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S Osváth

Publications and source records attributed to S Osváth.

13 recordsLinked to original sources
PubMed2001

Site specific labeling of Rhodobacter sphaeroides reaction centers with dye probes for surface pH measurements.

Covalently bound pH sensitive dyes are an important tool for characterizing the proteolytic reactions of protein complexes that play key roles in biological energy transduction. Here we demonstrate the feasibility of this method for photosynthetic reaction centers (RCs) for the first time, by the highly selective attachment of two thiol reactive derivatives of fluorescein to the two H subunit cysteines of the photosynthetic RC from Rhodobacter sphaeroides R-26 The pK(a) shifts of the dyes upon binding to the protein and in response to high salt were measured, and interpreted based on the structure of the RC. 2-[(5-fluoresceinyl)aminocarbonyl]ethyl-methanethiosulfonate was attached to Cys H156 and fluorescein-5-maleimide to Cys H234. By following the absorption changes of bound fluorescein (500 nm), and those of the hydrophilic pH indicator 8-hydroxypyrene-1,3,6-tris-sulfonic acid (468 nm), the surface and bulk pH were monitored separately with less than 5% crosstalk. Flash-induced proton uptake and external calibrations by mixing with aliquots of acid were measured in different redox states of the RCs. The results indicate that the charge in the quinone acceptor complex after flash activation (primary quinone acceptor (Q(A))- or secondary quinone acceptor (Q(B))-) has no effect on the surface pH and potential in the vicinity of these two attachment sites, between pH 6.5 and 9. Application of the method to other surface locations is discussed.

Arylsulfonates↗
PubMed1997

Coupling of cytochrome and quinone turnovers in the photocycle of reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides.

A minimal kinetic model of the photocycle, including both quinone (Q-6) reduction at the secondary quinone-binding site and (mammalian) cytochrome c oxidation at the cytochrome docking site of isolated reaction centers from photosynthetic purple bacteria Rhodobacter sphaeroides, was elaborated and tested by cytochrome photooxidation under strong continuous illumination. The typical rate of photochemical excitation by a laser diode at 810 nm was 2.200 s-1, and the rates of stationary turnover of the reaction center (one-half of that of cytochrome photooxidation) were 600 +/- 70 s-1 at pH 6 and 400 +/- 50 s-1 at pH 8. The rate of turnover showed strong pH dependence, indicating the contribution of different rate-limiting processes. The kinetic limitation of the photocycle was attributed to the turnover of the cytochrome c binding site (pH < 6), light intensity and quinone/quinol exchange (6 < pH < 8), and proton-coupled second electron transfer in the quinone acceptor complex (pH > 8). The analysis of the double-reciprocal plot of the rate of turnover versus light intensity has proved useful in determining the light-independent (maximum) turnover rate of the reaction center (445 +/- 50 s-1 at pH 7.8).

Animals↗
PubMed1994

Trapping magnetically oriented chloroplast thylakoid membranes in gels for electric measurements.

Electrochromic absorbance change and light gradient photovoltage measurements were carried out in chloroplast thylakoid membranes embedded in different compositions of gels. The goal was to find a system suitable for determining the dependence of the amplitude of the anomalous light gradient photovoltage signal, with opposite sign with respect to the ordinary signal, on the alignment of membranes. We found that polyacrylamide gel drastically increased the permeability of membranes which rendered electric measurements impossible in this gel. Agarose gel was successfully used in electric measurements. We will show that the light gradient photovoltage induced by a 580 nm laser flash depends strongly on the alignment of membranes with respect to the direction of the excitation beam. With face-aligned membranes the amplitude of the anomalous (negative) signal was drastically diminished, and the ordinary (positive) signal became clearly discernible. The observed differences in the decay of the two signals are tentatively explained by differences in the rates of ionic equilibration between different subcompartments of the inner aqueous phase of the thylakoid membrane system.

Acrylamides↗
PubMedDate not supplied

Intratubular germ cell tumours of pig testicles.

In the course for the histopathological examination of testicles of 30 castrated boars the authors have observed in 4 animals the intratubular growth of atypical germ cells. On the basis of the histological picture this change is equivalent to the human infertile testicles, observed as carcinoma in situ, resp. intratubular germ cell tumour. The described phenomenon occurred in atrophic testicles in 3 of the 4 old animals. The described change in the domestic pig, not known so far, raises the possibility of development of solid testis tumours in this species, as well.

Animals↗