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It is well known that effective exchange of metabolites between mitochondria and the cytoplasm is essential for cell physiology. The key step of the exchange is trans­port across the mitochondrial outer membrane, which is supported by the volt­age-dependent anion-selective channel (VDAC). Therefore, it is clear that the perme­ability of VDAC must be regulated to adjust its activity to the actual cell needs. VDAC-modulating activities, often referred to as the VDAC modulator, were identi­fied in the intermembrane space of different organism mitochondria but the responsi­ble protein(s) has not been identified as yet. Because the VDAC modulator was re­ported to act on VDAC of intact mitochondria when added to the cytoplasmic side it has been speculated that a similar modulating activity might be present in the cyto­plasm. To check the speculation we used mitochondria of the yeast Saccharomyces cerevisiae as they constitute a perfect model to study VDAC modulation. The mito­chondria contain only a single isoform of VDAC and it is possible to obtain viable mu­tants devoid of the channel (Δpor1). Moreover, we have recently characterised a VDAC-modulating activity located in the intermembrane space of wild type and Δpor1 S. cerevisiae mitochondria. Here, we report that the cytoplasm of wild type and Δpor1 cells of S. cerevisiae contains a VDAC-modulating activity as measured in a reconsti­tuted system and with intact mitochondria. Since quantitative differences were ob­served between the modulating fractions isolated from wild type and Δpor1 cells when they were studied with intact wild type mitochondria as well as by protein electrophoresis it might be concluded that VDAC may influence the properties of the involved cytoplasmic proteins. Moreover, the VDAC-modulating activity in the cytoplasm differs distinctly from that reported for the mitochondrial intermembranen space. Nevertheless, both these activities may contribute efficiently to VDAC regulation. Thus, the identification of the proteins is very important.
The purpose of this study was establishing the basic energetic parameters of amoeba Acanthamoeba castellaniimitochondria respiring with malate and their response to oxidative stress caused by hydrogen peroxide in the presence of Fe2+ions. It appeared that, contrary to a previous report (Trocha LK, Stobienia O (2007) Acta Biochim Polon 54: 797), H2O2-treated mitochondria of A. Castellanii did not display any substantial impairment. No marked changes in cytochrome pathway activity were found, as in the presence of an inhibitor of alternative oxidase no effects were observed on the rates of uncoupled and phosphorylating respiration and on coupling parameters. Only in the absence of the alternative oxidase inhibitor, non-phosphorylating respiration progressively decreased with increasing concentration of H2O2, while the coupling parameters (respiratory control ratio and ADP/O ratio) slightly improved, which may indicate some inactivation of the alternative oxidase. Moreover, our results show no change in membrane potential, Ca2+uptake and accumulation ability, mitochondrial outer membrane integrity and cytochrome crelease for 0.5–25 mM H2O2-treated versuscontrol (H2O2-untreated) mitochondria. These results indicate that short (5 min) incubation of A. Castellanii mitochondria with H2O2 in the presence of Fe2+ does not damage their basic energetics.
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