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P S Low

Publications and source records attributed to P S Low.

At least 19 recordsLinked to original sources
PubMed2006

The effects of three factor VII polymorphisms on factor VII coagulant levels in healthy Singaporean Chinese, Malay and Indian newborns.

Factor VII (FVII) is an independent risk factor for coronary artery disease. Three polymorphisms of the factor VII gene (F7) were studied in a group of healthy newborns comprising 561 Chinese, 398 Malays and 226 Asian Indians from Singapore. The allele frequencies of 3 polymorphisms (R353Q, Promoter 0/10bp Del/Ins and Intron 7) in the FVII gene were ascertained through genotyping by polymerase chain reaction and restriction digestion of amplified fragments. In Chinese the minor allele frequencies are Q: 0.04, Ins: 0.03, R7: 0.44; Malays, Q: 0.06, Ins: 0.10, R7: 0.41; and Indians, Q: 0.25, Ins: 0.23, R7: 0.43. Strong linkage disequilibrium (Delta > 0.7) is observed between the 0/10 bp and the R353Q sites in all ethnic groups. We conclude that: (i) the prevalence of the minor Q and Ins alleles of the R353Q and 0/10 bp polymorphisms are significantly higher in the Indian newborns than the Chinese and Malays; (ii) the Q allele is significantly associated (p = 0.01) with a lower plasma FVII coagulant level in the Indian and Malay neonates; and this polymorphism explains up to 3.8% of the variance in FVII coagulant levels; (iii) there is no significant difference in allele frequencies of the three polymorphisms between neonates with and without family histories of CAD.

Antigens↗
PubMed2004

MELAS: a case report.

INTRODUCTION: Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is a rare, neurodegenerative and fatal disease caused by mutations in the mitochondrial DNA. Multiple systems of the body, including the oral cavity, can be affected by this disease. An electronic search of Medline spanning the years 1985 to 2003 was carried out using the key words "MELAS, Dentistry." It yielded no literature on the dental aspects of MELAS. CLINICAL PICTURE: This report documents the case of a 6-year-old Chinese boy diagnosed with MELAS and highlights problems encountered in the multidisciplinary management of MELAS patients, including its dental management. TREATMENT AND OUTCOME: Dental management was successfully performed under general anaesthesia with close medical supervision by paediatrician and anaesthetist. CONCLUSIONS: There is no known treatment of the underlying disease and the clinical course is usually unpredictable. Preventive dental care is important in this group of patients as concurrent medical conditions can complicate dental care.

Child↗
PubMed2002

Synthesis and evaluation of taxol-folic acid conjugates as targeted antineoplastics.

A series of Taxol derivatives tethered at C2' and C-7 to glutamate and folate have been synthesized for evaluation as prodrugs which release Taxol via hydrolytic lability of their alpha-alkoxy and alpha-amino esters. The half-time for hydrolysis of these materials was determined in pH 7 and pH 5 buffer. The in vitro cytotoxicity has been assessed in cell culture against A-549 lung cancer, MCF-7 breast cancer, and HT-29 colon cancer. Selected agents were further screened for folate binding and competitive binding with free folic acid. One agent (54), further evaluated in animal studies was found to increase the lifespan in mice, but was less effective than Taxol itself.

Antineoplastic Agents↗
PubMed2002

Genotype associations of factor VII gene with plasma factor VII coagulant activity and antigen levels in healthy Chinese.

A raised plasma factor VII (FVII) level is one of the risk factors for coronary artery disease. The R353Q polymorphism at codon 353 and the 10 base pair (bp) insertion (0/10 bp) polymorphism of the FVII gene have been reported to be associated with plasma FVII levels in several populations. We investigated these two polymorphisms in 209 male and 214 female healthy Chinese. The allele frequencies of 10 bp and Q were 0.036 and 0.045, respectively. Strong linkage disequilibrium was observed between these two sites (Delta = 0.85, P < 0.001). There were significant genotype associations of these two loci with FVII coagulant activity (FVIIc) and antigen (FVIIAg) levels. Heterozygous individuals had lower FVIIc and FVIIAg levels than those homozygous for the common alleles. When analyzed separately by gender, the 0/10 bp polymorphism was strongly associated with FVIIAg levels in males and females. However, both polymorphisms were significantly associated with FVIIc levels only in the females. The effect of 0/10 bp polymorphism predominated over that of the R353Q polymorphism in a two-way analysis of variance procedure. In the Chinese, the 10 bp insertion may reduce transcription of the FVII gene, leading to the decreased synthesis of FVII protein and thus FVIIc.

Adult↗
PubMed2001

Effect of pH on the self-association of erythrocyte band 3 in situ.

The human erythrocyte anion exchanger (band 3) contains a cytoplasmic domain (cdb3) that exists in a reversible, pH-dependent structural equilibrium among three native conformations. To understand how this conformational equilibrium might influence the association state of band 3, we have incubated stripped erythrocyte membranes in solutions ranging from pH 6.0 to pH 10.5 and have examined the oligomeric state of the protein by size exclusion high performance liquid chromatography. We demonstrate that incubation of membranes in slightly acidic conditions favors dimer formation, whereas extended incubation at higher pHs (pH>9) leads to irreversible formation of an oligomeric species larger than the tetramer. Since the pH dependence of the conformational state of the cytoplasmic domain exhibits a similar pH profile, we suggest that the conformation of the cytoplasmic domain can modulate the self-association of band 3. Importantly, this modulation would appear to require the structural interactions present within the intact protein, since the isolated membrane-spanning domain does not display any pH dependence of association. The irreversible nature of the alkali-induced aggregation further suggests that a secondary reaction subsequent to band 3 association is required to stabilize the high molecular weight aggregate. Although we were able to eliminate covalent bond formation in this irreversible aggregation process, the exact nature of the secondary reaction remains to be elucidated.

Anion Exchange Protein 1, Erythrocyte↗
PubMed2001

Analysis of integral membrane protein contributions to the deformability and stability of the human erythrocyte membrane.

Three major hypotheses have been proposed to explain the role of membrane-spanning proteins in establishing/maintaining membrane stability. These hypotheses ascribe the essential contribution of integral membrane proteins to (i) their ability to anchor the membrane skeleton to the lipid bilayer, (ii) their capacity to bind and stabilize membrane lipids, and (iii) their ability to influence and regulate local membrane curvature. In an effort to test these hypotheses in greater detail, we have modified both the membrane skeletal and lipid binding interactions of band 3 (the major membrane-spanning and skeletal binding protein of the human erythrocyte membrane) and have examined the impact of these modifications on erythrocyte membrane morphology, deformability, and stability. The desired changes in membrane skeletal and protein-lipid interactions were induced by 1) reaction of the cells with 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), an inhibitor of band 3-mediated anion transport that dissociates band 3 into dimers (increasing its surface area in contact with lipid) and severs band 3 linkages to the membrane skeleton; 2) a fragment of ankyrin that ruptures the same ankyrin-band 3 bridge to the membrane skeleton, but drives the band 3 subunit equilibrium toward the tetramer (i.e. decreasing the band 3 surface area in contact with lipid); and 3) an antibody to the ankyrin-binding site on band 3 that promotes the same changes in band 3 skeletal and lipid interactions as the ankyrin fragment. We observed that although DIDS induced echinocytic morphological changes in the treated erythrocytes, it had little impact on either membrane deformability or stability. In contrast, resealing of either the ankyrin fragment or anti-band 3 IgG into erythrocytes caused spontaneous membrane fragmentation and loss of deformability/stability. Because these and other new observations cannot all be reconciled with any single hypothesis on membrane stability, we suggest that more than one hypothesis may be operative and provide an explanation of how each might individually contribute to net membrane stability.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed2001

Characterization of the reversible conformational equilibrium in the cytoplasmic domain of human erythrocyte membrane band 3.

The cytoplasmic domain of erythrocyte membrane band 3 (cdb3) serves as a center of membrane organization, interacting with such proteins as ankyrin, protein 4.1, protein 4.2, hemoglobin, several glycolytic enzymes, and a tyrosine kinase, p72syk. cdb3 exists in a reversible, pH-dependent conformational equilibrium characterized by large changes in Stokes radius (11 A) and intrinsic fluorescence (2-fold). Based on the crystallographic structure of the cdb3 dimer, we hypothesized that the above conformational equilibrium might involve the movement of flanking peripheral protein binding domains away from a shared dimerization domain. To test this hypothesis, we have mutated both donor (W105L) and acceptor (D316A) residues of a prominent H bond that bridges the above two domains and have examined the effect on the resulting conformational equilibrium. Analysis of the intrinsic fluorescence, Stokes radius, thermal stability, urea stability, and segmental mobility of these mutants reveals that the above H bond is indeed present in the low pH conformation of cdb3 and broken in a higher pH conformation. The data further reveal that cdb3 exists in three native pH-dependent conformations and that rupture of the aforementioned H bond occurs only during conversion of the low pH conformation to the mid-pH conformation. Conversion of the mid-pH conformation to the high pH conformation would now appear to involve structural changes primarily in the peripheral protein binding domain. Because ankyrin associates avidly with the low pH conformation of cdb3, ankyrin occupancy should strongly influence this structural equilibrium and thereby affect band 3 and perhaps global membrane properties.

Anion Exchange Protein 1, Erythrocyte↗
PubMed2001

Retargeting of viral vectors to the folate receptor endocytic pathway.

Viral vectors with high transfection efficiencies are not always those with optimal target cell binding specificities. As a consequence, virus pseudotyping has been developed to endow transfection competent viruses with improved cell binding specificities and affinities. We have hypothesized that chemical conjugation of a virus to a cell specific ligand might also alter its target cell specificity and produce a virus that would transfect only the desired cell type. To test this concept, an ecotropic replication-defective myeloproliferative sarcoma retrovirus and an amphotropic murine adenovirus containing the gene for beta-galactosidase were chemically derivatized with folic acid. As expected from its strong ecotropism, the unmodified retrovirus did not induce beta-galactosidase expression in nonhost KB cells, while the amphotropic adenovirus yielded high levels of gene expression in the same cell line. Surprisingly, although folate derivatization enabled avid binding of both viruses to folate receptor expressing KB cells, the folate conjugation did not promote retroviral gene expression and actually prevented the normal beta-galactosidase expression seen with the adenoviral vector. The fact that co-administration of excess free folic acid to block uptake by folate receptor-mediated endocytosis restored adenoviral gene expression to the level obtained with unmodified virus suggests that folate derivatization per se does not hamper viral activity. We, therefore, conclude that neither retroviral nor adenoviral delivery via the folate endocytosis pathway is compatible with viral gene expression in KB cells.

Adenoviridae↗
PubMed2001

Involvement of mitogen-activated protein kinase activation in the signal-transduction pathways of the soya bean oxidative burst.

The oxidative burst constitutes one of the most rapid defence responses characterized in the Plant Kingdom. We have observed that four distinct elicitors of the soya bean oxidative burst activate kinases of masses approximately 44 kDa and approximately 47 kDa. Evidence that these kinases regulate production of reactive oxygen species include: (i) their rapid activation by oxidative burst elicitors, (ii) their tight temporal correlation between activation/deactivation of the kinases and activation/deactivation of the oxidative burst, (iii) the identical pharmacological profile of kinase activation and oxidant production for 13 commonly used inhibitors, and (iv) the autologous activation of both kinases and oxidant production by calyculin A and cantharidin, two phosphatase inhibitors. Immunological and biochemical studies reveal that the activated 44 kDa and 47 kDa kinases are mitogen-activated protein (MAP) kinase family members. The kinases prefer myelin basic protein as a substrate, and they phosphorylate primarily on threonine residues. The kinases are themselves phosphorylated on tyrosine residues, and this phosphorylation is required for activity. Finally, both kinases are recognized by an antibody against activated MAP kinase immediately after (but not before) cell stimulation by elicitors. Based on these and other observations, a preliminary sequence of signalling steps linking elicitor stimulation, kinase activation and Ca(2+) entry, to initiation of oxidant production, is proposed.

Cells, Cultured↗
PubMed2001

Regulation of the glycophorin C-protein 4.1 membrane-to-skeleton bridge and evaluation of its contribution to erythrocyte membrane stability.

The band 3-ankyrin-spectrin bridge and the glycophorin C-protein 4.1-spectrin/actin bridge constitute the two major tethers between the erythrocyte membrane and its spectrin skeleton. Although a structural requirement for the band 3-ankyrin bridge is well established, the contribution of the glycophorin C-protein 4.1 bridge to red cell function remains to be defined. In order to explore this latter bridge further, we have identified and/or characterized five stimuli that sever the linkage in intact erythrocytes and have examined the impact of this rupture on membrane mechanical properties. We report here that elevation of cytosolic 2,3-bisphosphoglycerate, an increase in intracellular Ca(2+), removal of cell O(2), a decrease in intracellular pH, and activation of erythrocyte protein kinase C all promote dissociation of protein 4.1 from glycophorin C, leading to reduced retention of glycophorin C in detergent-extracted spectrin/actin skeletons. Significantly, where mechanical studies could be performed, we also observe that rupture of the membrane-to-skeleton bridge has little or no impact on the mechanical properties of the cell, as assayed by ektacytometry and nickel mesh filtration. We, therefore, suggest that, although regulation of the glycophorin C-protein 4.1-spectrin/actin bridge likely occurs physiologically, the role of the tether and the associated regulatory changes remain to be established.

2,3-Diphosphoglycerate↗
PubMed2001

An apoplastic Ca2+ sensor regulates internal Ca2+ release in aequorin-transformed tobacco cells.

Removal of Ca(2+) from tobacco suspension cell medium has two immediate effects on cytosolic Ca(2+) fluxes: (i) externally derived Ca(2+) influx (occurring in response to cold shock or hypo-osmotic shock) is inhibited, and (ii) organellar Ca(2+) release (induced by a fungally derived defense elicitor, caffeine, or hypo-osmotic shock) is elevated. We show here that the enhanced release of internal Ca(2+) is likely due to increased discharge from a caffeine-sensitive store in response to a signal transduced from an extracellular Ca(2+) sensor. Thus, chelation of extracellular Ca(2+) in the absence of any other stimulus directly activates release of intracellular Ca(2+) into the cytosol. Evidence that this chelator-activated Ca(2+) flux is dependent on a signaling pathway includes its abrogation by prior treatment with caffeine, and its inhibition by protein kinase inhibitors (K252a and staurosporine) and anion channel blockers (niflumate and anthracene-9-carboxylate). An unexpected characteristic of tobacco cell adaptation to low external Ca(2+) was the emergence of a new Ca(2+) compartment that was inaccessible to external EGTA, yet responsive to the usual stimulants of extracellular Ca(2+) entry. Thus, cells that are exposed to EGTA for 20 min lose sensitivity to caffeine and defense elicitors, indicating that their intracellular Ca(2+) pools have been depleted. Surprisingly, these same cells simultaneously regain their ability to respond to stimuli that usually activate extracellular Ca(2+) influx even though all external Ca(2+) is chelated. Because this gradual restoration of Ca(2+) influx can be inhibited by the same kinase inhibitors that block EGTA-activated Ca(2+) release, we propose that chelator-activated Ca(2+) release from internal stores leads to deposition of this Ca(2+) into a novel EGTA- and caffeine-insensitive compartment that can subsequently be activated by stimulants of extracellular Ca(2+) entry.

Aequorin↗
PubMed2001

Activation of the oxidative burst in aequorin-transformed Nicotiana tabacum cells is mediated by protein kinase- and anion channel-dependent release of Ca2+ from internal stores.

The source of Ca2+ involved in transducing an oxidative-burst defense signal was examined in aequorin-transformed tobacco (Nicotiana tabacum L.) cells using modulators of Ca2+ entry. Treatments that either increased or decreased the influx of Ca2+ from external stores were found to have little effect on the magnitude or kinetics of an osmotically stimulated oxidative burst. In contrast, treatments that reduced the discharge of Ca2+ from internal stores inhibited dilution-activated H2O2 production. Curiously, most of the modulators commonly employed in animal studies as internal Ca(2+)-release inhibitors were neither effective in blocking discharge of intracellular Ca2+ nor in preventing the oxidative burst. When three different biochemical elicitors of the oxidative burst were similarly examined, both the H2O2 production and Ca2+ fluxes stimulated were found to be sensitive to modulators of internal Ca2+ release, but neither was impacted by alterations in externally derived Ca2+ influx. We hypothesize, therefore, that the oxidative burst does not depend on the influx of external Ca2+, but instead may generally be mediated by the release of internal Ca2+ in a manner that depends on the proper function of kinases and anion channels. These Ca2+ pulses trigger downstream signaling events that include the activation of Ca(2+)-regulated protein kinases, which are required for stimulation of the oxidative burst.

Aequorin↗
PubMed2001

Measurement of stress-induced Ca(2+) pulses in single aequorin-transformed tobacco cells.

Signaling patterns measured in large cell populations are the sum of differing signals from separate cells, and thus, the detailed kinetics of Ca(2+) pulses can often be masked. In an effort to evaluate whether the cytosolic Ca(2+) pulses previously reported in populations of elicitor- and stress-stimulated tobacco cells accurately represent the pulses that occur in individual cells, a study of single cell Ca(2+) fluxes in stress-stimulated tobacco cells was undertaken. Individual aequorin-transformed cells were isolated from a tobacco suspension culture and placed directly on a sensitive photo-multiplier tube mounted in a dark chamber. Ca(2+)-dependent luminescence was then monitored after stimulation with hypo- or hyper-osmotic shock, cold shock, or defense elicitors (oligogalacturonic acid and harpin). Hypo-osmotic shock induced a biphasic Ca(2+) transient in 67% of the single cells tested that exhibited similar kinetics to the biphasic pulses measured repeatedly in 1ml cell suspensions. In contrast, 33% of the stimulated cells displayed Ca(2+) flux patterns that were not previously seen in cell suspension studies. Additionally, because only 29% of the cells tested responded with measurable Ca(2+) pulses to oligogalacturonic acid and 33% to the harpin protein, we conclude that not all cells in a suspension are simultaneously sensitive to stimulation with defense elicitors. In contrast, all cells tested responded with an immediate Ca(2+) influx after cold or hyperosmotic shock. We conclude that in many cases the Ca(2+) signaling patterns of single cells are accurately represented in the signaling patterns of large populations, but that single cell measurements are still required to characterize the Ca(2+) fluxes of the less prominent cell populations.

Aequorin↗
PubMed2001

Variations in the promoter region of the apolipoprotein A-1 gene influence plasma lipoprotein(a) levels in Asian Indian neonates from Singapore.

We studied the influence of two DNA polymorphisms (-75 bp G/A and +83 bp C/T) in the promoter region of the apolipoprotein A-1 (apoA1) gene on cord plasma level of lipoprotein(a) [Lp(a)] in 1076 newborns of both genders from the three major ethnic groups in Singapore-Chinese, Malays, and Asian Indians. The frequency of the A: allele at -75 bp in the Indians was significantly lower than the Chinese and Malays. There was no linkage disequilibrium between the two sites studied. Both polymorphic sites were not significantly associated with any lipid factors except for Lp(a) levels in the Asian Indians. The AA and CC homozygotes were significantly associated with lower Lp(a) levels. These associations were specific only to the male Indian neonates. The genetic variations at the -75 and +83 bp explained 6.9% and 7.2%, respectively, of the total variability of plasma Lp(a) levels at birth in the Asian Indians. The Lp(a) levels were also significantly different between composite genotypes in the order GG/TT > GA/CT > GG/CT > GA/CC > GG/CC > AA/CC. The effects of the two polymorphisms seem to be additive as the composite genotypes were able to explain 14% of the Lp(a) variance, equivalent to the sum of the two constituent sites. Our results showed that there is significant ethnic- and gender-specific influence of the apoA1 gene on plasma Lp(a) levels at birth that is inherent and independent of known gene-environment interactions.

Apolipoprotein A-I↗
PubMed2001

The use of vigabatrin in infantile spasms in Asian children.

BACKGROUND: The use of adrenocorticotrophic hormone (ACTH) and prednisolone in the management of infantile spasms has been well established, but is associated with significant morbidity and cannot be used as long-term medication. Since the introduction of vigabatrin, results have been promising with suggestions currently that it should be used as first-line management of infantile spasms. AIM: The aim of this study was to establish the efficacy, tolerability and problems associated with the use of vigabatrin, in Asian children with infantile spasms. MATERIALS AND METHODS: Eighteen local Asian patients with infantile spasms were given vigabatrin, in 8 as first-line monotherapy and in 10 patients as add-on therapy to pre-existing anti-convulsants. RESULTS: Thirty-nine per cent (7/18) showed 100% suppression of seizures at 2 weeks and 50% (9/18) showing complete suppression of seizures at 4 weeks after starting therapy. There was however, a high relapse rate (56%) in complete responders within the first 6 months of therapy. This was probably due to a lower maintenance dose in the patients, as those who relapsed were on an average dose of 59 mg/kg/day at the time of relapse and responded to a stepped up dose of 83 mg/kg/day. Vigabatrin was well tolerated and only 2 patients developed somnolence and irritability. CONCLUSION: A dose of at least 70 mg/kg/day may be necessary to achieve adequate control and yet avoid the common adverse affects of vigabatrin.

Anticonvulsants↗
PubMed2001

Diagnosing and prognosticating acute meningitis in young infants within 24 hours of admission.

INTRODUCTION: The early diagnosis and prognosis of acute meningitis in young infants (infants 90 days old or younger) have not been well studied. We therefore investigated the diagnostic and prognostic factors for acute meningitis obtainable within 24 hours of admission. METHODS: Data were obtained through a retrospective case review of 55 young infants from 1991 to 1999 inclusive. RESULTS: The 3 commonest symptoms of acute meningitis were fever, abnormal activity and decreased feeding. The 3 commonest signs were temperature > 38.0 degrees C, irritability/crying and abnormal tone/reflexes. The best predictor of acute bacterial meningitis (ABM) was the cerebrospinal fluid (CSF)-to-blood glucose ratio. A glucose ratio of < or = 0.8 can be used to diagnose ABM with 100% sensitivity and 100% negative predictive value. Furthermore, a ratio < or = 0.3 can be used to diagnose ABM with 100% specificity and 100% positive predictive value. The best predictor of unfavourable neurological outcome (UFNO) was also the CSF-to-blood glucose ratio. A glucose ratio of < or = 0.3 again can be used to prognosticate for UFNO with 100% sensitivity and 100% negative predictive value. CONCLUSIONS: Diagnosis of acute meningitis by history and physical examination alone is difficult. However, with the aid of laboratory tests, in particular the CSF-to-blood glucose ratio, one can diagnose ABM and prognosticate for unfavourable neurological outcome with high sensitivity and high negative predictive value within 24 hours of admission.

Acute Disease↗