Induction of calcium efflux from isolated rat-liver mitochondria by 1,2-dibromoethane

Conjugative metabolism of 1,2-dibromoethane in mitochondria: disruption of oxidative phosphorylation and alkylation of mitochondrial DNA

1,2-Dibromoethane (DBE) is an environmental contaminant that is metabolized by glutathione S-transferases to a haloethane-glutathione conjugate. Since haloethane-glutathione conjugates are known to alkylate nuclear DNA and cytoplasmic proteins, these effects were investigated in isolated rat liver mitochondria exposed to DBE by measuring guanine adducts and several aspects of oxidative phosphorylation including respiratory control ratios, respiratory enzyme activity, and ATP levels. Mitochondrial large-amplitude swelling and glutathione status were assessed to evaluate mitochondrial membrane integrity and function. When exposed to DBE, mitochondria became uncoupled rapidly, yet no large-amplitude swelling or extramitochondrial glutathione was observed. Mitochondrial GSH was depleted to 2-53% of controls after a 60-min exposure to micromolar quantities of DBE; however, no extramitochondrial GSH or GSSG was detected. The depletion of mitochondrial glutathione corresponded to an increase of an intramitochondrial GSH-conjugate which, based on HPLC elution profiles and retention times, appeared to be S,S'-(1,2-ethanediyl)bis(glutathione). Activities of the NADH oxidase and succinate oxidase respiratory enzyme systems were inhibited 10-74% at micromolar levels of DBE, with succinate oxidase inactivation occurring at lower doses. ATP concentrations in DBE-exposed mitochondria in the presence of succinate were 5-90% lower than in the controls. The DNA adduct S-[2-(N(7)-guanyl)ethyl]glutathione was detected by HPLC in mtDNA isolated from DBE-exposed mitochondria. The results suggest that respiratory enzyme inhibition, glutathione depletion, decreased ATP levels, and DNA alkylation in DBE-exposed mitochondria occur via the formation of an S-(2-bromoethyl)glutathione conjugate, the precursor of the episulfonium ion alkylating species of DBE.

Release of Ca(2+) from intracellular stores and entry of extracellular Ca(2+) are involved in sea squirt sperm activation

A rise in intracellular free Ca(2+) concentration ([Ca(2+)](i)) is required to activate sperm of all organisms studied. Such elevation of [Ca(2+)](i) can occur either by influx of extracellular Ca(2+) or by release of Ca(2+) from intracellular stores. We have examined these sources of Ca(2+) in sperm from the sea squirt Ascidia ceratodes using mitochondrial translocation to evaluate activation and the Ca(2+)-sensitive dye fura-2 to monitor [Ca(2+)](i) by bulk spectrofluorometry. Sperm activation artificially evoked by incubation in high-pH seawater was inhibited by reducing seawater [Ca(2+)], as well as by the presence of high [K(+)](o) or the Ca channel blockers pimozide, penfluridol, or Ni(2+), but not nifedipine or Co(2+). The accompanying rise in [Ca(2+)](i) was also blocked by pimozide or penfluridol. These results indicate that activation produced by alkaline incubation involves opening of plasmalemmal voltage-dependent Ca channels and Ca(2+) entry to initiate mitochondrial translocation. Incubation in thimerosal or thapsigargin, but not ryanodine (even if combined with caffeine pretreatment), evoked sperm activation. Activation by thimerosal was insensitive to reduced external calcium and to Ca channel blockers. Sperm [Ca(2+)](i) increased upon incubation in high-pH or thimerosal-containing seawater, but only the high-pH-dependent elevation in [Ca(2+)](i) could be inhibited by pimozide or penfluridol. Treatment with the protonophore CCCP indicated that only a small percentage of sperm could release enough Ca(2+) from mitochondria to cause activation. Inositol 1,4,5-trisphosphate (IP(3)) delivered by liposomes or by permeabilization increased sperm activation. Both of these effects were blocked by heparin. We conclude that high external pH induces intracellular alkalization that directly or indirectly activates plasma membrane voltage-dependent Ca channels allowing entry of external Ca(2+) and that thimerosal stimulates release of Ca(2+) from IP(3)-sensitive intracellular stores.

Molecular mechanisms of dibromoalkane cytotoxicity in isolated rat hepatocytes

The cytotoxicity of dibromoalkanes to isolated hepatocytes was proportional to the dibromoalkane concentration and increasing chain length of the dibromoalkane (C2-C6). The rapid hepatocyte glutathione (GSH) depletion which occurred upon addition of the dibromoalkanes was also dependent on the concentration and chain length of the dibromoalkane. When added to hepatocytes, dibromoalkanes also caused a loss in protein sulfhydryl groups. After a lag period, lipid peroxidation occurred before the onset of cytotoxicity. Antioxidants or removing the oxygen from the medium markedly delayed dibromoalkane cytotoxicity. Bromoaldehydic metabolites formed by cytochrome P450-dependent mixed-function oxidases were probably responsible for lipid peroxidation as deuterated 1,2-dibromoethane (d4-DBE) induced less lipid peroxidation and was less cytotoxic even though GSH was depleted as rapidly and as effectively. Hepatocytes were also more resistant to dibromoalkanes if cytochrome P450 isoenzymes were inactivated with SKF 525A or methyl pyrazole. Furthermore, hepatocyte susceptibility to dibromoalkanes was increased markedly if aldehyde dehydrogenase was inactivated with disulfiram, cyanamide or chloral hydrate. Cytochrome P450-induced hepatocytes isolated from pyrazole-, phenobarbital- or 3-methylcholanthrene-pretreated rats were also more susceptible to dibromoalkanes. These results suggest that dibromoalkane-induced cell lysis is due to lipid peroxidation as well as cytochrome P450-dependent formation of toxic bromoaldehydic metabolites which can bind with cellular macromolecules. Dibromoethane GSH conjugates also contribute to DBE cytotoxicity as depleting hepatocyte GSH beforehand increased hepatocyte resistance to DBE but not other dibromoalkanes.

Early mitochondrial disfunction in bromobenzene treated mice: a possible factor of liver injury

The membrane potential of liver mitochondria isolated from bromobenzene treated mice was studied. Specifically, the efficiency of the energy-transducing mitochondrial membrane was measured during the phase between the occurrence of a massive loss of hepatic GSH, after 2-3 hr of bromobenzene intoxication, and the appearance of lipid peroxidation and cell death (12-15 hr after treatment). Partial uncoupling of oxidative phosphorylation was observed in mitochondria during the early period of intoxication (3-9 hr). These anomalies in oxidative metabolism did not result in irreversible damage to the mitochondrial inner membrane. The possibility that phenolic metabolites of bromobenzene are responsible for the uncoupling effects was examined. Orto- and especially para-bromphenol reproduced the alterations of mitochondrial function when added to normal mitochondria at concentrations comparable to those found in the livers of the intoxicated animals. Since the concentration of the bromophenols (especially p-bromophenol) largely increases after the intoxication times as tested here, mitochondrial uncoupling may represent a mechanism of liver damage acting synergistically with or even independently of other factors such as oxidative stress and lipid peroxidation.

Biochemical mechanism of GSH depletion induced by 1,2-dibromoethane in isolated rat liver mitochondria. Evidence of a GSH conjugation process

HPLC measurements of GSH and GSSG levels in isolated rat liver mitochondria, on addition of 1,2-dibromoethane (DBE), revealed the presence of a glutathione (GSH)-conjugating pathway of DBE. This process required the structural integrity of the mitochondrial matrix and inner membrane complex and was inhibited by the uncouplers of oxidative phosphorylation, particularly 2,4-dinitrophenol. On the other hand it was not affected by the energetic state of the mitochondria, since other mitochondrial inhibitors like KCN and oligomycin did not have any effect on it. This process also did not require the involvement of mitochondrial inner membrane transport systems, based on the measurement of the mitochondrial transmembrane potential. The involvement of mitochondrial GSH-S-transferases, located either in the matrix or in the intermembrane space, is discussed.

The effect of a ferric iron complex on isolated rat-liver mitochondria. III. Mechanistic aspects of iron-induced calcium efflux

Addition of iron(III)-gluconate complex to isolated rat liver mitochondria induced a net efflux of Ca2+ which was not inhibited by ruthenium red. This process resulted in the enhancement of Ca2+ cycling and a consequent membrane potential drop. Under these experimental conditions the content of mitochondrial glutathione did not appear to be critically modified, whereas an extensive oxidation of mitochondrial pyridine nucleotides was parallelly detected. Iron failed to induce appreciable changes in the oxidation level of pyridine nucleotides in mitochondria isolated from rats fed a selenium deficient diet, a condition in which mitochondrial glutathione peroxidase resulted inhibited by 80%. The iron-induced Ca2+ release in Se-deficient mitochondria appeared largely delayed and the membrane potential of these mitochondrial did not present gross alterations. Iron was also found to induce a transient increase in the mitochondrial cyanide-insensitive oxygen consumption. This effect was largely prevented by the addition of the hydrogen peroxide scavenger catalase. It was concluded that iron induced the activation of a specific Ca2+ efflux pathway via the oxidation of pyridine nucleotides due to the hydrogen peroxide metabolism by glutathione enzyme system.