Proteasome-dependent turnover of protein disulfide isomerase in oxidatively stressed cells

Generalized increases in protein oxidation and protein degradation in response to mild oxidative stress have been widely reported, but only a few individual proteins have actually been shown to undergo selective, oxidation-induced proteolysis. Our goal was to find such proteins in Clone 9 liver cells exposed to hydrogen peroxide. Using metabolic radiolabeling of intracellular proteins with [35S]cysteine/methionine, and analysis by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE), we found at least three labeled proteins ("A," "B," and "C") whose levels were decreased significantly more than the generalized protein loss after mild oxidative stress. "Protein C" was excised from 2-D PAGE and subjected to N-terminal amino acid microsequencing. "Protein C" was identified as Protein Disulfide Isomerase or PDI (E.C. 5.3.4.1), and this identity was reconfirmed by Western blotting with a C-terminal anti-PDI monoclonal antibody. A combination of quantitative radiometry and Western blotting in 2-D PAGE revealed that PDI was selectively degraded and then new PDI was synthesized, following H2O2 exposure. PDI degradation was blocked by inhibitors of the proteasome, and by cell treatment with proteasome C2 subunit antisense oligonucleotides, indicating that the proteasome was largely responsible for oxidation-induced PDI degradation.

Reduced neutrophil sequestration in lung tissue after laparoscopic lavage in a rat peritonitis model

Laparoscopy to treat abdominal infections is becoming more and more popular. The effects of the CO(2) pneumoperitoneum have not yet been completely clarified. In a rat peritonitis model, therefore, we investigated the influence of laparoscopic lavage in comparison with the conventional technique. A defined multibacterial fecal specimen was installed in the abdominal cavities of 80 rats. These animals were randomized to three groups: group 1 (n = 32), no intervention; group 2 (n = 24), conventional; group 3 (n = 24), laparoscopic lavage. At 1, 2, and 8 hours after the surgical intervention, animals were killed and autopsied. The main outcome measures were bacteremia, interleukin-6 (IL-6) in plasma and ascites, changes in the blood count, and myeloperoxidase (MPO) activity in lung, liver, kidney, and pancreas. Differences of bacteremia were not found. In the ascites a marked increase in IL-6 was observed after 8 hours, which was lower in the treatment groups than in the controls (p <0.025). MPO activity as a measure of the granulocytes present in the tissue showed significant changes only in lung tissue. Two hours after the surgical intervention, the MPO in the lung in the laparoscopy group was significantly lower than that in the controls and the laparotomy group. In conclusion, conventional and laparoscopic lavage reduce inflammation. In this model, laparoscopic lavage with a CO(2) pneumoperitoneum appeared to have no negative influence on the inflammatory reaction during the early postoperative phase. Reduced neutrophil sequestration in lung tissue following laparoscopic lavage reflects the lower level of trauma caused by laparoscopy.

Towards specific functions of lysosomal cysteine peptidases: phenotypes of mice deficient for cathepsin B or cathepsin L

The lysosomal cysteine peptidases cathepsin B and cathepsin L are abundant and ubiquitously expressed members of the papain family, and both enzymes contribute to the terminal degradation of proteins in the lysosome. However, there is accumulating evidence for specific functions of lysosomal proteases in health and disease. The generation of 'knock out' mouse strains that are deficient in lysosomal proteases provides a valuable tool for evaluation of existing hypotheses and gaining new insights into the in vivo functions of these proteases. In this minireview, we summarise and discuss the findings obtained by analysis of mice that are devoid of cathepsin B or cathepsin L. In brief, cathepsin L appears to be critically involved in epidermal homeostasis, regulation of the hair cycle, and MHC class II-mediated antigen presentation in cortical epithelial cells of the thymus. Cathepsin B plays a major role in pathological trypsinogen activation in the early course of experimental pancreatitis and contributes significantly to TNF-alpha induced hepatocyte apoptosis.

Differential oxidative injury in extrapancreatic tissues during experimental pancreatitis: modification of lung proteins by 4-hydroxynonenal

Oxidative stress is considered to be a pathogenic factor for multisystem organ failure during acute pancreatitis. Infusion of 3% and 5% sodium taurocholate into the pancreatic duct of rats resulted in a 24-hr lethality of 8% and 82%, respectively. Kidney tissue showed a long-lasting significant elevation of malondialdehyde (lipid peroxidation). Only small amounts of this aldehyde were formed in the liver. In the lung malondialdehyde was increased during the first 6 hr after pancreatitis induction. Malondialdehyde levels were not different for pancreatitis initiated by 3% or 5% taurocholate. Protein-bound carbonyls (protein oxidation) in the tissues were not significantly changed at any time point. However, after infusion of 5% taurocholate, lung proteins were oxidatively modified by the product of lipid peroxidation, 4-hydroxynonenal. Another parameter characteristic for pancreatitis with high lethality was the high number of neutrophils in the lungs. We conclude that oxidative stress is important for the injury of extrapancreatic tissues during pancreatitis, but survival is determined by the degree of systemic inflammation.

Oxidative stress in lung tissue induced by CO(2) pneumoperitoneum in the rat

BACKGROUND

Clinical trials have found that the pneumoperitoneum has potentially hazardous side effects. The biochemical basis of organ injury induced by pneumoperitoneum is, however, not well defined. Since oxidative stress is believed to play an important role in many pathological conditions, we set out to examine oxidative stress markers in the lung, liver, kidney, and pancreas by using a rat model of laparoscopy with CO(2) pneumoperitoneum and comparing it to a group with gasless laparoscopy.

METHODS

Malondialdehyde (for lipid peroxidation), protein-bound carbonyls (for protein oxidation), reduced and oxidized glutathione, and the neutrophil marker myeloperoxidase were evaluated in tissue homogenates at 2 h, 6 h, and 18 h after laparoscopy. Immunoblotting was used to analyze the modification of lung proteins by 4-hydroxynonenal at 6 h.

RESULTS

Significant lipid peroxidation was found selectively in lungs at 2 h and 6 h after CO(2) pneumoperitoneum. This was accompanied by a loss of glutathione but only minor protein oxidation. Further, lung proteins were clearly modified by the aldehydic product of lipid peroxidation 4-hydroxynonenal. Myeloperoxidase in lungs increased continuously up to 18 h in both experimental groups, but there were higher levels in the group with pneumoperitoneum.

CONCLUSION

Oxidative stress is likely to contribute to the impairment of pulmonary function after laparoscopic operations using a CO(2) pneumoperitoneum.

Role of cathepsin B in intracellular trypsinogen activation and the onset of acute pancreatitis

Autodigestion of the pancreas by its own prematurely activated digestive proteases is thought to be an important event in the onset of acute pancreatitis. The mechanism responsible for the intrapancreatic activation of digestive zymogens is unknown, but a recent hypothesis predicts that a redistribution of lysosomal cathepsin B (CTSB) into a zymogen-containing subcellular compartment triggers this event. To test this hypothesis, we used CTSB-deficient mice in which the ctsb gene had been deleted by targeted disruption. After induction of experimental secretagogue-induced pancreatitis, the trypsin activity in the pancreas of ctsb(-/-) animals was more than 80% lower than in ctsb(+/+) animals. Pancreatic damage as indicated by serum activities of amylase and lipase, or by the extent of acinar tissue necrosis, was 50% lower in ctsb(-/-) animals. These experiments provide the first conclusive evidence to our knowledge that cathepsin B plays a role in intrapancreatic trypsinogen activation and the onset of acute pancreatitis.

Differential impairment of 20S and 26S proteasome activities in human hematopoietic K562 cells during oxidative stress

The 20S proteasome and the 26S proteasome are major components of the cytosolic and nuclear proteasomal proteolytic systems. Since proteins are known to be highly susceptible targets for reactive oxygen species, the effect of H(2)O(2) treatment of K562 human hematopoietic cells toward the activities of 20S and 26S proteasomes was investigated. While the ATP-independent degradation of the fluorogenic peptide suc-LLVY-MCA was not affected by H(2)O(2) concentrations of up to 5 mM, the ATP-stimulated degradation of suc-LLVY-MCA by the 26S proteasome began to decline at 400 microM and was completely abolished at 1 mM oxidant treatment. A combination of nondenaturing electrophoresis and Western blotting let us believe that the high oxidant susceptibility of the 26S proteasome is due to oxidation of essential amino acids in the proteasome activator PA 700 which mediates the ATP-dependent proteolysis of the 26S-proteasome. The activity of the 26S-proteasome could be recovered within 24 h after exposure of cells to 1 mM H(2)O(2) but not after 2 mM H(2)O(2). In view of the specific functions of the 26S proteasome in cell cycle control and other important physiological functions, the consequences of the higher susceptibility of this protease toward oxidative stress needs to be considered.

Adaptation of protein carbonyl detection to the requirements of proteome analysis demonstrated for hypoxia/reoxygenation in isolated rat liver mitochondria

The key technique in proteome analysis is high-resolution two-dimensional (2D) electrophoretic separation of proteins from biological samples. This method combines isoelectric focusing (IEF) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Derivatization of protein carbonyls with 2, 4-dinitrophenylhydrazine (DNPH) and subsequent anti-dinitrophenyl (DNP) immunoblotting is widely used for the detection of oxidatively modified proteins. In previous studies on adapting this method to 2D electrophoresis the derivatization step was carried out before and after the 2D procedure, resulting in an altered spot pattern and high background staining, respectively. The aim of the present experiments was to develop a method for protein derivatization with DNPH between the IEF and the SDS-PAGE steps. Mitochondria were exposed to 10 min hypoxia and 5 min reoxygenation. After IEF using immobilized pH gradients the gel strips were incubated in DNPH/trifluoroacetic acid/SDS for 20 min and neutralized, and SDS-PAGE was performed. Proteins were either stained with Coomassie dye or subjected to Western blotting using anti-DNP IgG. Gels and blots were scanned and matched to a master gel, and the relative carbonyl content of each spot was calculated and compared for five experiments. Importantly, the spot patterns in DNPH-treated and untreated gels were not different. Protein carbonyls could be detected in 59 of the 125 matched spots. Although there was no significant increase in the total protein carbonyl content after hypoxia/reoxygenation, eighteen 2D spots exhibited an increase in carbonyl content. However, most protein spots did not show a change or even a decline (4 spots) in protein carbonyls.

Endoscopic treatment of clinically symptomatic leaks of thoracic esophageal anastomoses

BACKGROUND

The mortality of thoracic anastomotic leakage following esophageal reconstruction has been reported to be as high as 60%. Early septic fulminant suture line leaks require rethoracotomy. In addition, however, clinically symptomatic leaks may also occur 2 to 7 days after resection of the esophagus.

METHODS

Among 80 esophageal reconstructions performed between January 1994 and July 1998, a total of 7 (8.75%) clinically apparent leaks of thoracic anastomoses were observed. The standard treatment consisted of endoscopic lavage, drainage and subsequent closure of the defect by repeated intraluminal and submucosal applications of fibrin glue. In 2 patients a novel approach permitting rapid closure by plugging the fistula with a Vicryl-cylinder was tried. In 4 patients the effect of endoscopic treatment on the HLA-DR expression on monocytes was investigated and compared to 6 patients with intact anastomoses.

RESULTS

All 7 patients were successfully treated via endoscopy. The cylinder plug achieved immediate closure of the leak. The measured change in HLA-DR expression reflected the improvement in the inflammatory response and thus documented the success of endoscopic treatment.

CONCLUSIONS

Endoscopic management of thoracic leakages represents a safe and relatively noninvasive therapeutic option.

Degradation of hypochlorite-damaged glucose-6-phosphate dehydrogenase by the 20S proteasome

Glucose-6-phosphate dehydrogenase (G6PD) was treated with various concentrations of hypochlorite, which is produced by myeloperoxidase and is one of the most important oxidants during inflammatory processes. Inhibition of enzymatic activity, protein fragmentation, and proteolytic susceptibility toward the isolated 20S proteasome of G6PD were investigated. With rising hypochlorite concentrations, an increased proteasomal degradation of G6PD was measured. This occurred at higher hypochlorite concentrations than G6PD inactivation and at lower levels than G6PD fragmentation. The proteolytic activities of the 20S proteasome itself was determined by degradation of oxidized model proteins and cleavage of the synthetic proteasome substrate suc-LLVY-MCA. Proteasome activities remained intact at hypochlorite concentrations in which G6PD is maximally susceptible to proteasomal degradation. Only higher hypochlorite concentrations could decrease the proteolytic activities of the proteasome, which was accompanied by disintegration and fragmentation of the proteasome and proteasome subunits. Therefore, we conclude that the 20S proteasome can degrade proteins moderately damaged by hypochlorite and could contribute to an increased protein turnover in cells exposed to inflammatory stress.

Poly-ADP ribose polymerase activates nuclear proteasome to degrade oxidatively damaged histones

The 20S proteasome has been shown to be largely responsible for the degradation of oxidatively modified proteins in the cytoplasm. Nuclear proteins are also subject to oxidation, and the nucleus of mammalian cells contains proteasome. In human beings, tumor cells frequently are subjected to oxidation as a consequence of antitumor chemotherapy, and K562 human myelogenous leukemia cells have a higher nuclear proteasome activity than do nonmalignant cells. Adaptation to oxidative stress appears to be one element in the development of long-term resistance to many chemotherapeutic drugs and the mechanisms of inducible tumor resistance to oxidation are of obvious importance. After hydrogen peroxide treatment of K562 cells, degradation of the model proteasome peptide substrate suc-LLVY-MCA and degradation of oxidized histones in nuclei increases significantly within minutes. Both increased proteolytic susceptibility of the histone substrates (caused by modification by oxidation) and activation of the proteasome enzyme complex occur independently during oxidative stress. This rapid up-regulation of 20S proteasome activity is accompanied by, and depends on, poly-ADP ribosylation of the proteasome, as shown by inhibitor experiments, 14C-ADP ribose incorporation assays, immunoblotting, in vitro reconstitution experiments, and immunoprecipitation of (activated) proteasome with anti-poly-ADP ribose polymerase antibodies. The poly-ADP ribosylation-mediated activated nuclear 20S proteasome is able to remove oxidatively damaged histones more efficiently and therefore is proposed as an oxidant-stimulatable defense or repair system of the nucleus in K562 leukemia cells.

Evaluation of UVA-mediated oxidative damage to proteins and lipids in extracorporeal photoimmunotherapy

The combination of UVA and 8-methoxypsoralen (8-MOP) is known for the ability to produce reactive oxygen species (ROS) that react subsequently with DNA, lipids and proteins. In most studies concerned with UVA effects mediated by free radicals, UVA doses higher than those exhibiting beneficial clinical results in extracorporeal photoimmunotherapy (ECPI) were used. The present study was undertaken to determine markers of oxidative stress in plasma and cells from the buffy coat using conditions relevant for ECPI (cumulative UVA dose at the sample level < or = 2 J/cm2). Plasma exposed to UVA of 20 J/cm2 resulted in protein oxidation as well in crosslinking and fragmentation revealed by electrophoresis. Exposure of the buffy coat and plasma to considerably lower doses of UVA (up to 2 J/cm2) combined with various 8-MOP concentrations resulted neither in an increase of malondialdehyde as a marker of lipid peroxidation nor in a changed electrophoretic protein pattern. In these same experiments the total antioxidative capacity decreased to 65% of the initial value, suggesting that the antioxidative defense of plasma is able to cope with oxidative stress under ECPI conditions. These results were confirmed by data from 10 patients with scleroderma or cutaneous T-cell lymphoma during ECPI treatment. The present results suggest that, although ROS are formed during ECPI, gross oxidative damage does not occur. It is, however, possible, that specific effects mediated by oxygen radicals may co-trigger the photoimmunomodulatory effects of ECPI.

Oxidative stress affects pancreatic proteins during the early pathogenesis of rat caerulein pancreatitis

Rats received up to four subcutaneous injections of caerulein (20 microg/kg) in hourly intervals to induce a mild edematous pancreatitis. A single dose of caerulein resulted in significant oxidative modification of proteins in pancreatic homogenates as compared to saline-injected controls (p < 0.01). Repeated injections of the secretagogue were unable to induce a further increase in modified proteins. Protein oxidation preceded the formation of the lipid peroxidation product malondialdehyde as well as edema and the increase in serum amylase by 1-2.5 h. Western blotting for oxidatively modified proteins confirmed the results of the quantitative measurements and did not reveal individual, selectively modified proteins. These findings indicate that oxidative stress and oxidative protein modification are very early events in the initial period of caerulein pancreatitis. This may explain the poor success rate of most studies that administered antioxidants therapeutically during or after initiation of pancreatitis.

Comparative resistance of the 20S and 26S proteasome to oxidative stress

Oxidatively modified ferritin is selectively recognized and degraded by the 20S proteasome. Concentrations of hydrogen peroxide (H2O2) higher than 10 micromol/mg of protein are able to prevent proteolytic degradation. Exposure of the protease to high amounts of oxidants (H2O2, peroxynitrite and hypochlorite) inhibits the enzymic activity of the 20S proteasome towards the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-methylcoumarylamide (Suc-LLVY-MCA), as well as the proteolytic degradation of normal and oxidant-treated ferritin. Fifty per cent inhibition of the degradation of the protein substrates was achieved using 40 micromol of H2O2/mg of proteasome. No change in the composition of the enzyme was revealed by electrophoretic analysis up to concentrations of 120 micromol of H2O2/mg of proteasome. In further experiments, it was found that the 26S proteasome, the ATP- and ubiquitin-dependent form of the proteasomal system, is much more susceptible to oxidative stress. Whereas degradation of the fluorogenic peptide, Suc-LLVY-MCA, by the 20S proteasome was inhibited by 50% with 12 micromol of H2O2/mg, 3 micromol of H2O2/mg was enough to inhibit ATP-stimulated degradation by the 26S proteasome by 50%. This loss in activity could be followed by the loss of band intensity in the non-denaturing gel. Therefore we concluded that the 20S proteasome was more resistant to oxidative stress than the ATP- and ubiquitin-dependent 26S proteasome. Furthermore, we investigated the activity of both proteases in K562 cells after H2O2 treatment. Lysates from K562 cells are able to degrade oxidized ferritin at a higher rate than non-oxidized ferritin, in an ATP-independent manner. This effect could be followed even after treatment of the cells with H2O2 up to a concentration of 2mM. The lactacystin-sensitive ATP-stimulated degradation of the fluorogenic peptide Suc-LLVY-MCA declined, after treatment of the cells with 1mM H2O2, to the same level as that obtained without ATP stimulation. Therefore, we conclude that the regulation of the 20S proteasome by various regulators takes place during oxidative stress. This provides further evidence for the role of the 20S proteasome in the secondary antioxidative defences of mammalian cells.

Comparison of protein oxidation and aldehyde formation during oxidative stress in isolated mitochondria

Oxidative stress is known to cause oxidative protein modification and the generation of reactive aldehydes derived from lipid peroxidation. Extent and kinetics of both processes were investigated during oxidative damage of isolated rat liver mitochondria treated with iron/ascorbate. The monofunctional aldehydes 4-hydroxynonenal (4-HNE), n-hexanal, n-pentanal, n-nonanal, n-heptanal, 2-octenal, 4-hydroxydecenal as well as thiobarbituric acid reactive substances (TBARS) were detected. The kinetics of aldehyde generation showed a lag-phase preceding an exponential increase. In contrast, oxidative protein modification, assessed as 2,4-dinitrophenylhydrazine (DNPH) reactive protein-bound carbonyls, continuously increased without detectable lag-phase. Western blot analysis confirmed these findings but did not allow the identification of individual proteins preferentially oxidized. Protein modification by 4-HNE, determined by immunoblotting, was in parallel to the formation of this aldehyde determined by HPLC. These results suggest that protein oxidation occurs during the time of functional decline of mitochondria, i.e. in the lag-phase of lipid peroxidation. This protein modification seems not to be caused by 4-HNE.

Differentially expressed genes in hippocampal cell cultures in response to an excitotoxic insult by quinolinic acid

The NMDA-type glutamate receptor agonist quinolinic acid (QA), which causes tissue lesions in the rat brain as well as cell loss in neuronal cultures, is widely used in models of glutamate excitotoxicity. The aim of this study was to evaluate the alterations in gene expression in a primary hippocampal cell culture after exposure to QA. By means of differential mRNA display, we were able to pinpoint as many as 23 bands which appeared to be upregulated after a 6-h treatment with quinolinic acid. The differential expression of 13 cDNAs could be confirmed by dot blot and/or Northern analysis. Of the cDNAs, the p112 regulatory subunit of the 26S proteasome, a PDGF-associated protein and the glia-derived protease nexin PN-1 could be identified. The results provide emphasis to the participation of proteolysis and protease inhibition in neurodegenerative processes.

Occurrence of oxidatively modified proteins: an early event in experimental acute pancreatitis

Free radical-mediated injury is believed to play a key role in the pathogenesis of acute pancreatitis (AP). Therefore, oxidative damage of proteins may be an important event in the development of AP. The present study was performed to investigate oxidative protein modification, quantified as 2,4-dinitrophenylhydrazine-reactive protein-carbonyls, during the time course of taurocholate-induced pancreatitis of the rat and to analyze oxidatively modified proteins by Western blotting. Protein modification in pancreatic homogenates was found as early as 30 min after induction of severe AP with 3% taurocholate preceding the elevation of serum amylase activity and the increase of malondialdehyde in the tissue. A correlation of protein-carbonyl contents to a score of pancreatic macroscopic alterations (r = .69) and to the wet weight/dry weight ratio (r = .65) was found. Infusion of 5% taurocholate resulted in fulminant AP with high lethality during the 24 h of the experiment. However, rats surviving showed significantly lower level of protein-carbonyls than animals that died between 20-24 h after AP induction. The quantitative data were confirmed by the intensity of immunostained protein-carbonyls. The present data show a rather uniform increase in the staining pattern not revealing single, selectively damaged proteins. The aldehydic product of lipid peroxidation 4-hydroxynonenal (HNE) is known for its reactivity towards proteins. Interestingly, an antibody raised against protein-bound HNE did not indicate an increased protein modification by this aldehyde. In conclusion, experimental AP is characterized by an early oxidative protein modification, possibly contributing to functional impairment of the pancreas. This protein alteration may not be mediated by HNE.

Short-term impairment of energy production in isolated rat liver mitochondria by hypoxia/reoxygenation: involvement of oxidative protein modification

The aim of the present study was to elucidate the role of mitochondria in liver impairment after ischaemia/reperfusion. It is commonly assumed that mitochondria are in part responsible for tissue damage by impaired oxidative phosphorylation as a consequence of the attack of radicals generated within the mitochondria. The principal support for this hypothesis was found by exposing isolated mitochondria to temporary hypoxia in combination with alterations of substrate supply. Rat liver mitochondria treated in this way responded with impaired ADP-stimulated respiration after reoxygenation, which decreased with time of hypoxia and reoxygenation. The decline of the activity of the NADH-cytochrome c-oxidoreductase complex found under these conditions is likely to cause the drop in active respiration. No changes in the content of respiratory chain complexes, determined by Blue Native PAGE, could be demonstrated. However, oxidative modifications of mitochondrial proteins, indicated by carbonyl formation, were found. Likewise, products of lipid peroxidation, such as lipid peroxides and malondialdehyde, were formed. Mitochondria were still able to build up a transmembrane potential and did not show drastic changes in membrane conductivity after hypoxia/reoxygenation stress. The presence of water-soluble antioxidants exhibited a beneficial effect, diminishing the decline of active respiration after 5 min of hypoxia and 10 min of reoxygenation. These observations strongly suggest that mitochondria play a pathogenic role in ischaemia/reperfusion injury, which is at least in part mediated by an oxygen-derived free-radical-linked mechanism.

Role of endogenous and exogenous antioxidants in the defence against functional damage and lipid peroxidation in rat liver mitochondria

Mitochondria are cellular organelles where the generation of reactive oxygen species may be high. They are, however, effectively protected by their high capacities of antioxidative systems, as enzymes and either water or lipid soluble low molecular weight antioxidants. These antioxidative defence systems can be effectively regenerated after or during an oxidative stress as long as the mitochondria are in an energized state. Energization of mitochondria mainly depends on the availability of suitable respiratory substrates which can provide hydrogen for the reduction of either the glutathione- or alpha-tocopherol-system, since GSH is regenerated by glutathione reductase with the substrate NADPH and the alpha-tocopheroxyl-radical likely by reduced coenzyme Q. It was shown that mitochondria do not undergo damages as long as they can keep a high energy state. The delicate balance between prooxidative/antioxidative activities can be shifted towards oxidation, if experimentally prooxidants were added. After exhaustion of the antioxidative defence systems damages of mitochondrial functions become expressed followed by membrane injuries along with the oxidation and degradation of mitochondrial lipids and proteins leading finally to the total degradation of the mitochondria. Extramitochondrial antioxidants may assist the mitochondrial antioxidative defence systems in a complex way, whereby particularly ascorbic acid can act both as prooxidant and as antioxidant.

Micromolar calcium prevents isolated rat liver mitochondria from anoxia-reoxygenation injury

Rat liver mitochondria were exposed to extramitochondrial free calcium between 0 and 5 microM and/or 5 minutes of anoxia followed by 10 minutes of reoxygenation. At concentrations higher than 4 microM, the membrane potential collapsed indicating the permeability transition of the mitochondrial membrane. Anoxia-reoxygenation shifted this transition to lower calcium concentrations. Anoxia-reoxygenation alone resulted in the decrease of ADP stimulated respiration down to about 40% of its initial value. Between 1 and 2 microM, a protective effect in terms of respiration and oxidative protein modification was found. It is concluded that calcium may suppress the formation of reactive oxygen species during anoxia-reoxygenation before permeability transition occurs.