Characterization of Adducts of Ethanol Metabolites with Cytochrome c

An optimized method for the measurement of acetaldehyde by high-performance liquid chromatography

BACKGROUND

Acetaldehyde is produced during ethanol metabolism predominantly in the liver by alcohol dehydrogenase and rapidly eliminated by oxidation to acetate via aldehyde dehydrogenase. Assessment of circulating acetaldehyde levels in biological matrices is performed by headspace gas chromatography and reverse phase high-performance liquid chromatography (RP-HPLC).

METHODS

We have developed an optimized method for the measurement of acetaldehyde by RP-HPLC in hepatoma cell culture medium, blood, and plasma. After sample deproteinization, acetaldehyde was derivatized with 2,4-dinitrophenylhydrazine (DNPH). The reaction was optimized for pH, amount of derivatization reagent, time, and temperature. Extraction methods of the acetaldehyde-hydrazone (AcH-DNP) stable derivative and product stability studies were carried out. Acetaldehyde was identified by its retention time in comparison with AcH-DNP standard, using a new chromatography gradient program, and quantitated based on external reference standards and standard addition calibration curves in the presence and absence of ethanol.

RESULTS

Derivatization of acetaldehyde was performed at pH 4.0 with an 80-fold molar excess of DNPH. The reaction was completed in 40 minutes at ambient temperature, and the product was stable for 2 days. A clear separation of AcH-DNP from DNPH was obtained with a new 11-minute chromatography program. Acetaldehyde detection was linear up to 80 μM. The recovery of acetaldehyde was >88% in culture media and >78% in plasma. We quantitatively determined the ethanol-derived acetaldehyde in hepatoma cells, rat blood and plasma with a detection limit around 3 μM. The accuracy of the method was <9% for intraday and <15% for interday measurements, in small volume (70 μl) plasma sampling.

CONCLUSIONS

An optimized method for the quantitative determination of acetaldehyde in biological systems was developed using derivatization with DNPH, followed by a short RP-HPLC separation of AcH-DNP. The method has an extended linear range, is reproducible and applicable to small-volume sampling of culture media and biological fluids.

Liver proteome analysis in a rodent model of alcoholic steatosis

Alcoholic steatosis (AS) is the initial pathology associated with early stage alcoholic liver disease (ALD) and is characterized by the accumulation of fat in the liver. AS is considered clinically benign because it is reversible, and the progression of AS to alcoholic steatohepatitis (ASH) is a key step in the development of ALD. A two-dimensional gel electrophoresis (2DE)-mass spectrometry (MS) proteomic approach was used to investigate the protein expression pattern underlying AS, as the first step toward determining liver tissue biomarkers for early stage ALD. Several proteins involved in fatty acid and amino acid metabolism were up-regulated in 3- and 6-week ethanol-fed rats relative to isocaloric controls, which suggest a higher energy demand upon chronic exposure to ethanol. In addition, the expression of two proteins associated with alcohol-induced oxidative stress, peroxiredoxin 6 (PRDX6) and aldehyde dehydrogenase 2 (ALDH2), was down-regulated in ethanol fed rats, and suggests an increase in reactive oxygen species and oxidative stress. To investigate if irreversible protein modification arising from oxidative stress during AS impacts protein levels, the extent of carbonylated proteins in the ethanol and isocaloric groups was identified using mass spectrometry. The detection of modified proteins involved in antioxidant functions further supports the notion that oxidative modification of these proteins leads to protein turnover during AS. In addition, the carbonylation of betaine-homocysteine S-methyltransferase, a protein implicated in fatty liver development, in 3-week and 6-week ethanol exposed samples suggests that this protein could be a marker for early stage AS.

Acetaldehyde-induced mitochondrial dysfunction sensitizes hepatocytes to oxidative damage

Acetaldehyde (Ac), the main metabolite of ethanol oxidation, is a very reactive compound involved in alcohol-induced liver damage. In the present work, we studied the effect of Ac in mitochondria functionality. Mitochondria from Wistar rats were isolated and treated with Ac. Ac decreased respiratory control by 50% which was associated with a decrease in adenosine triphosphate content (28.5%). These results suggested that Ac could be inducing changes in cell redox status. We determined protein oxidation, superoxide dismutase (SOD) activity, and glutathione ratio, indicating that Ac induced an enhanced oxidation of proteins and a decrease in SOD activity (90%) and glutathione/oxidized GSH ratio (36%). The data suggested that Ac-induced oxidative stress mediated by mitochondria dysfunction can lead to cell sensitization and to a second oxidative challenge. We pretreated hepatocytes with Ac followed by treatment with antimycin A, and this experiment revealed a noticeable decrease in cell viability, determined by neutral red assay, in comparison with cells treated with Ac alone. Our data demonstrate that Ac impairs mitochondria functionality generating oxidative stress that sensitizes cells to a second damaging signal contributing to the development of alcoholic liver disease.

The mechanism of elevated toxicity in HepG2 cells due to combined exposure to ethanol and ionizing radiation

Ethanol and ionizing radiation exposure are independently known to cause tissue damage through various mechanisms. The non-enzymatic and enzymatic metabolism of ethanol, the latter via the cytochrome P(450) 2E1-dependent pathway produces free radicals, which deplete cellular glutathione (GSH). Ionizing radiation exposure has been shown to induce lipid peroxidation, DNA damage, protein oxidation and GSH depletion. It was postulated that cells sensitized by ethanol will be susceptible to additional insult, such as by radiation through increased oxidative stress. In this investigation, cultured liver cells (HepG2, human hepatocellular liver carcinoma) were exposed to ethanol, followed by ionizing radiation. The antioxidant status of the cells was evaluated by an array of techniques. Levels of glutathione, cysteine (CYS), and malondialdehyde (MDA) were measured by HPLC. Activities of antioxidant enzymes, catalase and glutathione reductase (GR) were determined enzymatically. Apoptosis was evaluated by the caspases-3 assay and fluorescence microscopy. The data showed that combined treatment with ethanol and radiation resulted in the lowest levels of GSH, and highest MDA level compared with the control. The catalase activity was lower in the combined exposure groups, when compared with the single agent exposure groups, and the glutathione reductase activity was the highest in the combined exposure groups and lowest in the control. These findings suggest that a combination of ethanol and ionizing radiation results in greater toxicity in vitro through elevated oxidative stress.

Peroxidases: a role in the metabolism and side effects of drugs

Current safety screening of drug candidates or new chemical entities for reactive metabolite formation focuses on the role of cytochrome P450. However, peroxidases also have a major role in drug metabolism, and peroxidase-catalyzed drug oxidation could lead to reactive metabolite formation, resulting in oxidative stress and cytotoxicity. Here, the different classes of human peroxidases are summarized and the molecular mechanisms of peroxidase-catalyzed drug metabolism are discussed. In addition, evidence is presented that indicates a role of these enzymes in drug toxicity.

Binding of Acetaldehyde to a Glutathione Metabolite: Mass Spectrometric Characterization of an Acetaldehyde-Cysteinylglycine Conjugate

BACKGROUND

Ethanol administration decreases hepatic glutathione levels and increases urinary sulfhydryl excretion. Ethanol-induced liver injury is blunted by the administration of glutathione precursors. Acetaldehyde generated in the metabolism of ethanol binds to a number of amino acid residues in proteins and peptides, but it does not react readily with glutathione. Due to the possible role of acetaldehyde in cysteine and glutathione homeostasis, we investigated the reaction of acetaldehyde to cysteinylglycine, the dipeptide generated in vivo in the hydrolysis of glutathione by gamma-glutamyltransferase.

METHODS

A conjugate between acetaldehyde and cysteinylglycine was generated under physiologically relevant conditions, both in vitro and in vivo. It was separated by a new reverse-phase high-performance liquid chromatography method and identified by electrospray ionization/ion trap tandem mass spectrometric analysis.

RESULTS

The conjugate with a stoichiometry of 1:1 between cysteinylglycine and acetaldehyde is most rapidly generated in vitro and was identified by mass spectroscopy as 2-methyl-thiazolidine-4-carbonyl-glycine. This thiazolidine derivative is stable in vitro and in biological fluids of rats. The conjugate was present in high concentrations in the bile of rats pretreated with ethanol and an inhibitor of aldehyde dehydrogenase.

CONCLUSIONS

The sequestering of cysteinylglycine by acetaldehyde occurs rapidly under physiologic conditions. Long-lived sulfur-containing biomolecules that incorporate acetaldehyde might affect cysteine and glutathione homeostasis and may also play a protective role by reducing circulating acetaldehyde levels. The acetaldehyde conjugate or its metabolic products could potentially serve as markers of ethanol consumption.

The peroxidase activity of cytochrome c-550 from Paracoccus versutus

Next to their natural electron transport capacities, c-type cytochromes possess low peroxidase and cytochrome P-450 activities in the presence of hydrogen peroxide. These catalytic properties, in combination with their structural robustness and covalently bound cofactor make cytochromes c potentially useful peroxidase mimics. This study reports on the peroxidase activity of cytochrome c-550 from Paracoccus versutus and the loss of this activity in presence of H2O2. The rate-determining step in the peroxidase reaction of cytochrome c-550 is the formation of a reactive intermediate, following binding of peroxide to the haem iron. The reaction rate is very low compared to horseradish peroxidase (approximately one millionth), because of the poor accessibility of the haem iron for H2O2, and the lack of a base catalyst such as the distal His of the peroxidases. This is corroborated by the linear dependence of the reaction rate on the peroxide concentration up to at least 1 M H2O2. Steady-state conversion of a reducing substrate, guaiacol, is preceded by an activation phase, which is ascribed to the build-up of amino-acid radicals on the protein. The inactivation kinetics in the absence of reducing substrate are mono-exponential and shown to be concurrent with haem degradation up to 25 mM H2O2 (pH 8.0). At still higher peroxide concentrations, inactivation kinetics are biphasic, as a result of a remarkable protective effect of H2O2, involving the formation of superoxide and ferrocytochrome c-550.

Alcoholic liver disease

Important mechanisms responsible for alcohol-induced liver injury include mitochondrial damage and loss of ATP, formation of acetaldehyde-and other aldehyde-protein adducts, release of reactive oxygen species (ROS) from mitochondrial electron transfer chain, CYP2E1 , and activated Kupffer cells (KCs); weakening of antioxidant defense systems; and increased intestinal permeability with endotoxemia. Endotoxin interacts with ethanol and/or acetaldehyde, and such interaction leads to a complex cascade of autocrine and paracrine pathways that involve the release of cytokines (proinflammatory, anti-inflammatory, and mutagenic), chemokines, and eicosanoids. These pathways are mediated by activation of KCs, induction of proliferation, and other phenotype changes in hepatic stellate cells (HSCs) leading to transformation to myofibroblasts (the latter is responsible for fibrogenesis, chemotaxis, and contractility, therefore contributing to portal hypertension, angiogenic response, and release of additional cytokines), and stimulation of sinusoidal cells (SECs) to release adhesive molecules and cytokines. Recent data implicate a likely role of apoptosis as a mechanism of hepatocyte cell death in alcoholic liver disease.

Disruptive behavior disorders and substance use disorders in adolescents

Disruptive behaviors disorders in the form of conduct disorder, oppositional defiant disorder and/or attention-deficit hyperactivity disorder are found in a majority of adolescents with substance use disorders These disorders influence the risk for and the course of substance use disorders in adolescents and potentially provide important targets for intervention. Interventions such as family therapy and multisystemic therapy can focus on important environmental factors that help to produce and sustain substance use, related problems and disruptive/deviant social behavior. Researchers and clinicians are increasingly utilizing multimodal approaches that use several psychosocial approaches in addition to medication, if indicated. This article reviews our current understanding of the relationship between disruptive behavior disorders and substance use disorders in adolescents and the importance of this understanding in the prevention, assessment and treatment of adolescents with substance use disorders.

Potentiation by chronic ethanol treatment of the mitochondrial permeability transition

Mitochondria isolated from rats chronically fed ethanol were more sensitive to induction of the mitochondrial permeability transition (MPT) by a variety of agents than mitochondria isolated from isocalorically matched controls. The agents utilized have been implicated in both necrotic (Ca(2+)) and apoptotic (ceramide, GD3 ganglioside, and Bax) forms of cell killing and help promote pore opening by differing mechanisms. In each case it was found that concentrations of the inducing agents which promoted little or no pore opening in mitochondria isolated from pair matched controls produced massive MTP opening in mitochondria from chronically ethanol fed rats as evidenced by swelling. In all cases induction of the MPT was prevented by the presence of cyclosporin A.