8,347 research outputs found
Characterization of theThreshold for NAD(P)H:quinone Oxidoreductase Activity in Intact Sulforaphane-treated Pulmonary Arterial Endothelial Cells
Treatment of bovine pulmonary arterial endothelial cells in culture with the phase II enzyme inducer sulforaphane (5 μM, 24 h; sulf-treated) increased cell-lysate NAD(P)H:quinone oxidoreductase (NQO1) activity by 5.7 ± 0.6 (mean ± SEM)-fold, but intact-cell NQO1 activity by only 2.8 ± 0.1-fold compared to control cells. To evaluate the hypothesis that the threshold for sulforaphane-induced intact-cell NQO1 activity reflects a limitation in the capacity to supply NADPH at a sufficient rate to drive all the induced NQO1 to its maximum activity, total KOH-extractable pyridine nucleotides were measured in cells treated with duroquinone to stimulate maximal NQO1 activity. NQO1 activation increased NADP+ in control and sulf-treated cells, with the effect more pronounced in the sulf-treated cells, in which the NADPH was also decreased. Glucose-6-phosphate dehydrogenase (G-6-PDH) inhibition partially blocked NQO1 activity in control and sulf-treated cells, but G-6-PDH overexpression via transient transfection with the human cDNA alleviated neither the restriction on intact sulf-treated cell NQO1 activity nor the impact on the NADPH/NADP+ ratios. Intracellular ATP levels were not affected by NQO1 activation in control or sulf-treated cells. An increased dependence on extracellular glucose and a rightward shift in the Km for extracellular glucose were observed in NQO1-stimulated sulf-treated vs control cells. The data suggest that glucose transport in the sulf-treated cells may be insufficient to support the increased metabolic demand for pentose phosphate pathway-generated NADPH as an explanation for the NQO1 threshold
Magnetic-Field Amplification in the Thin X-ray Rims of SN1006
Several young supernova remnants (SNRs), including SN1006, emit synchrotron
X-rays in narrow filaments, hereafter thin rims, along their periphery. The
widths of these rims imply 50 to 100 G fields in the region immediately
behind the shock, far larger than expected for the interstellar medium
compressed by unmodified shocks, assuming electron radiative losses limit rim
widths. However, magnetic-field damping could also produce thin rims. Here we
review the literature on rim width calculations, summarizing the case for
magnetic-field amplification. We extend these calculations to include an
arbitrary power-law dependence of the diffusion coefficient on energy, . Loss-limited rim widths should shrink with increasing photon
energy, while magnetic-damping models predict widths almost independent of
photon energy. We use these results to analyze Chandra observations of SN 1006,
in particular the southwest limb. We parameterize the full widths at half
maximum (FWHM) in terms of energy as FWHM . Filament
widths in SN1006 decrease with energy; to , implying
magnetic field amplification by factors of 10 to 50, above the factor of 4
expected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules
out magnetic damping models. It also favors short mean free paths (small
diffusion coefficients) and strong dependence of on energy ().Comment: Accepted by ApJ, 49 pages, 10 figure
GRB Spikes Could Resolve Stars
GRBs vary more rapidly than any other known cosmological phenomena. The lower
limits of this variability have not yet been explored. Improvements in
detectors would reveal or limit the actual rate of short GRBs. Were microsecond
"spike" GRBs to exist and be detectable, they would time-resolve stellar mass
objects throughout the universe by their gravitational microlensing effect.
Analyzing the time structure of sufficient numbers of GRB spikes would reveal
or limit , , and/or .Comment: 18 pages, 2 figures, in press: ApJ (Letters
An X-ray and Radio Study of the Varying Expansion Velocities in Tycho's Supernova Remnant
We present newly obtained X-ray and radio observations of Tycho's supernova
remnant using {\it Chandra} and the Karl G. Jansky Very Large Array in 2015 and
2013/14, respectively. When combined with earlier epoch observations by these
instruments, we now have time baselines for expansion measurements of the
remnant of 12-15 year in the X-rays and 30 year in the radio. The remnant's
large angular size allows for proper motion measurements at many locations
around the periphery of the blast wave. We find, consistent with earlier
measurements, a clear gradient in the expansion velocity of the remnant,
despite its round shape. The proper motions on the western and southwestern
sides of the remnant are about a factor of two higher than those in the east
and northeast. We showed in an earlier work that this is related to an offset
of the explosion site from the geometric center of the remnant due to a density
gradient in the ISM, and using our refined measurements reported here, we find
that this offset is towards the northeast. An explosion center
offset in such a circular remnant has implications for searches for progenitor
companions in other remnants.Comment: Accepted for publication in ApJ Letter
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