Effect of hydrazine upon vitamin B12-dependent methionine synthase activity and the sulphur amino acid pathway in isolated rat hepatocytes

Betaine treatment attenuates chronic ethanol-induced hepatic steatosis and alterations to the mitochondrial respiratory chain proteome

Introduction. Mitochondrial damage and disruption in oxidative phosphorylation contributes to the pathogenesis of alcoholic liver injury. Herein, we tested the hypothesis that the hepatoprotective actions of betaine against alcoholic liver injury occur at the level of the mitochondrial proteome. Methods. Male Wister rats were pair-fed control or ethanol-containing liquid diets supplemented with or without betaine (10 mg/mL) for 4-5 wks. Liver was examined for triglyceride accumulation, levels of methionine cycle metabolites, and alterations in mitochondrial proteins. Results. Chronic ethanol ingestion resulted in triglyceride accumulation which was attenuated in the ethanol plus betaine group. Blue native gel electrophoresis (BN-PAGE) revealed significant decreases in the content of the intact oxidative phosphorylation complexes in mitochondria from ethanol-fed animals. The alcohol-dependent loss in many of the low molecular weight oxidative phosphorylation proteins was prevented by betaine supplementation. This protection by betaine was associated with normalization of SAM : S-adenosylhomocysteine (SAH) ratios and the attenuation of the ethanol-induced increase in inducible nitric oxide synthase and nitric oxide generation in the liver. Discussion/Conclusion. In summary, betaine attenuates alcoholic steatosis and alterations to the oxidative phosphorylation system. Therefore, preservation of mitochondrial function may be another key molecular mechanism responsible for betaine hepatoprotection.

Inhibition of cobalamin-dependent methionine synthase by substituted benzo-fused heterocycles

The cobalamin-dependent cytosolic enzyme, methionine synthase (EC.2.1.1.13), catalyzes the remethylation of homocysteine to methionine using 5-methyltetrahydrofolate as the methyl donor. The products of this remethylation--methionine and tetrahydrofolate--participate in the active methionine and folate pathways. Impaired methionine synthase activity has been implicated in the pathogenesis of anaemias, cancer and neurological disorders. Although the need for potent and specific inhibitors of methionine synthase has been recognized, there is a lack of such agents. In this study, we designed, synthesized and evaluated the inhibitory activity of a series of substituted benzimidazoles and small benzothiadiazoles. Kinetic analysis revealed that the benzimidazoles act as competitive inhibitors of the rat liver methionine synthase, whilst the most active benzothiadiazole (IC(50) = 80 microm) exhibited characteristics of uncompetitive inhibition. A model of the methyltetrahydrofolate-binding site of the rat liver methionine synthase was constructed; docking experiments were designed to elucidate, in greater detail, the binding mode and reveal structural requirements for the design of inhibitors of methionine synthase. Our results indicate that the potency of the tested compounds is related to a planar region of the inhibitor that can be positioned in the centre of the active site, the presence of a nitro functional group and two or three probable hydrogen-bonding interactions.

Statistical Search Space Reduction and Two-Dimensional Data Display Approaches for UPLC−MS in Biomarker Discovery and Pathway Analysis

A new analytical strategy for biomarker recovery from directly coupled ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC Tof MS) data on biofluids is presented and exemplified using a study on hydrazine-induced liver toxicity. A key step in the strategy involves a novel procedure for reducing the spectroscopic search space by differential analysis of cohorts of normal and pathological samples using an orthogonal projection to latent structures discriminant analysis (O-PLS-DA). This efficiently sorts principal discriminators of toxicity from the background of thousands of metabolic features commonly observed in data sets generated by UPLC-MS analysis of biological fluids and is thus a powerful tool for biomarker discovery.

Involvement of apoptosis in hydrazine induced toxicity in rat primary hepatocytes

The current study was undertaken to investigate the role of apoptosis in hydrazine induced hepatotoxicity. Hepatocytes were exposed to hydrazinium nitrate (HzN) at two doses (50 and 75 mM) for 2 h then placed in fresh HzN-free media and cultured for an additional 24 h. Post-exposure, cell viability was evaluated at several time points by lactate dehydrogenase (LDH) leakage and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction. Markers of apoptosis (mitochondrial membrane potential, annexin binding, DNA fragmentation, caspase activation, and cytochrome c release) were measured 24 h post-exposure. The viability data showed time dependent increase in LDH leakage at 75 mM of HzN, with only a slight increase at 50 mM. MTT reduction showed a decrease in mitochondrial activity at both doses immediately after the 2 h continuous exposure. However, MTT reduction returned to normal at 50 mM while at 75 mM, MTT reduction initially recovered but then deteriorated to approximately 50% of controls at 24 h post-exposure. Based on viability data, exposure to 50 mM HzN for 2 h is a marginally toxic dose while 75 mM is a significantly toxic dose. The results for apoptosis biomarkers showed a reduction in mitochondrial membrane potential, an increase in annexin binding, an increase in total caspase activity, moderate activation of caspase-3, and release of cytochrome c. However, the appearance of DNA fragmentation in HzN exposed cells was very low compared to positive controls (cadmium and cyclosporine). The possibility that HzN induces apoptosis without the involvement of DNA fragmentation can not be ruled out. The present results, overall, suggest that apoptosis may be a contributing factor in acute HzN toxicity.

Methionine synthase activity and sulphur amino acid levels in the rat liver tumour cells HTC and Phi-1

Methionine dependence has been reported in tumour cells and suggested as a possible target for chemotherapeutic drugs. The underlying defect has not been extensively researched, nor have levels of sulphur amino acids been examined in these cells. This study compared two rat liver tumour cell lines. One was found to be methionine dependent (HTC) and the other found to be methionine independent (Phi-1). The methionine-dependent cell line (HTC) was discovered to contain markedly less methionine synthase activity, the enzyme activity being less responsive to methionine concentration than in the methionine-independent cells (Phi-1). HTC cells had lower cysteine requirements and contained larger concentrations of reduced glutathione (GSH) and taurine than the Phi-1 cells. Also, in contrast to Phi-1 cells, no glutathione was found in the media of the HTC cells, although large quantities of cysteinylglycine were detected. These results suggested that differences in methionine synthase activity might be partly responsible for methionine dependence and that methionine-dependent cells may have different metabolic requirements for other sulphur amino acids.

In vitro toxicity assessment of a new series of high energy compounds

Hydrazine is an aircraft fuel and propellant used by the US Air Force. Due to its toxicity the Propulsion Directorate of the Air Force Research Laboratory (AFRL/PR) has investigated alternative chemicals to replace hydrazine. AFRL/PR has synthesized a series of high energy chemicals (HECs), primarily hydrazine derivatives and amino containing compounds such as hydrazinium nitrate (HZN), 2-hydroxyethyl-hydrazine nitrate (HEHN), diethyl hydrazine nitrate (DEHN), ethanolamine nitrate (EAN), histamine dinitrate (HDN) and methoxylamine nitrate (MAN) to study as alternative chemical candidates. Although HECs are reliable constituents of powered propellant systems, they constitute an important class of toxic agents to which military and civilian personnel can be exposed. The current study was undertaken to examine the toxicity of HECs in primary hepatocytes in vitro. The effects of short-term exposure (4 h) of hepatocytes to HECs were investigated with reference to viability, mitochondrial function and oxidative stress markers. The results showed a decrease in mitochondrial activity, increase in lactate dehydrogenase (LDH) leakage and depletion of reduced glutathione (GSH) levels. The levels of reactive oxygen species (ROS) increased dose dependently in HZN, MAN and HDN exposed cells. However, there was no induction of ROS generation in EAN, DEHN and HEHN exposed cells. Depletion of GSH in hepatocytes by buthionine sulfoximine (BSO) prior to exposure to HZN increased its toxicity. The results suggest that at least one mechanism of HEC toxicity is mediated through oxidative stress.