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Biomedical subjects

A Cheng

Publications and source records attributed to A Cheng.

At least 127 records · Page 7Linked to original sources
PubMed1992

Multicenter evaluation of a new enzyme immunoassay for detection of Clostridium difficile enterotoxin A.

The Premier Clostridium difficile toxin A enzyme immunoassay (PTA EIA) (Meridian Diagnostics, Inc., Cincinnati, Ohio) for rapid diagnosis of antibiotic-associated colitis (AAC) was evaluated in a multicenter study. Stool samples from 421 patients suspected of having AAC were tested for toxin A by the PTA EIA and for toxin B by three tissue culture assays (TCA) employing WI-38 cells (New England Deaconess Hospital) in conventional tubes or foreskin fibroblasts (Children's Hospital) or Vero cells (Beth Israel Hospital) in microwells. The tubes and plates were examined at 24 and 48 h for cytotoxicity. Clinical criteria, repeat testing at another site, and culture of frozen stool samples for C. difficile were used to evaluate discrepant results. Of 504 samples, 66 were positive and 409 were negative by both tests. Eight samples had indeterminate PTA EIA results and were excluded from this analysis. Of 21 discrepancies, 9 were PTA EIA positive and TCA negative and 12 were PTA EIA negative TCA positive. Following resolution of the discrepancies, 11 of 12 PTA EIA-negative-TCA-positive and 5 of 9 PTA EIA-positive-TCA-negative samples were considered true positive for AAC. The sensitivity and specificity were, respectively, 86.6 and 99.0% for the PTA EIA and 93.9 and 99.8% for TCA. The predictive values of positive and negative tests were, respectively, 94.7 and 97.4% for the PTA EIA and 98.7 and 98.8% for TCA. We conclude that the PTA EIA is a rapid, simple EIA technique whose accuracy in detecting enterotoxin A approaches that of reference TCA methods for detection of cytotoxin B.

Bacterial Proteins↗
PubMed1992

Limited value of the resting electrocardiogram in assessing patients with recent onset chest pain: lessons from a chest pain clinic.

OBJECTIVE: To evaluate a clinic set up specifically to assess patients with recent onset chest pain, particularly those presenting with a normal resting electrocardiogram. DESIGN: Retrospective review of case notes. SETTING: Cardiac department of a tertiary referral cardiothoracic centre. PATIENTS: 250 consecutive patients with recent onset chest pain seen within 24 hours of general practitioner referral. OUTCOME MEASURES: Clinical diagnosis and management. RESULTS: 40% of patients were seen within seven days of the onset of symptoms. Twenty seven per cent had non-cardiac symptoms and could be discharged while 60% were considered to have cardiac pain. Sixty six patients (26%) were admitted directly from the clinic and 48 of these underwent coronary angiography within three weeks. Seventy patients (28%) have so far undergone intervention (angioplasty or coronary artery surgery), 22 within one month of presentation. One hundred and nine patients (44%) presented with a normal resting electrocardiogram, 21 of whom were considered to have unstable angina. Forty one of these patients were investigated of whom 37 were found to have significant coronary disease and 26 have undergone intervention. CONCLUSIONS: This experience highlights the inadequacy of a routine electrocardiogram reporting service in patients with recent onset of chest pain. An alternative facility offering immediate and complete cardiac assessment produced patient benefit with early diagnosis and intervention. Investigation of these patients, however, accounted for 5% of cardiac catheterisation laboratory throughput; this was a significant additional and unscheduled workload.

Adult↗
PubMed1992

Diagnosis of tuberculous meningitis by detection of tuberculostearic acid in cerebrospinal fluid.

Tuberculous meningitis still poses a major diagnostic problem. Recently, several new techniques for rapid diagnosis of tuberculous meningitis have been developed. One of these newer techniques is the detection of mycobacterial fatty acids, using gas chromatography--mass spectroscopy. In this study we evaluated the sensitivity and specificity of this recently reported method in a blind trial. The sensitivity was found to be 100%, specificity only 91%. The number of false positive results limits the value of this test in the primary diagnosis of tuberculous meningitis. Nonetheless, we consider this method to be a valuable tool for establishing the diagnosis in culture-negative patients presenting with signs and symptoms suggestive of tuberculous meningitis.

Diagnosis, Differential↗
PubMed1991

Sequence of human glucose-6-phosphate dehydrogenase cloned in plasmids and a yeast artificial chromosome.

The sequence of 20,114 bp of DNA including the human glucose-6-phosphate dehydrogenase (G6PD) gene was determined. The region included a prominent CpG island, starting about 680 nucleotides upstream of the transcription start site, extending about 1050 nucleotides downstream of the start site, and ending just at the start of the first intron. The transcribed region from the start site to the poly(A) addition site covers 15,860 bp. The sequence of the 13 exons agreed with published cDNA sequence and for the 11 exons tested, with the corresponding sequence in a yeast artificial chromosome (YAC). The latter confirms YAC cloning fidelity at the DNA sequence level. Sixteen Alu sequences constitute 24% of the total sequence tract. Four were outside the borders of the mRNA transcript of the gene; all the others were found in a large (9858 bp) intron between exons 2 and 3. Two Alu clusters each contain Alus lying between the monomers of another.

Amino Acid Sequence↗
PubMed1991

Deep venous thrombosis in the baboon: an experimental model.

Experimental models of deep venous thrombosis, heretofore, have not been available for laboratory studies. This investigation establishes a novel model of venous thrombosis by inhibiting the protein C system combined with venous stasis and subtle venous injury. Ten adolescent baboons were studied in pairs, with one animal receiving saline solution (B2, B4, B6, B8, B10) and one being exposed to thrombogenic reagents (B1, B3, B5, B7, B9). These reagents represented a combination of a monoclonal antibody (HPC4) to protein C, 1 to 4 mg/kg administered over 5 minutes, and tumor necrosis factor administered over 3 minutes at a dose of 150 micrograms/kg through a catheter placed into the left superficial femoral vein with distal ligation. To encourage stasis, a pediatric size blood pressure cuff was inflated to 40 mm Hg on the right thigh for 50 minutes of every hour during the first experimental day (day 1) in B5 to B10. The animals were observed for a 6-hour period on day 1 and then for an 11- to 15-day period until sacrifice. Hemodynamic and hematologic parameters were recorded along with duplex imaging of the iliac veins and inferior vena cava on a daily basis. Venography was performed on day 1, day 4, and the day of sacrifice. At sacrifice the entire iliac and vena caval system was carefully dissected, opened, and photographed. Experimental animals given the HPC4 and tumor necrosis factor developed left iliac vein thrombosis extending into the inferior vena cava. Duplex imaging, venography, and autopsy revealed that control animals receiving saline solution never developed comparable thrombus. Experimental subjects exhibited thrombus on duplex imaging by day 4 (B1), day 3 (B3), day 2 (B5), 120 minutes (B7), and 360 minutes (B9) after receiving HPC4 and tumor necrosis factor. Venograms performed on day 1 exhibited thrombus in B5, B7, and B9. The extent of thrombus, the timing of its occurrence, and its effect on the animals' left leg followed a dose-dependent relationship for the animals in which the occlusive blood pressure cuff was used. Significantly greater declines in blood pressure, white blood cell count, and platelet count were found in affected animals given HPC4 and tumor necrosis factor reagents as compared to control subjects. All affected animals demonstrated the appearance of fibrin split products and a markedly prolonged prothrombin time. This investigation, for the first time, establishes a reproducible model of deep venous thrombosis involving inhibition of protein C that will facilitate further laboratory studies on venous thrombosis.

Animals↗
PubMed1990

Functional analysis of the murine T-cell receptor beta enhancer and characteristics of its DNA-binding proteins.

The minimal T-cell receptor (TCR) beta-chain (TCR beta) enhancer has been identified by transfection into lymphoid cells. The minimal enhancer was active in T cells and in some B-lineage cells. When a larger fragment containing the minimal enhancer was used, its activity was apparent only in T cells. Studies with phytohemagglutinin and 4 beta-phorbol-12,13-dibutyrate revealed that the enhancer activity was increased by these agents. By a combination of DNase I footprinting, gel mobility shift assay, and methylation interference analysis, seven different motifs were identified within the minimal enhancer. Furthermore, competition experiments showed that some of these elements bound identical or similar factors that are known to bind to the TCR V beta promoter decamer or to the immunoglobulin enhancer kappa E2 or muEBP-E motif. These shared motifs may be important in the differential gene activity among the different lymphoid subsets.

Animals↗
PubMed1989

Continuous aortic regurgitation in severely dysfunctional native hearts after heterotopic cardiac transplantation.

Doppler techniques were used to investigate the frequency and characteristics of aortic regurgitation (AR) in the severely deteriorated native heart after heterotopic cardiac transplantation. Ten patients were studied in whom the native left ventricular fractional shortening was less than 6%. AR was detected by Doppler in 6 patients. The AR was continuous throughout the cardiac cycle in 3 patients, continuous when present (but not occurring with every beat) in 2 patients and present throughout diastole and continued into midsystole in the remaining patient. AR was associated with abnormalities of aortic valve opening (either its complete or intermittent absence) and with poor forward flow and even reversed flow through the native heart. It is postulated that AR in this group mainly occurs as a result of abnormal locking of the aortic valve due to severely impaired left ventricular ejection. Abnormal left ventricular diastolic function and enlargement may also be contributory.

Adult↗
PubMed1989

Low dose alcohol contracts the human umbilical artery in vitro.

Beginning at very low concentrations (0.001 g/100 ml), alcohol elicited dose-dependent contractions of the human umbilical artery in vitro. Additionally, 16 of the 108 arteries tested had a 5-10-min spasm in response to alcohol. Alcohol (0.2 g/100 ml) also increased tension developed in response to all angiotensin II doses, but had no effect on serotonin-induced contractions. These results suggest that alcohol may increase umbilicoplacental resistance in vivo, thus decreasing fetal-placental blood flow.

Culture Techniques↗
PubMed1989

The bacteriology of bronchiectasis in Hong Kong investigated by protected catheter brush and bronchoalveolar lavage.

Bacteria often colonize the lower respiratory tract of patients with bronchiectasis. Although the role of these bacteria in the pathogenesis of the disease is uncertain, their accurate identification is important for epidemiologic and treatment purposes. Therefore, the aims of this study were: (1) to identify these bacteria in patients with bronchiectasis without cystic fibrosis using the protected catheter brush (PCB) in order to avoid oropharyngeal contamination, and (2) to compare the results of bronchoalveolar lavage (BAL) with PCB. Quantitative culture was performed on PCB and BAL specimens obtained from the most severely affected lobes of 23 patients with bronchiectasis. Results of PCB showed no significant growth (less than 10(3) colony-forming units [cfu]/ml) in nine patients and 17 significant isolates (greater than 10(3) cfu/ml) in the rest: H. influenzae, 5; P. aeruginosa, 4; K. ozaenae, 2; S. aureus, 2; P. fluorescens, 1; S. pneumoniae, 1; Veillonella, 1; and coag.-ve Staph., 1. For BAL, the results were the same (20 isolates) regardless of whether 10(4) or 10(5) cfu/ml was chosen as the cutoff point. More organisms were cultured from BAL specimens, and these included all but one of the organisms cultured from PCB. We conclude that the bacteriology of bronchiectasis in Hong Kong is different from that reported in sputum studies in the West (mainly H. influenzae, S. pneumoniae, and S. aureus), and with 10(4) cfu/ml as the cutoff point, BAL gives comparable results to PCB.

Adult↗
PubMed1988

Allosteric nucleotide specificity of phosphorylase kinase: correlation of binding, conformational transitions, and activation. Utilization of lin-benzo-ADP to measure the binding of other nucleoside diphosphates, including the phosphorothioates of ADP.

Recent work has shown that ADP is an allosteric activator of nonphosphorylated phosphorylase kinase from rabbit skeletal muscle (Cheng, A., Fitzgerald, T. J., and Carlson, G. M. (1985) J. Biol. Chem. 260, 2535-2542). The specificity of the allosteric site for nucleoside diphosphates is further investigated in this study. Only purine nucleoside diphosphates are capable of causing allosteric activation, and an amino group at position 2 or 6 of the purine ring is required. Comparisons are made of the abilities of 5'-diphosphate analogs of ADP, including phosphorothioates, to activate, to bind, and to induce in the enzyme's beta subunits conformational changes associated with activation. Binding is measured by competition titrations utilizing fluorescence polarization of lin-benzo-ADP, itself an allosteric activator; and conformational changes are measured by partial proteolysis and chemical cross-linking. When measured at an identical percentage of saturation at the allosteric site, the abilities of ADP analogs to induce conformational changes in the beta subunits parallel their abilities to activate the holoenzyme. An unmodified beta-phosphate of ADP, although not necessary for binding at the allosteric site, is needed to fully drive the activating conformational transition. The activating nucleoside diphosphate appears to be the free species, as opposed to its Mg2+ complex.

Adenosine Diphosphate↗
PubMed1988

Competition between nucleoside diphosphates and triphosphates at the catalytic and allosteric sites of phosphorylase kinase.

The interactions of nucleotides at the allosteric and catalytic sites of phosphorylase kinase were examined. Binding of nucleoside triphosphates at the nucleoside diphosphate allosteric activation site inhibited enzymatic activity; this was observed with either ATP or GTP. Increasing concentrations of ADP caused a biphasic response: low concentrations activated and higher concentrations inhibited. Inhibition was due to the binding of ADP at the catalytic site, as opposed to an allosteric inhibitory site. GDP activated at low concentrations, but did not inhibit even at relatively high concentrations, and is therefore a specific probe for the allosteric site. Maximal activity of the nonactivated holoenzyme at pH 6.8 is achieved at an optimal ratio of ATP to ADP, such that the inhibitory actions of ATP at the allosteric site and of ADP at the catalytic site are balanced. Various potential molecular mechanisms to explain the allosteric activation by ADP were examined and ruled out, thus strengthening our previous conclusion that the activation is predominantly caused by a conformational transition in the beta subunits directly induced by the binding of ADP (Cheng, A., Fitzgerald, T. J., and Carlson, G. M. (1985) J. Biol. Chem. 260, 2535-2542; Trempe, M. R., and Carlson, G. M. (1987) J. Biol. Chem. 262, 4333-4340; Cheng, A., Fitzgerald, T. J., Bhatnager, D., Roskoski, R., Jr., and Carlson, G. M. (1988) J. Biol. Chem. 263, 5534-5542). The catalytic site exhibited high stereospecificity for inhibition by the Rp and Sp epimers of adenosine 5'-O-(1-thiodiphosphate), with the Rp epimer (Ki = 0.5 microM) being 136-fold more effective than its Sp counterpart. This can readily explain the inability of the Rp epimer to be an effective allosteric activator.

Adenosine Diphosphate↗
PubMed1988

Effects of addition of a 2-methyl group to ethyl nipecotates (beta-meperidines) on receptor affinities and opiate agonist/antagonist activities.

A series of 2-methyl-3-carbethoxy-3-(m-hydroxyphenyl)piperidine opiates (13a-d) with N-substituent variations have been synthesized, and their receptor affinities and in vivo agonist and antagonist activities and energy-conformational profiles have been determined. These are racemates of the alpha-epimer at the C-2 position, with a methyl group cis to the 3-phenyl group. One of the main goals of this study was to compare the conformational and pharmacological behavior of these 2-methyl "beta-meperidine" analogues to their 2-desmethyl racemic counterparts (14a-c) previously reported in the literature. The 2-desmethyl and 2-methyl analogues were found to have very similar phenyl equatorial conformers as their lowest energy forms with the addition of a 2-methyl group diminishing conformational flexibility. The presence of the 2-methyl group appears to diminish affinity at the mu-receptor and also to somewhat diminish already weak antinociceptic agonist activity. Given the similarity in lowest energy conformation, this reduction is most likely caused by the unfavorable interaction of the methyl group itself with a local mu-receptor binding site. Superposition of the phenol OH and protonated amine nitrogen NH of either 2-methyl enantiomer of 13a in its lowest energy conformer with the same OH and NH groups of metazocine, used as a high affinity rigid analogue, leads to reasonable overlap. However, the N-substituents and the piperidine and phenyl rings do not overlap in this proposed pharmacophore, perhaps accounting for the rather poor affinities found for these 3-phenylpiperidines and the lack of N-substituent modulation of affinity and efficacy as in fused ring opioids.

Animals↗
PubMed1987

Prevalence survey of infection in a Hong Kong hospital using a standard protocol and microcomputer data analysis.

A 1-day prevalence survey of hospital infection was performed in June 1985 at a new general teaching hospital in Hong Kong. The 1980 British national survey protocol was used, and the results were analysed by microcomputer. The major part of the survey was carried out by nine people over a period of 3 days and was completed within 6 weeks. The rate of hospital-acquired infection was 8.9% and of community-acquired infection was 16.5%. Antibiotics were mainly used in infected patients. The British protocol is suitable for Hong Kong hospitals, and with microcomputer data analysis such surveys can be completed quickly and accurately even with limited resources.

Adolescent↗
PubMed1986

N-substituent modulation of opiate agonist/antagonist activity in resolved 3-methyl-3-(m-hydroxyphenyl)piperidines.

A series of 3-methyl-3-(m-hydroxyphenyl)piperidines with N-substituent variations have been synthesized and resolved, and an X-ray crystal structure of one analogue was determined. The compounds have been characterized, pharmacologically, by detailed opiate receptor binding studies and determination of in vivo analgesia and opiate antagonism. The results indicate that all compounds bind with high selectivity and moderate affinity to mu-receptors with no qualitative difference between enantiomeric pairs. By contrast a striking difference in activities is found, with the (-) enantiomers being pure agonists and the (+) enantiomers having both agonist and antagonist activity. The effect of N-substituents on relative agonist and antagonist potency does not mimic that of fused ring opiates with the N-phenethyl compound, the most potent antagonist. These results together with the X-ray structure obtained suggest that agonist and antagonist activity is initiated by a bimodel binding of the compounds in two different orientations at the mu-receptor site.

Animals↗
PubMed1985

Adenosine 5'-diphosphate as an allosteric effector of phosphorylase kinase from rabbit skeletal muscle.

Equilibrium binding and activity studies indicate that adenosine 5'-diphosphate binds to phosphorylase kinase with high affinity at a site, or sites, distinct from the catalytic site. Equilibrium dialysis at pH 6.8 and 8.2, with and without Mg2+, and with phosphorylated and nonphosphorylated enzyme preparations revealed approximately 8 ADP binding sites per alpha 4 beta 4 gamma 4 delta 4 hexadecamer, with Kd values ranging from 0.26 to 17 microM. Decreasing the pH from 8.2 to 6.8 or removing the Mg2+ enhanced the affinity for ADP. At pH 6.8, ADP stimulated the phosphorylase conversion and autophosphorylation activities of the nonactivated enzyme. Analogs of ADP with modifications at the 2'-, 3'-, and 5'-positions allowed determination of structural requirements for the stimulation of activity. ADP seems to alter the conformation of the beta subunit because addition of the nucleotide inhibits its dephosphorylation by phosphoprotein phosphatase and its chemical cross-linking by 1,5-difluoro-2,4-dinitrobenzene. The binding affinities and effects of ADP suggest that it may function physiologically as an allosteric effector of phosphorylase kinase.

Adenosine Diphosphate↗