Ethane formation of isolated rat hepatocytes due to carbon tetrachloride

Pathobiochemical mechanisms of hepatocellular damage following lipid peroxidation

In hepatocytes, cytotoxic events induced by haloalkanes or acute iron-overload exhibit neither a quantitative nor a temporal correlation to lipid peroxidation or covalent binding. Thus, secondary pathological mechanisms have been postulated linking initial focal reactions of free radicals and end stage pathological consequences. Due to the crucial role of plasma-membrane integrity in the cytotoxic process it has to be supposed that relevant secondary pathological mechanisms finally impair the physico-chemical and functional properties of this membrane. Based on recent developments a chain of causality is proposed as a two-step activation of phospholipase A2 producing cytolytic amounts of lysophosphatides. In this cascade, the initial activating step is a decrease of membrane lipid fluidity induced by lipid peroxidation and/or by calcium binding and intramembranous formation of 4-hydroxynonenal. This enzyme activation is further amplified by the early rise of cytosolic calcium. Consequently, increasing amounts of lysophosphatides progressively impair membrane configuration thus improving the substrate accessibility for phospholipase A2 in a second activation step. Finally, the lysophosphatides reach plasma membrane-lytic concentrations by this autocatalytic enzyme activation.

Silver affects viability and structure of cultured mouse peritoneal macrophages and peroxidative capacity of whole mouse liver

The effects of silver on cultured mouse peritoneal macrophages were examined by estimation of cell survival and by light and electron microscopy. Addition of silver lactate to the culture medium at final concentrations of 40 and 80 microM resulted in coagulation necrosis and rapid cell death. At lower concentrations cell structure appeared normal. However, the rate of cell death at 20 microM silver lactate was increased as compared to controls. Silver, visualized by physical development/autometallography, was invariably located in lysosomes. The production of malondialdehyde in mouse liver of silver-treated mice as compared to controls was also examined. This lipid peroxidation product had increased to the same amount in animals treated with silver for either 3 days or with only one silver injection 4 h before examination. This study has demonstrated that silver affects viability and structure of cultured macrophages, possibly due to induction of lipid peroxidation, as demonstrated to occur in the liver of silver-exposed mice.

Lipid peroxidation and mechanisms of toxicity

Aerobic organisms by definition require oxygen, and the importance of iron in aerobic respiration has long been recognized, but despite their beneficial roles, these elements can pose a real threat to the organism. During oxygen reduction, reactive species such as O2-. and H2O2 are formed readily. Iron can combine with these species, or with molecular oxygen itself, to generate free radicals which will attack the polyunsaturated fatty acids of membrane lipids. This oxidative deterioration of membrane lipids is known as lipid peroxidation. To protect itself against this form of attack, the organism possesses several types of defense mechanisms. Under normal conditions, these defenses appear to offer adequate protection for cell membranes, but the possibility exists that certain foreign compounds may interfere with or even overwhelm these defenses, and herein could lie a general mechanism of toxicity. This possible cause of toxicity is discussed in relation to other suggested causes.

CCl4-induced toxicity in isolated hepatocytes: the importance of direct solvent injury

CCl4 is proposed to induce cellular injury through its metabolites that are generated by a cytochrome P-450 dependent step. These free radical products can interact with membrane structures, thereby generating lipid peroxides. The latter process has been implicated as a major mechanism of CCl4 hepatoxicity, although this relationship has been difficult to demonstrate when using isolated hepatocyte preparations. This report demonstrates that there are at least two mechanisms by which CCl4 induces injury in isolated hepatocytes. One occurs within minutes of exposure to CCl4 and is characterized by modest malondialdehyde formation and no decline in cellular-reduced glutathione. SKF 525A, metyrapone and promethazine did not protect against this early damage. A second phase of damage, evident particularly after 3 hr, is characterized by a marked increase in malondialdehyde formation, a fall in cellular glutathione and substantial further cellular damage. These changes could be moderated by the cytochrome P-450 inhibitors and promethazine, and antioxidant. Further examination of the initial phase of damage reveals an immediate dose-related inhibition of O2 consumption. This could not be prevented by SKF 525A or metyrapone and was associated with loss of ability to stimulate mitochondrial respiration with dinitrophenol. Rapid recovery to initial O2 consumption rates occurred with time as CCl4 evaporated from the incubation system. This was associated with a partial return of dinitrophenol stimulation of mitochondrial O2 consumption despite significant glutamate dehydrogenase release. A portion of this recovery could be inhibited by SKF 525A, suggesting that some O2 consumption was due to CCl4 metabolism and ensuing lipid peroxidation. These data suggest that early CCl4 toxicity is a direct consequence of its solvent properties and is partially reversible; subsequent damage may be mediated by lipid peroxidation. This solvent injury has not been previously recognized and may have relevance not only to a reversible toxicity as demonstrated with isolated hepatocytes but also in vivo as well.

Absence of lipid peroxidation as determined by ethane exhalation in rats treated with 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)

The exhalation of ethane is widely used as an indicator of in vivo lipid peroxidation. To test the hypothesis that lipid peroxidative events are involved in the toxicity of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), we administered a lethal dose of TCDD (60 micrograms/kg), IP to male Sprague-Dawley rats (160-180 g) and measured by gas chromatography the exhalation of ethane into the atmosphere of a closed all-glass exposure chamber. TCDD-treated rats exhaled only slightly more ethane than control rats at a single time point 7 days following TCDD administration. Since the exhalation of ethane is the net result of the endogenous production of the gas and its metabolic degradation, the latter was quantified by measuring the clearance of exogenous ethane (initial concentration = 100 ppm) introduced to the atmosphere of the exposure chamber. The clearance of ethane in TCDD-treated rats was markedly decreased, reaching a minimum 7 days following TCDD treatment. Apparently, the slight increase in exhaled ethane was due to an inhibition of ethane metabolism caused by TCDD. However, rats obviously intoxicated and having lost considerable body weight might be impaired in their ability to transport ethane. To bypass this problem we injected ethane (0.2 ml) directly into the rats IP. Here also the metabolic clearance in TCDD-treated rats was diminished. In a further experiment, rats treated with dithiocarb at a dose where ethane metabolism was totally inhibited exhaled more ethane than did TCDD-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation of ethane in vitro by rat liver homogenates following glutathione depletion in vivo

Depletion of hepatic glutathione (GSH) in rats by treatment with phorone (diisopropylidene acetone) in vivo led to a dramatic enhancement of spontaneous ethane production by liver post-mitochondrial fractions. Exogenous GSH added in vitro abolished this effect in a concentration-dependent manner. The same inhibitory action was seen with both dithiocarb or (+)-catechin. The concentrations yielding 50% inhibition (I50) were in the micromolar range for all three compounds.

Mechanisms of chemical toxicity--a unifying hypothesis

Most cases of chemical toxicity involve one or both of the fundamental pathological processes of "acute lethal injury" and "autoxidative cellular injury." The process of "acute lethal injury" by which toxic chemicals interfere with cellular energy metabolism, leading ultimately to cell death and tissue necrosis, is well known and reasonably understood. Inhibition of glycolysis, mitochondrial respiration, or oxidative phosphorylation, resulting in lack of ATP synthesis or inhibition of ATPase and other enzymes, leads to decreased efficiency of the sodium pump, hydropic degeneration, lipid accumulation, and eventually cell death. Less well known are the mechanisms whereby toxic chemicals initiate autoxidation, leading to "autoxidative cellular injury," disrupting cell membranes, and resulting in increased autophagocytosis, cell death, and mutations. Many reactive intermediates of toxic chemicals are electrophiles, free radicals, or free-radical generators, which may potentiate the toxicity of tissue oxygen, depleting intracellular glutathione and biological antioxidants, resulting in membrane damage, impairment of the calcium pump, cell death, and damage to DNA. The mechanisms of oxygen toxicity and chemical-mediated oxygen toxicity are discussed, with particular reference to the microsomal mixed-function oxidase system and its role in the detoxication and activation of environmental chemicals. The dependence of tissue oxygen concentration, the rates of oxidative activation of chemicals, and the extents of autoxidative cellular injury on the size of the animal species is considered, and the importance of this to the scientific evaluation of chemical toxicity is discussed.

Lipid peroxidation in isolated rat hepatocytes measured by ethane and n-pentane formation

Isolated rat hepatocytes (1 X 10(7) cells/ml) were aerobically incubated in Eagle's Minimum Essential Medium which contained 2.0% albumin. As potential parameters of lipid peroxidation ethane and n-pentane formed were measured in samples obtained from the gas phase above the incubation mixture. 15-30 nmol ethane or n-pentane were produced by 10(7) hepatocytes within 90 min. Carbon tetrachloride (CCl4) or ADP-complexed ferrous ions stimulated ethane and n-pentane formation considerably, depending on the concentrations of the compounds. With CCl4 10(7) cells formed max 180 nmol ethane and 140 nmol n-pentane within 90 min incubation, whereas with Fe(II) max 130 nmol ethane and 220 nmol n-pentane could be detected. When n-pentane was added to the gas phase above the incubation mixture containing either medium or medium plus hepatocytes its amount decreased by 30% within the first 5 min of incubation. However, afterwards only minor amounts of n-pentane disappeared, even in the presence of hepatocytes. This indicates that n-pentane equilibrates with the cells suspension under the conditions used. Cell viability, as determined by the release of lactate dehydrogenase into the medium and by the uptake of trypan blue by the cells, and the recovery of the cells decreased only in presence of relatively high concentrations of CCl4, or Fe(II) respectively. However, a maximal effect on ethane and n-pentane formation was reached already with lower concentration.

Comparison of methods of assessment of metal-induced lipid peroxidation in isolated rat hepatocytes

There is some controversy about which method of assessment of lipid peroxidation in isolated hepatocytes is most appropriate. The present study was undertaken primarily to compare measurement of concentrations of thiobarbituric acid (TBA) reacting substances with measurement of ethane concentrations in the gas phase of the incubation flask as indicators of lipid peroxidation. Four metal salts, FeCl2, NaVO3, CdCl2, and MnCl2, were selected as agents that interact with the lipid peroxidation process. Furthermore, reduced glutathione (GSH) concentrations and enzyme leakage were assayed to determine whether there was a consistent pattern of interaction between lipid peroxidation and change in GSH concentrations and enzyme leakage. The effects of the metal ions on the concentration of TBA reactants estimated in the whole cell suspension and on the gaseous ethane concentration were similar. However, the assessment of TBA reactants was a little more sensitive, and ethane concentrations continued to climb with incubation time rather than leveling off as observed for TBA reactants. In general, the results of both assays were in good agreement. No consistent pattern of interaction between lipid peroxidation, GSH, and enzyme leakage was discernible in the results of the present study. It is suggested that perhaps the best approach to an experimental situation where lipid peroxidation is thought to be of central importance is measurement of both parameters, TBA reactants and ethane concentrations.

Toxic drug effects associated with oxygen metabolism: redox cycling and lipid peroxidation

Various endogenous and exogenous compounds exert cytotoxic effects via oxygen reduction. In general, these are reduced by intracellular enzymes (reductases of various kinds) in one-electron transfer reactions, before they in turn reduce O2 to O2, the superoxide anion radical. Thus, a cycle is formed of O2 uptake at the expense of cellular reducing equivalents, notably NADPH, generating further active oxygen species (figs 1,2). Structures capable of 'redox cycling' include catechols and other quinone compounds, iron chelates, and aromatic nitro compounds. Several anticancer agents, and also some mutagens, operate on this principle, and their toxic effects may be explained by redox cycling. The particular importance of hypoxic conditions for deleterious O2 effects is given by the concomitant flux through reductive as well as oxidative pathways. Toxic effects include membrane damage resulting from peroxidative reactions of polyunsaturated fatty acids (lipid peroxidation), as well as the attack of reactive oxygen species on proteins (enzymes) and nucleic acids; thus O2 metabolism is linked to carcinogenicity and mutagenicity. Lipid peroxidation is also induced by various halogenated compounds such as carbon tetrachloride. Again, hypoxic conditions are particularly critical because, on the one hand, metabolic activation leading to the free radical is enhanced and, on the other hand, oxygen required for the maintenance of lipid peroxidation is still available. - Powerful antioxidant systems of the cell maintain low steady state concentrations of oxygen metabolites, and toxic effects may, in part, also be explained by the constant drain of reducing equivalents resulting from redox cycling.