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D G Lewis

Publications and source records attributed to D G Lewis.

At least 19 recordsLinked to original sources
PubMed1994

Optimization of a polarized source for in vivo x-ray fluorescence analysis of platinum and other heavy metals.

The Monte Carlo method was used to optimize a polarized photon source for the x-ray fluorescence analysis of platinum and other heavy metals in vivo. The source consisted of a 140 kVp, 25 mA x-ray tube with the photons plane-polarized by 90 degrees scattering. The use of plane-polarized photons results in a significant reduction in background when the fluorescent radiation is measured along the direction of polarization. A Monte Carlo computer programme was written to simulate the production and interaction of polarized photons in order to determine the optimal polarizing material and dimensions, together with beam width and geometrical arrangement of source, polarizer and beam collimators. Calculated photon energy distributions are compared with experimental data to test the validity of the model. The best configuration of the polarization system for the in vivo analysis of platinum consisted of a 20 mm Cu polarizing block with a secondary collimator subtending a 0.1 radian angle at the polarizer.

Humans↗
PubMed1992

A megavoltage CT scanner for radiotherapy verification.

We have further developed a system for generating megavoltage CT images immediately prior to the administration of external beam radiotherapy. The detector is based on the scanner of Simpson (Simpson et al 1982)--the major differences being a significant reduction in dose required for image formation, faster image formation and greater convenience of use in the clinical setting. Attention has been paid to the problem of ring artefacts in the images. Specifically, a Fourier-space filter has been applied to the sinogram data. After suitable detector calibration, it has been shown that the device operates close to its theoretical specification of 3 mm spatial resolution and a few percent contrast resolution. Ring artefacts continue to be a major source of image degradation. A number of clinical images have been presented. The next stage of this work is to use the system to make clinical measurements of patient set-up inaccuracies building on our work making such measurements from digital portal images (Evans et al 1992).

Evaluation Studies as Topic↗
PubMed1992

Localization of diphtheria toxin nuclease activity to fragment A.

We describe a series of experiments that aimed to establish whether nuclease activity is actually associated with diphtheria toxin (DTx) and its A subunit (DTA), as we originally reported (M. P. Chang, R. L. Baldwin, C. Bruce, and B. J. Wisnieski, Science 246:1165-1168, 1989). Here we show that (i) trypsinization of DTx does indeed produce nucleolytically active DTA, (ii) reduction of electroeluted, unreduced, cleaved DTx (58 kDa) yields nuclease-active DTA (24 kDa), and (iii) fractionation of DTx and DTA by anion-exchange chromatography leads to coelution of nuclease activity with both forms of the toxin, even though each form elutes at a distinct salt concentration. In addition, we show that Escherichia coli-derived DTA also expresses nuclease activity. These studies confirm our initial assertion that the nuclease activity observed in DTx preparations is intrinsic to the DTA portion of DTx.

Chromatography, Ion Exchange↗
PubMed1988

Structure of the pseudomonad fungal antibiotic phenazine-1-carboxylic acid.

C13H8N2O2, Mr = 224.2, monoclinic, Cc, a = 3.955 (1), b = 19.278 (4), c = 13.468 (1) A, beta = 98.90 (2) degrees, V = 1015 (2) A3, Z = 4, D chi = 1.468 Mg m-3, lambda (Mo K alpha) = 0.7107 A, mu = 0.061 mm-1, F(000) = 464, T = 293 (2) K, R = 0.047 for 571 observed reflections. The crystal-structure determination of the title compound, a phenazine antibiotic from Pseudomonas fluorescens 2-79 (NRRL B-15132), confirms its structure as phenazine-1-carboxylic acid. The molecular packing is described by discrete stacks of molecules parallel to the a axis with the distance between the essentially planar molecules being ca 3.96 A; there are no significant intermolecular contacts in the lattice.

Anti-Bacterial Agents↗
PubMed1981

The structure determination of rabbit phosphoglucomutase.

Tetragonal crystals of rabbit phosphoglucomutase have been grown from solutions containing ammonium sulphate, polyethylene glycol solution and enzyme. There are two molecules, each of relative molecular mass 64 000 per asymmetric unit. A rotation function suggests that these are related by a twofold axis. X-ray diffraction data for five heavy-atom derivatives and native crystals have been collected by using oscillation photography. A tentative and partial solution of the KAu(CN)2 sites has been obtained. The enzyme in the native crystals is phosphorylated, but the phosphate can be removed without harm to the crystals. Similarly the essential Mg2+ ion can be removed or replaced by Zn2+. The enzyme is active in the native crystals.

Animals↗
PubMed1980

Thyrotropin-releasing hormone-receptor interaction in GH3 pituitary cells.

The interaction between TRH and GH3 pituitary tumor cells was studied in monolayer cultures or membrane-containing fractions. In intact cells, the apparent dissociation constant (Kd) was approximately 10 nM, and the total number of binding sites was approximately 1.4 pmol/mg protein at temperatures of 0-37 C. In broken cell preparations, the number of sites occupied at saturating TRH concentrations was reduced by half when the temperature was increased from 0 to 30 C. Linear Scatchard plots were obtained under all conditions. The rate of dissociation of TRH was temperature dependent, and first order plots were nonlinear. The half-times for dissociation at 0 C were over 240 min in cells and membranes, while at 37 C, the half-time values were 24 min (cells) and less than 0.5 min (membranes). Identical dissociation kinetics were obtained by dilution alone or dilution with excess unlabeled hormone. When cultures which had been incubated with TRH were lightly fixed with glutaraldehyde, dissociation at 37 C became immeasurably slow. However, TRH dissociated immediately when sodium dodecyl sulfate, ethanol, or acetone was added, indicating that the tripeptide was not covalently coupled to cell proteins. The data indicate that binding of TRH to high affinity GH3 receptors is not cooperative, and that the majority of TRH bound after short incubations is dissociable.

Animals↗
PubMed1978

Solubilization of pituitary receptors for thyrotropin-releasing hormone.

Receptors for thyrotropin-releasing hormone were solubilized by Triton X-100. Membrane fractions from GH3 pituitary tumor cells were incubated with thyrotropin-releasing hormone in order to saturate specific receptor sites before the addition of detergent. The amount of protein-bound hormone solubilized by Triton X-100 was proportional to the fractional saturation of specific membrane receptors. Increasing detergent:protein ratios from 0.5 to 20 led to a progressive loss of hormone . receptor complex from membrane fractions with a concomitant increase in soluble protein-bound hormone. The soluble hormone . receptor complex was not retained by 0.22 micron filters and remained soluble after ultracentrifugation. Following incubation with high (2.5--10%) concentrations of Triton X-100 and other non-ionic detergents, or following repeated detergent extraction, at least 18% of specifically bound thyrotropin-releasing hormone remained associated with particulate material. Unlike the hormone receptor complex, the free hormone receptor was inactivated by Triton X-100. A 50% loss of binding activity was obtained with 0.01% Triton X-100, corresponding to a detergent:protein ratio of 0.033. The hormone . receptor complex was included in Sepharose 6B and exhibited an apparent Stoke radius of 46 A in buffers containing Triton X-100. The complex aggregated in detergent-free buffers. Soluble hormone receptors were separated from excess detergent and thyrotropin-releasing hormone by chromatography on DEAE-cellulose. Thyrotropin-releasing hormone dissociated from soluble receptors with a half-time of 120 min at 0 degrees C, while the membrane hormone . receptor complex was stable for up to 5 at 0 degrees C.

Cell Membrane↗
PubMed1978

Mosaicism and the trisomy 8 syndrome.

Three new cases of trisomy 8 mosaicism are presented; two have features corresponding with those usually found in this syndrome, whereas one is highly atypical. In view of the almost universal mosaicism of these patients, the literature is reviewed with an emphasis on the patterns of mosaicism found. There is little correlation between degree of mosaicism and extent of clinical abnormality. The degree of mosaicism differs in different tissues, fibroblasts being more informative of aneuploidy than lymphocytes, and there is some evidence that the degree of mosaicism varies with time. The reasons for these findings are discussed, with particular reference to the raised paternal age found in a proportion of the reported cases.

Abnormalities, Multiple↗
PubMed1976

Electronic tooth stimulator for pain research.

It is not possible to measure pain but it is possible to measure the stimulus that causes it. Electrical stimuli are probably the most suitable since they can be readily produced and accurately measured. Applying stimuli to the teeth of human subjects is convenient and does not cause damage. This paper describes a tooth stimulator which is considered to be an advance on other available equipment, being easier to use and more accurate, as well as reassuring for the human patient or experimental subject.

Electric Stimulation↗