Apoptosis regulates human vascular calcification in vitro: evidence for initiation of vascular calcification by apoptotic bodies. (41/518)

The mechanisms involved in the initiation of vascular calcification are not known, but matrix vesicles, the nucleation sites for calcium crystal formation in bone, are likely candidates, because similar structures have been found in calcified arteries. The regulation of matrix vesicle production is poorly understood but is thought to be associated with apoptotic cell death. In the present study, we investigated the role of apoptosis in vascular calcification. We report that apoptosis occurs in a human vascular calcification model in which postconfluent vascular smooth muscle cell (VSMC) cultures form nodules spontaneously and calcify after approximately 28 days. Apoptosis occurred before the onset of calcification in VSMC nodules and was detected by several methods, including nuclear morphology, the TUNEL technique, and external display of phosphatidyl serine. Inhibition of apoptosis with the caspase inhibitor ZVAD.fmk reduced calcification in nodules by approximately 40%, as measured by the cresolphthalein method and alizarin red staining. In addition, when apoptosis was stimulated in nodular cultures with anti-Fas IgM, there was a 10-fold increase in calcification. Furthermore, incubation of VSMC-derived apoptotic bodies with (45)Ca demonstrated that, like matrix vesicles, they can concentrate calcium. These observations provide evidence that apoptosis precedes VSMC calcification and that apoptotic bodies derived from VSMCs may act as nucleating structures for calcium crystal formation.  (+info)

The tetra-manganese complex of photosystem II during its redox cycle - X-ray absorption results and mechanistic implications. (42/518)

Using X-ray absorption spectroscopy (XAS), relevant information on structure and oxidation state of the water-oxidizing Mn complex of photosystem II has been obtained for all four semi-stable intermediate states of its catalytic cycle. We summarize our recent XAS results and discuss their mechanistic implications. The following aspects are covered: (a) information content of X-ray spectra (pre-edge feature, edge position, extended X-ray absorption fine-structure (EXAFS), dichroism in the EXAFS of partially oriented samples); (b) S(1)-state structure; (c) X-ray edge results on oxidation state changes; (d) EXAFS results on structural changes during the S-state cycle; (e) a structural model for the Mn complex in its S(3)-state; (f) XAS-based working model for the S(2)-S(3) transition; (g) XAS-based working model for the S(0)-S(1) transition; (h) potential role of hydrogen atom abstraction by the Mn complex. Finally, we present a specific hypothesis on the mechanism of dioxygen formation during the S(3)-(S(4))-S(0) transition. According to this hypothesis, water oxidation is facilitated by manganese reduction that is coupled to proton transfer from a substrate water to bridging oxides.  (+info)

Probing the Mn oxidation states in the OEC. Insights from spectroscopic, computational and kinetic data. (43/518)

Results from a variety of experimental techniques which have been used to define the oxidation levels of Mn and other components in the S states of the water oxidising complex in Photosystem II are reviewed. A self-consistent interpretation of Mn X-ray absorption near edge spectroscopy, UV-visible and near infrared spectroscopic data suggests that Mn oxidation occurs only on the S0-->S1 transition, and that all four Mn centres have formal oxidation state III thereafter. Ligand oxidation occurs on the transitions to S2 and S3. This is supported by high level quantum chemical calculations and an analysis of the kinetics of substrate water exchange, as recently determined by Wydrzynski et al. (this journal). One type of model for the catalytic site structure and water oxidation mechanism, consistent with these conclusions, is discussed. This model invokes magnetically separate oxo bridged dimers with water oxidation occurring by a concerted 2H+/2e- transfer mechanism, with one H transfer to a bridge oxygen on each dimer.  (+info)

On the enzymatic activation of NADH. (44/518)

Atomic (1 A) resolution x-ray structures of horse liver alcohol dehydrogenase in complex with NADH revealed the formation of an adduct in the active site between a metal-bound water and NADH. Furthermore, a pronounced distortion of the pyridine ring of NADH was observed. A series of quantum chemical calculations on the water-nicotinamide adduct showed that the puckering of the pyridine ring in the crystal structures can only be reproduced when the water is considered a hydroxide ion. These observations provide fundamental insight into the enzymatic activation of NADH for hydride transfer.  (+info)

Mobilization of selenite by Ralstonia metallidurans CH34. (45/518)

Ralstonia metallidurans CH34 (formerly Alcaligenes eutrophus CH34) is a soil bacterium characteristic of metal-contaminated biotopes, as it is able to grow in the presence of a variety of heavy metals. R. metallidurans CH34 is reported now to resist up to 6 mM selenite and to reduce selenite to elemental red selenium as shown by extended X-ray absorption fine-structure analysis. Growth kinetics analysis suggests an adaptation of the cells to the selenite stress during the lag-phase period. Depending on the culture conditions, the medium can be completely depleted of selenite. Selenium accumulates essentially in the cytoplasm as judged from electron microscopy and energy-dispersive X-ray analysis. Elemental selenium, highly insoluble, represents a nontoxic storage form for the bacterium. The ability of R. metallidurans CH34 to reduce large amounts of selenite may be of interest for bioremediation processes targeting selenite-polluted sites.  (+info)

Corrosion resistance of the Pt-Fe-Nb magnets for dental-casting. (46/518)

Magnetic attachments have been used in clinical dental practice, but there is some difficulties associated with removable bridges. One possible solution is to make whole bridges of Pt-Fe magnet alloys and its abutment out of magnetic stainless steel by casting. In terms of castability and magnetic properties, the promising composition of the Pt-Fe-Nb magnet alloy is Pt-30.0 mass% Fe-0.6 mass% Nb and Pt-30.0 mass% Fe-0.5 mass% Nb-0.03 mass% Si. In the present study, the corrosion resistance of these alloys was investigated based on the elusion test, electrochemical behavior and surface characterization by EPMA analysis. The released elements from the Pt-Fe-Nb magnets were mainly Fe ions in quantities similar to that of stainless steel for biomedical use, and the Pt-Fe-Nb magnet alloy, the Pt-Fe-Nb-Si magnet alloy and platinum resembled each other in electrochemical behavior. The present findings suggest, that the Pt-Fe-Nb magnet alloy provides excellent corrosion resistance and has important clinical dental applications.  (+info)

WDX study of resin-dentin interface on wet vs. dry dentin. (47/518)

The purpose of the present study was to determine the resin-dentin Interface conditions in Wet vs. Dry Dentin. Dentin disks were prepared from extracted human premolars. Sectioned dentin surfaces were used for SEM studies of wet vs. dried acid-etched dentin. These specimens were cut perpendicular to the surface into two equal halves. One-half of the sectioned specimen was observed by SEM in three treatment groups and the other half was observed for micromorphological differences in the resin-dentin interface using Wavelength Dispersive X-ray Spectrometer (WDX). SEM photomicrographs of the dentin surface showed the collapse of collagen fibrils in the demineralized layer and enlargement of the tubule orifices. A collagen rich layer approximately 8-10 microns thick (WDX) was observed at the resin-dentin interface when treated with the conventional dry-bonding technique. Dentin surfaces treated by the wet-bonding technique (SB), as observed by SEM, showed an uncollapsed collagen layer, while the collagen-rich layer was approximately 1-2 microns thick (WDX). The present findings suggest that moist bonding is required for optimum infiltration of adhesive resin into the demineralized layer.  (+info)

Surface composition and structure of titanium polished with aqueous slurry of ferric oxide. (48/518)

Cast plates were prepared from commercial titanium. The plates were polished with a slurry of fine ferric oxide powder. The surface composition and structure were investigated by electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). In the high pressure-polished surface, iron was non-uniformly distributed but oxygen was mostly uniformly distributed, while in a light pressure-polished surface, iron and oxygen were uniformly distributed though at lower and higher concentrations, respectively. EPMA state analysis and XPS suggested that the iron might exist as Fe2+ in the outermost surface, while it might be in a metallic state in the inner surface layer.  (+info)