Biomedical subjects

Maria Stoimenova

Publications and source records attributed to Maria Stoimenova.

7 recordsLinked to original sources

PubMed2006

Scaling procedures in colloidal electro-optics.

In a series of recent papers, we demonstrate the possibility to use the complete electro-optic saturation curves (or arbitrary part of them) to analyse the polarization mechanisms of charged colloidal particles. Scaling of the electro-optic saturation curves by field intensity permits to find the frequency range in which the functional dependence of the effects, hence the orientation mechanism remains unchanged. Similar procedure can be applied to verify the changes of this dependence with variation of particle electric parameters. When different polarization mechanisms are involved the new scale helps their analysis. The aim of the present paper is to summarize the basic applications and advantages of the scaling method in the electro-optic investigation of charged colloids.

Colloids↗

PubMed2006

Steady electro-optic characteristics of noninteracting colloidal particles.

In previous papers on the electro-optic effects of beta-FeOOH particles we proposed a new procedure for analysis of the low frequency behavior of charged particles. The procedure is based on comparison of characteristic field intensity curves on an appropriate scale and helps to test the dependence of the slow effects on particle surface electric polarizability (relaxing in the kilohertz domain). The results stimulated us to test the applicability of the method to other samples and to reconsider the literature data on the electro-optic behavior of charged colloids in the hydrodynamic domain. The aim of the present paper is to demonstrate on a series of samples similar features of the electro-optic responses of charged particles in the relaxation interval of particle rotation. The analysis leads to a new hypothesis for explanation of the complicated low frequency behavior of charged particles. The superposition of two slow effects (linear and quadratic with field intensity), relaxing in the relaxation interval of particle rotation, can explain the complicated frequency curves in this domain. One of the slow effects is observed for all polarizable particles. It is of negative sign and displays the features of an induced dipole effect dependent on the "kilohertz" induced moment. It corresponds to a slow stage of the surface polarization process related to electrokinetic charge. The linear slow effect shows permanent dipole like behavior and appears only at certain ionic content of the medium. It shows no direct dependence on counterion mobility and on the "kilohertz" induced moment and is probably due to surface charge nonuniformity.

Journal Article↗

PubMed2005

Class-1 hemoglobin and antioxidant metabolism in alfalfa roots.

In the course of nitric oxide (NO) scavenging, hemoglobin (Hb) turnover is linked to antioxidant metabolism and affects the cellular redox level. The influence of Hb presence on the ascorbate-glutathione cycle enzymes and the levels of H(2)O(2) and ascorbate was investigated in alfalfa root cultures transformed to over-express (Hb+) or down-regulate (Hb-) class-1 Hb. Hb+ lines had substantially increased ascorbate levels as well as elevated monodehydroascorbate reductase and ascorbate peroxidase activities. Hb- lines showed significant increases in dehydroascorbate reductase and glutathione reductase activities. The observed changes in ascorbate and ascorbate-glutathione cycle enzymes were pronounced both at high (40 kPa) and low (3 kPa) O(2) pressures. Hb- lines had significantly reduced levels of the NO- and H(2)O(2)-sensitive enzyme, aconitase, as compared to Hb+ lines. This reduced activity was likely due the higher levels of NO in Hb- lines, as treatment of plant extracts with the NO-donor DEANO also affected aconitase activity. The H(2)O(2) levels were not significantly different amongst the lines and showed no variation with change in oxygen partial pressure. In conclusion, the expression of class-1 Hb improves the antioxidant status through increased ascorbate levels and increased activity of enzymes involved in H(2)O(2) removal.

Aconitate Hydratase↗

PubMed2005

In higher plants, only root mitochondria, but not leaf mitochondria reduce nitrite to NO, in vitro and in situ.

At oxygen concentrations of < or =1%, even completely nitrate reductase (NR)-free root tissues reduced added nitrite to NO, indicating that, in roots, NR was not the only source for nitrite-dependent NO formation. By contrast, NR-free leaf slices were not able to reduce nitrite to NO. Root NO formation was blocked by inhibitors of mitochondrial electron transport (Myxothiazol and SHAM), whereas NO formation by NR-containing leaf slices was insensitive to the inhibitors. Consistent with that, mitochondria purified from roots, but not those from leaves, reduced nitrite to NO at the expense of NADH. The inhibitor studies suggest that, in root mitochondria, both terminal oxidases participate in NO formation, and they also suggest that even in NR-containing roots, a large part of the reduction of nitrite to NO was catalysed by mitochondria, and less by NR. The differential capacity of root and leaf mitochondria to reduce nitrite to NO appears to be common among higher plants, since it has been observed with Arabidopsis, barley, pea, and tobacco. A specific role for nitrite to NO reduction in roots under anoxia is discussed.

Methacrylates↗

PubMed2005

The haemoglobin/nitric oxide cycle: involvement in flooding stress and effects on hormone signalling.

BACKGROUND: Class 1 haemoglobins (Hbs) are induced in plant cells under hypoxic conditions. They have a high affinity for oxygen, which is two orders of magnitude lower than that of cytochrome oxidase, permitting the utilization of oxygen by the molecule at extremely low oxygen concentrations. Their presence reduces the levels of nitric oxide (NO) that is produced from nitrate ion during hypoxia and improves the redox and energy status of the hypoxic cell. SCOPE: The mechanism by which Hb interacts with NO under hypoxic conditions in plants is examined, and the effects of Hb expression on metabolism and signal transduction are discussed. CONCLUSIONS: The accumulated evidence suggests that a metabolic pathway involving NO and Hb provides an alternative type of respiration to mitochondrial electron transport under limited oxygen. Hb in hypoxic plants acts as part of a soluble, terminal, NO dioxygenase system, yielding nitrate ion from the reaction of oxyHb with NO. NO is mainly formed due to anaerobic accumulation of nitrite. The overall reaction sequence, referred to as the Hb/NO cycle, consumes NADH and maintains ATP levels via an as yet unknown mechanism. Hb gene expression appears to influence signal transduction pathways, possibly through its effect on NO, as evidenced by phenotypic changes in normoxic Hb-varying transgenic plants. Ethylene levels are elevated when Hb gene expression is suppressed, which could be a factor leading to root aerenchyma formation during hypoxic stress.

Disasters↗

PubMed2004

Electro-optic characteristics of optically interacting beta-FeOOH particles.

In this article the influence of multiple light scattering on the basic electro-optic parameters of optically dense colloidal particles is analyzed. The model system is an aqueous suspension of monodisperse ellipsoidal beta-FeOOH particles that displays large electric light scattering variations, including sign reversal, at very low particle volume fractions (two orders of magnitude below the critical concentration of particle electric interactions). The scaling method permits the relative variations in particle electric polarizability to be followed and its relaxation frequency to be determined. Particle rotational relaxation frequency and the phase shift of the responses at this frequency are obtained by the alternating component of the effects. Characteristic field intensity curves in the low-frequency range are used to follow the relative changes induced by the slow electrokinetic effect. The experimental results show that, despite the drastic variations in the effects with volume fraction, the basic electro-optic parameters are independent of multiple scattering and can be adequately determined for any particle concentration, excluding a narrow range in the vicinity of the electro-optic sign reversal. The investigation demonstrates that the dependence of the frequency behavior of aqueous beta-FeOOH on particle volume fraction reported in the literature is due not to optical interactions but to variation of particle surface electric state in the process of dilution.

Journal Article↗

PubMed2004

Electro-optic characteristics of aqueous beta-FeOOH particles.

This paper presents a detailed analysis of the electro-optic behavior of suspensions of noninteracting monodisperse beta-FeOOH particles. The electro-optic parameters are determined for aqueous suspensions of the oxide particles and the influences of surface charge and Debye layer thickness are verified. Since the conventional method of frequency analysis is inconsistent in the low-frequency range, new electro-optic parameters are introduced to define the frequency variation of the effects. Electric polarizability is determined with precision to a constant, and its relative variations are followed. As reported for other oxides, electric polarizability correlates with charge variations in the diffuse part of the particle surface electric layer, and its relaxation frequency increases with surface charge density, indicating a Maxwell-Wagner type of surface polarization. The alternating component of the responses yields particle relaxation frequency and the phase shift of the responses at this frequency. For all studied samples the phase shift at particle relaxation frequency is 45 degrees. The relative changes in the steady component of the responses in the low-frequency range are followed by field intensity curves at characteristic frequencies of the samples. Electrophoretic rotation is the process consistent with our data for the low-frequency effect. The results show that it is enhanced by the combined actions of low or slowly relaxing polarizability and significant electrophoretic mobility.

Journal Article↗