1,048 research outputs found
Highly Purified Liver Microsomal Cytochrome P450: Properties and Catalytic Mechanism
Recent studies in this laboratory on two forms of cytochrome
P450 purified to homogeneity from rabbit liver microsomes are
reviewed. The two forms, phenobarbital-inducible P450LM2 and
5,6-benzoflavone-inducible P450LM4, differ in subunit molecular
weight, identity of the C-terminal amino acid, optical and EPR
spectra, and other properties. As isolated, oxidized P450LM2 is in
the low spin state, whereas P450LM4 is largely, but non entirely, in
the high spin state. Mechanistic studies have shown the following:
(a) P450LM2 may accept two electrons, calculated per heme, from
dithionite or NADPH in the presence of catalytic amounts of the
reductase, and may donate two electrons to various oxidizing agents,
including molecular oxygen. (b) Hydrogen peroxide is formed in
the reconstituted system in the presence of NADPH and oxygen, and
the amount varies with the substrate added. (c) Hydrogen peroxide
and other hydroperoxides apparently donate the oxygen atom
inserted into substrate during hydroxylation in the absence of 0 2
and an external donor. (d) Stopped flow spectrophotometry has
provided evidence for two distinct oxygenated complexes of the
reduced cytochrome. The reductase and cytochrome b5 may play
an effector role in increasing the rate of decomposition of the
second complex during oxygen insertion into substrate. A scheme
is proposed for the mechanism of action of purified P450LM2, based
on these and other findings
Highly Purified Liver Microsomal Cytochrome P450: Properties and Catalytic Mechanism
Recent studies in this laboratory on two forms of cytochrome
P450 purified to homogeneity from rabbit liver microsomes are
reviewed. The two forms, phenobarbital-inducible P450LM2 and
5,6-benzoflavone-inducible P450LM4, differ in subunit molecular
weight, identity of the C-terminal amino acid, optical and EPR
spectra, and other properties. As isolated, oxidized P450LM2 is in
the low spin state, whereas P450LM4 is largely, but non entirely, in
the high spin state. Mechanistic studies have shown the following:
(a) P450LM2 may accept two electrons, calculated per heme, from
dithionite or NADPH in the presence of catalytic amounts of the
reductase, and may donate two electrons to various oxidizing agents,
including molecular oxygen. (b) Hydrogen peroxide is formed in
the reconstituted system in the presence of NADPH and oxygen, and
the amount varies with the substrate added. (c) Hydrogen peroxide
and other hydroperoxides apparently donate the oxygen atom
inserted into substrate during hydroxylation in the absence of 0 2
and an external donor. (d) Stopped flow spectrophotometry has
provided evidence for two distinct oxygenated complexes of the
reduced cytochrome. The reductase and cytochrome b5 may play
an effector role in increasing the rate of decomposition of the
second complex during oxygen insertion into substrate. A scheme
is proposed for the mechanism of action of purified P450LM2, based
on these and other findings
Kondo Screening and Magnetic Ordering in Frustrated UNi4B
UNi4B exhibits unusual properties and, in particular, a unique
antiferromagnetic arrangement involving only 2/3 of the U sites. Based on the
low temperature behavior of this compound, we propose that the remaining 1/3 U
sites are nonmagnetic due to the Kondo effect. We derive a model in which the
coexistence of magnetic and nonmagnetic U sites is the consequence of the
competition between frustration of the crystallographic structure and
instability of the 5f moments.Comment: 4 pages, 2 figure
Domain Wall Spin Dynamics in Kagome Antiferromagnets
We report magnetization and neutron scattering measurements down to 60 mK on
a new family of Fe based kagome antiferromagnets, in which a strong local spin
anisotropy combined with a low exchange path network connectivity lead to
domain walls intersecting the kagome planes through strings of free spins.
These produce unfamiliar slow spin dynamics in the ordered phase, evolving from
exchange-released spin-flips towards a cooperative behavior on decreasing the
temperature, probably due to the onset of long-range dipolar interaction. A
domain structure of independent magnetic grains is obtained that could be
generic to other frustrated magnets.Comment: 5 pages, 4 figure
Kinetics of nitrite reduction and peroxynitrite formation by ferrous heme in human cystathionine β-synthase
Cystathionine β-synthase (CBS) is a pyridoxal phosphate-dependent enzyme that catalyzes the condensation of homocysteine with serine or with cysteine to form cystathionine and either water or hydrogen sulfide, respectively. Human CBS possesses a noncatalytic heme cofactor with cysteine and histidine as ligands, which in its oxidized state is relatively unreactive. Ferric CBS (Fe(III)-CBS) can be reduced by strong chemical and biochemical reductants to Fe(II)-CBS, which can bind carbon monoxide (CO) or nitric oxide (NO·), leading to inactive enzyme. Alternatively, Fe(II)-CBS can be reoxidized by O2 to Fe(III)-CBS, forming superoxide radical anion (O2 -·). In this study, we describe the kinetics of nitrite (NO2·-) reduction by Fe(II)-CBS to form Fe(II)NO·-CBS. The second order rate constant for the reaction of Fe(II)-CBS with nitrite was obtained at low dithionite concentrations. Reoxidation of Fe(II)NO·-CBS by O2 showed complex kinetic behavior and led to peroxynitrite (ONOO-) formation, which was detected using the fluorescent probe, coumarin boronic acid. Thus, in addition to being a potential source of superoxide radical, CBS constitutes a previously unrecognized source of NO· and peroxynitrite
Formation of collective spins in frustrated clusters
Using magnetization, specific heat and neutron scattering measurements, as
well as exact calculations on realistic models, the magnetic properties of the
\lacuvo compound are characterized on a wide temperature range. At high
temperature, this oxide is well described by strongly correlated atomic =1/2
spins while decreasing the temperature it switches to a set of weakly
interacting and randomly distributed entangled pseudo spins and
. These pseudo-spins are built over frustrated clusters, similar to
the kagom\'e building block, at the vertices of a triangular superlattice, the
geometrical frustration intervening then at different scales.Comment: 10 page
Candida albicans Hypha Formation and Mannan Masking of β-Glucan Inhibit Macrophage Phagosome Maturation
Received 28 August 2014 Accepted 28 October 2014 Published 2 December 2014 This is an open-access article distributed under the terms of the Creative Commons Attribution 3.0 Unported license. ACKNOWLEDGMENTS We thank Janet Willment, Aberdeen Fungal Group, University of Aberdeen, for kindly providing the soluble Dectin-1-Fc reporter. All microscopy was performed with the assistance of the University of Aberdeen Core Microscopy & Histology Facility, and we thank the IFCC for their assistance with flow cytometry. We thank the Wellcome Trust for funding (080088, 086827, 075470, 099215, 097377, and 101873). E.R.B. and A.J.P.B. are funded by the European Research Council (ERC-2009-AdG-249793), and J.L. is funded by a Medical Research Council Clinical Training Fellowship.Peer reviewedPublisher PD
Hidden magnetic frustration by quantum relaxation in anisotropic Nd-langasite
The static and dynamic magnetic properties of the NdGaSiO
compound, which appears as the first materialization of a rare-earth
kagome-type lattice, were re-examined, owing to contradictory results in the
previous studies. Neutron scattering, magnetization and specific heat
measurements were performed and analyzed, in particular by fully taking account
of the crystal electric field effects on the Nd ions. One of the novel
findings is that the peculiar temperature independent spin dynamics observed
below 10 K expresses single-ion quantum processes. This would short-circuit the
frustration induced cooperative dynamics, which would emerge only at very low
temperature
Magnetic susceptibility of diluted pyrochlore and SCGO antiferromagnets
We investigate the magnetic susceptibility of the classical Heisenberg
antiferromagnet with nearest-neighbour interactions on the geometrically
frustrated pyrochlore lattice, for a pure system and in the presence of
dilution with nonmagnetic ions. Using the fact that the correlation length in
this system for small dilution is always short, we obtain an approximate but
accurate expression for the magnetic susceptibility at all temperatures. We
extend this theory to the compound SrCr_{9-9x}Ga_{3+9x}O_{19} (SCGO) and
provide an explanation of the phenomenological model recently proposed by
Schiffer and Daruka [Phys. Rev. B56, 13712 (1997)].Comment: 4 pages, Latex, 4 postscript figures automatically include
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