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Biomedical subjects

C M Chen

Publications and source records attributed to C M Chen.

At least 271 records · Page 15Linked to original sources
PubMed1983

Cloning and expression of R-factor mediated arsenate resistance in Escherichia coli.

The resistance transfer factor R773 confers inducible arsenate, arsenite and antimony resistance on Escherichia coli. The genes for these resistances were cloned into the EcoRI site of plasmid pBR322 to produce a 33 kilobase plasmid. pUM1. Bacterial strains transformed with pUM1 synthesized a polypeptide of the apparent molecular weight 64,000 daltons when induced with arsenite. This polypeptide could be visualized on sodium dodecyl sulfate polyacrylamide gels stained with Coomassie blue. It was observed both in the membrane and cytosol fractions but not among the periplasmic proteins present in osmotic shock fluid. Minicells isolated from strain JR410(pUM1) incorporated [35S]methionine into an inducible 64,000 dalton polypeptide, as demonstrated on autoradiographs of electrophoresed [35S]-labeled minicell lysates, confirming that this polypeptide is a plasmid gene product. A 4.3 kilobase HindIII fragment of pUM1 was subcloned into the HindIII site of pBR322, producing recombinant plasmid pUM3. This plasmid conferred constitutive resistance ot arsenite and arsenate. Extensive synthesis of two polypeptides of 64,000 and 16,000 daltons was observed both in Coomassie stained gels of whole cells and autoradiographs of gels of [35S]methionine-labeled minicells. Synthesis of both polypeptides was constitutive.

Arsenates↗
PubMed1983

Leucinopine, a characteristic compound of some crown-gall tumors.

An unusual compound has been found in crowngall tumors induced by those Agrobacterium tumefaciens strains that utilize neither octopine nor nopaline. The compound has been isolated and shown by proton and carbon-13 NMR spectroscopy and by synthesis to be N(2)-(1,3-dicarboxypropyl)-L-leucine, which also exists in a cyclized (i.e., lactam) form. This compound, which we name "leucinopine," was not detected in octopine tumors, nopaline tumors, or the tumors induced by A. tumefaciens strains 181 and EU6.

Journal Article↗
PubMed1983

Effects of hyperkalaemia on sinus nodal function in dogs: sino-ventricular conduction.

The effects of hyperkalaemia on electrograms recorded from the sinus node, crista terminalis, Bachmann bundle, right and left atria and His bundle were studied in anaesthetised dogs. Increasing hyperkalaemia up to about 8.5 mol X litre-1 produced: 1) a gradual prolongation of the sino-crista terminalis interval, and 2) a shift in sinus pacemaker location. Hyperkalaemia between 8.5 and 10.0 mmol X litre-1 produced arrest of most of the atria but persistence of electrical activity of the sinus node, crista terminalis, Bachmann bundle, His bundle and ventricles and sustained sino-ventricular conduction. During the course of increasing hyperkalaemia, right atrial electrograms from sites closer to the crista terminalis disappeared later than those from sites more remote from the crista terminalis. Decremental conduction through sites progressively remote from the axis of the crista terminalis (and possibly also other "preferential pathways") seemed to be the basis of hyperkalaemic atrial arrest.

Animals↗
PubMed1983

Evidence that AV nodal re-entrant tachycardia does not require participation of the entire AV node.

Electrophysiologic studies in a case of AV nodal re-entrant tachycardia showed that a programmed atrial premature depolarization induced during the tachycardia did not change the tachycardia cycle but caused a delay in the following atrial echo. Analysis of such a phenomenon suggests that the atrial premature depolarization was conducted to the upper part of the AV node but not to the site of the re-entry. Therefore, AV nodal re-entry can persist without the participation of the upper part of the AV node. This case illustrates that the upper common pathway connecting the dual AV nodal pathways cranially is most likely located within the AV node and consists of AV nodal tissue.

Aged↗
PubMed1983

Production and characterization of monoclonal antibodies specific for a glycosylated polypeptide of human cytomegalovirus.

Nine hybrid cell lines producing antibodies specific for cytomegalovirus (CMV) antigen were obtained after fusion of P3/X63-Ag8 myeloma cells with spleen cells from BALB/c mice immunized with CMV complement-fixing antigen. By the immunoblot technique, five of nine antibodies (4D11, 7B4, 7D2, 8E3, and 8E10) were identified as being reactive to a CMV glycosylated polypeptide with molecular weight of 66,000 (GP66). Four other antibodies (1B8, 8E9, 4D2, and 7E2) appeared to be reactive with CMV antigen(s) only if the antigen was not denatured by sodium dodecyl sulfate. These remain unassigned until further studies are done. With the enzyme-linked immunosorbent assay (ELISA), competitive bindings were performed with a constant amount of horseradish peroxidase-conjugated antibody and various concentrations of unconjugated homologous and heterologous antibodies on CMV antigen-coated ELISA wells, and the antigenic determinant specific for each antibody was determined. The nine antibodies could be classified into six different groups, each group reacting with a different epitope or a different region with two or more antigenic determinants which are so close to each other that they cause binding inhibition. They are groups A (4D11), B (7B4, 8E10), C (7D2), D (4D2, 7E2, 8E9), E (8E3), and F (1B8). The extent of competition among antibodies within each group was the same. By using the two antibodies that reacted with different epitopes on GP66, a double-antibody sandwich ELISA method was developed. The method was sensitive enough to detect as little as 50% of the antigen present in one infected cell or 0.000245 U of CMV complement-fixing antigen per test well. Other strains of CMV (David, Kerr, Espilat, C-87, and five clinical isolates) gave positive results, whereas herpes simplex virus types 1 and 2, varicella-zoster virus and Epstein-Barr virus nuclear antigen preparations did not. By the indirect immunofluorescence assay, antibodies 4D11 and 8E3 were able to detect GP66 in the nucleus of CMV-infected F-5000 human embryonic fibroblasts as early as 2 h postinfection and were superior in this respect to the remaining seven antibodies tested. By the double-antibody sandwich ELISA, the presence of GP66 in CMV-infected cells was detected as early as 2 h postinfection.

Antibodies, Monoclonal↗
PubMed1983

Recording of diastolic slope from the junctional area in dogs with junctional rhythm.

Stable junctional rhythm was induced in 14 of 25 anesthetized dogs by exclusion of the sinus node and surrounding atrial tissue. In 12 of the 14 dogs diastolic slope was recorded through a unipolar lead consisting of a terminal of a plunge or catheter electrode placed in the His bundle area coupled with an indifferent superior vena caval electrode. Reversed polarity, high-gain amplification (0.1 mV/cm), and low-pass filters (0.1 to 50 Hz) were used for the unipolar recording. The diastolic slope (0.15 +/- 0.07 mV/sec) was recorded from a localized area close to the His bundle and preceded the bipolar His bundle deflection. Overdrive atrial stimulation was followed by slowing of the junctional rate and decrease in the diastolic slope. A strong negative correlation (r values from -.78 to -.95) was found between the junctional cycle lengths and the diastolic slopes after atrial pacing. Isoproterenol infusion increased the junctional rate and the diastolic slope, whereas vagal stimulation and verapamil injection decreased them. The effects of verapamil were partly reversed by CaCl2 injection. The responses of the diastolic slope to overdrive pacing, vagal stimulation, and isoproterenol suggest that the deflection is indeed due to membrane current responsible for phase 4 depolarization. The responses of the junctional rhythm and the diastolic slope to verapamil and CaCl2 suggest that AV nodal automaticity is responsible for the rhythm. Clinical application of this recording technique should help in the identification and characterization of automatic junctional rhythm.

Animals↗
PubMed1981

Metabolism of Cytokinin : DEPHOSPHORYLATION OF CYTOKININ RIBONUCLEOTIDE BY 5'-NUCLEOTIDASES FROM WHEAT GERM CYTOSOL.

Two forms (F-I and F-II) of 5'-nucleotidases (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) which catalyze the dephosphorylation of N(6)-(Delta(2)-isopentenyl)adenosine 5'-monophosphate and AMP to form the corresponding nucleosides were partially purified from the cytosol of wheat (Triticum aestivum) germ. Both the F-I (molecular weight, 57,000) and F-II (molecular weight, 110,000) 5'-nucleotidases dephosphorylate the ribonucleotides at an optimum pH of 7. The K(m) values for the cytokinin nucleotide are 3.5 micromolar (F-I enzyme) and 12.8 micromolar (F-II enzyme) in 100 millimolar Tris-maleate buffer (pH 7) at 37 C. The F-I enzyme is less rapidly inactivated by heating than is the F-II enzyme. Both nucleotidases hydrolyze purine ribonucleoside 5'-phosphates, AMP being the preferred substrate. N(6)-(Delta(2)-isopentenyl)Adenosine 5'-monophosphate is hydrolyzed at a rate 72 and 86% that of AMP by the F-I and F-II nucleotides, respectively. Phenylphosphate and 3'-AMP are not substrates for the enzymes. It is proposed that dephosphorylation of cytokinin nucleotide by cytosol 5'-nucleotidases may play an important role in regulating levels of "active cytokinin" in plant cells.

Journal Article↗
PubMed1981

Metabolism of cytokinin: deribosylation of cytokinin ribonucleoside by adenosine nucleosidase from wheat germ cells.

Adenosine nucleosidase (adenosine ribohydrolase, EC 3.2.2.7) which catalyzes the deribosylation of N(6)-(Delta(2)-isopentenyl)adenosine and adenosine to form the corresponding bases was partially purified from wheat germ. This enzyme (molecular weight 59,000 +/- 3,000) deribosylates the ribonucleosides at an optimum pH of 4.7 K(m) values for the cytokinin nucleoside and adenosine are 2.38 and 1.43 micromolar, respectively, in 50 millimolar Tris-citrate buffer (pH 4.7) at 30 C. The presence of adenosine and other cytokinin nucleosides inhibited the hydrolysis of N(6)-(Delta(2)-isopentenyl)adenosine but this reaction was insensitive to guanosine, uridine, or 3'-deoxyadenosine. It is hypothesized that an adequate level of "active cytokinin" in plant cells may be provided through the deribosylation of cytokinin riboside in concert with other cytokinin metabolic enzymes.

Journal Article↗
PubMed1979

Effects of verapamil on rapid Na channel-dependent action potentials of K+-depolarized ventricular fibers.

We studied the effects of 1 to 3 micrograms/ml of verapamil on Vmax and plateau duration in guinea-pig papillary muscles depolarized from -90 to -55 mV by increasing extracellular potassium from 5.4 to 20 mM. Under the conditions of our experiments, we found that verapamil did not influence the steady-state or recovery characteristics of Vmax at any of the studied K levels even when Vmax was less than 20 V/sec. Thus in the absence of rapid sodium channel blockade, the value of Vmax cannot be used to identify slow channel-dependent action potentials. Verapamil caused no shortening of plateau or total action potential duration when potassium was less than 7.5 mM. Above this level, verapamil caused progressive shortening of plateau and total action potential duration, due to the increase in potassium and not the associated decrease in resting membrane potential. Increasing extracellular calcium shortened plateau duration at all levels of potassium before verapamil but lengthened plateau duration in the high K, verapamil-treated fibers. These results, which can be explained by known effect of verapamil on the slow outward as well as slow inward currents, provide a mechanism whereby verapamil may increase Vmax of K-depolarized but rapid sodium current-dependent premature action potentials.

Action Potentials↗
PubMed1978

Cytokinin biosynthesis in cultured rootless tobacco plants.

Biosynthesis of cytokinin in shoots was examined by growing rootless tobacco (Nicotiana tabacum) plants in vitro. The rootless plants were originated by culturing tobacco callus on a high cytokinin-low auxin medium to induce the formation of plantlets which were then grown on medium without exogenous cytokinin and auxin. The rootless plants supplied with [(14)C]adenine synthesized ethanol-ethyl acetate-water-soluble radioactive components, portions of which had the same chromatographic and electrophoretic mobilities as N(6)-(Delta(2)-isopentenyl)adenine, N(6)-(Delta(2)-isopentenyl)adenosine, 6-(4-hydroxy-3-methyl-2-butenylamino)purine and 6-(4-hydroxy-3-methyl-2-butenylamino)-9-beta-d-ribofuranosylpurine. The total amount of these four major cytokinins was estimated to be present at a concentration of 14 to 23 nanomoles per kilogram of rootless plant. These data indicate that adenine serves as a precursor of the purine moiety of cytokinin molecules and that the cytokinin biosynthetic sites are also located in the shoot in addition to the presumed root sites.

Journal Article↗
PubMed1978

Metabolism of cytokinin: ribosylation of cytokinin bases by adenosine phosphorylase from wheat germ.

As part of the study of cytokinin metabolic pathways, an enzyme, adenosine phosphorylase (EC 2.4.2.-), which catalyzed the ribosylation of N(6)-(Delta(2)-isopentenyl)adenine, N(6)-furfuryladenine, and adenine to form the corresponding nucleosides, was partially purified from wheat (Triticum aestivum) germ. The pH optimum for the ribosylation of the cytokinins and adenine was from 6.5 to 7.8; for guanine and hypoxanthine it was from 7.0 to 8.5 At pH 7.2 (63 millimolar N-2-hydroxyethyl piperazine-N'-ethanesulfonic acid) and 37 C the K(m) for N(6)-(Delta(2)-isopentenyl)adenine was 57.1 micromolar; N(6)-furfuryladenine, 46.5 micromolar; adenine, 32.2 micromolar; and the V(max) for N(6)-(Delta(2)-isopentenyl)adenine, N(6)-furfuryladenine, and adenine were 134.7, 137.1, and 193.1 nanomoles per milligram protein per minute, respectively. The equilibrium constants of the phosphorolysis of N(6)-(Delta(2)-isopentenyl)adenosine and adenosine by this enzyme indicated that the reaction strongly favored nucleoside formation. This enzyme was shown to be distinct from inosine-guanosine phosphorylase based on the differences in the Sephadex G-100 gel filtration behaviors, pH optima, and the product and p-hydroxymercuribenzoate inhibitor studies. These results suggest that adenosine phosphorylase may play a significant role in the regulation of cytokinin metabolism.

Journal Article↗