acid was frequently present. The insoluble pigment component was considerably less but the lower swelling ratio suggested more cross linkage compared with black gallbladder stones. Differences in chemical composition between pigment gallbladder and bile duct calculi support the concept that Western type cholelithiasis and Oriental cholangitis occur as separate disease entities in Singapore patients.Publication Plasminogen activators in the endometrium; a preliminary study in subfertile subjects(1987) Koh, S.C.L.; Wong, P.C.; Yuen, R.; BIOCHEMISTRY; OBSTETRICS & GYNAECOLOGYLow or absent fibrinolytic activity in the mid-cycle endometrium is suggested as a prerequisite for the adhesion of fertilized ova. Plasminogen activator (PA) activity was evaluated in acid-treated endometrial curettings of subfertile females with 12 cases each in the proliferative and late secretory and 16 cases in the early secretory phase. PA activity varied widely with a mean of 70 ± 67.3 iu in the proliferative, 50.8 ± 38.5 iu in the late secretory and 25.9 ± 29.3 urokinase iu/0.5g tissue in the early secretory endometrium. Compared with proliferative endometrium, early secretory endometrium showed significantly lower activity (p < 0.01); the reduction in late secretory endometrium was not statistically significant. Unlike the normal patient, 12 out of 16 subfertile patients showed PA activity in the early secretory endometrium (4 to 85 urokinase iu/0.5g tissue). The presence of PA activity in early secretory endometrium may therefore be a possible factor related to the patients' continued infertility.Publication Purification and partial characterization of stonustoxin (lethal factor) from Synanceja horrida venom(1991) Poh, C.H.; Yuen, R.; Khoo, H.E.; Chung, M.; Gwee, M.; Gopalakrishnakone, P.; PHARMACOLOGY; ANATOMY; BIOCHEMISTRY1. The lethal factor of the stonefish (Synanceja horrida) venom, designated as the stonustoxin, was purified to homogeneity by a two-step procedure on Sephacryl S-200 High Resolution (HR) gel permeation and DEAE Bio-Gel A anion exchange chromatography. 2. Stonustoxin has a native mol. wt of 148,000 and an isoelectric point of 6.9. 3. SDS-polyacrylamide gel electrophoresis revealed two subunits (designated α and β) with mol. wts of 71,000 and 79,000, respectively. 4. The amino acid composition of both subunits and the N-terminal amino acid sequence of the β subunit were also determined. 5. Purified stonustoxin had an LD50 of 0.017 μg/g which is 22-fold more potent than that of the crude venom. 6. The toxin exhibited potent haemolytic activity in vitro and endema-inducing activity with a minimum edema dose (MED) of 0.15 μg in mouse paw. The edema effect was not antagonized by diphenhydramine.Publication Stonustoxin: A highly potent endothelium-dependent vasorelaxant in the rat(1993) Low, K.S.Y.; Gwee, M.C.E.; Yuen, R.; Gopalakrishnakone, P.; PHARMACOLOGYStonefish venom has been documented to cause marked hypotension and respiratory difficulties in evenomed animals. Stonustoxin, a lethal protein recently isolated from the venom of the stonefish Synanceja horrida produced hypotension and, at concentrations above 20 μg/kg, death in anaesthetized rats, with no observable effects on nerve-evoked twitches of the tibialis and diaphragm muscles. Stonustoxin (20-160 ng/ml) induced endothelium-dependent relaxations of rat thoracic aortae precontracted with noradrenaline. Higher concentrations induced relaxations followed by contractions. Methylene blue, haemoglobin and the specific NO-synthase inhibitor L-N(G)-nitro arginine methyl ester inhibited stonustoxin-induced relaxations, while the cyclooxygenase inhibitor indomethacin was without effect. The results of the present study show that stonustoxin causes marked vasorelaxation of the rat isolated aorta, which appearss to be due to the release of endothelium-derived relaxing factor (probably nitric oxide or nitric oxide-yielding substances) from the vascular endothelium, and this may be responsible for the in vivo hypotensive and lethal actions of stonustoxin and of stonefish venom.Publication Plasminogen activators and inhibitors in normal late pregnancy, postpartum and in the postnatal period(1992) Koh, C.L.S.; Viegas, O.A.C.; Yuen, R.; Chua, S.E.; Ng, B.L.; Ratnam, S.S.; OBSTETRICS & GYNAECOLOGYTissue plasminogen activator ( t-PA), urokinase plasminogen activator (u-PA) and plasminogen activator inhibitors (PAI) are elevated in late pregnancy with t-PA and u-PA remaining so at 6 weeks postnatal. PAI-2 remains at postpartum but was absent by 6 weeks postnatal unlike PAI activity which was absent at postpartum and returned to nonpregnant level at postnatal. The potential fibrinolytic response to stress is much reduced in pregnancy thus increasing the risk of thromboembolism.