Inhibition of cystathionine-γ-lyase leads to loss of glutathione and aggravation of mitochondrial dysfunction mediated by excitatory amino acid in the CNS

Oxidative stress has been implicated in the pathogenesis and progression of neurodegenerative disorders and antioxidants potentially have a major role in neuroprotection. Optimum levels of glutathione (γ-glutamylcysteinyl glycine), an endogenous thiol antioxidant are required for the maintenance of the redox status of cells. Cystathionine γ-lyase is the rate-limiting enzyme for the synthesis of cysteine from methionine and availability of cysteine is a critical factor in glutathione synthesis. In the present study, we have examined the role of cystathionine γ-lyase in maintaining the redox homeostasis in brain, particularly with reference to mitochondrial function since the complex I of the electron transport chain is sensitive to redox perturbation. Inhibition of cystathionine γ-lyase by L-propargylglycine caused loss of glutathione and decrease in complex I activity in the brain although the enzyme activity in mouse brain was 1% of the corresponding hepatic activity. We then examined the effect of this inhibition on the neurotoxicity mediated by the excitatory amino acid, L-β-oxalyl amino-L-alanine, which is the causative factor of a type of motor neuron disease, neurolathyrism. L-β-oxalyl amino-L-alanine toxicity was exacerbated by L-propargylglycine measured as loss of complex I activity indicating the importance of cystathionine γ-lyase in maintaining glutathione levels and in turn the mitochondrial function during excitotoxicity. Oxidative stress generated by L-β-oxalyl amino-L-alanine itself inhibited cystathionine γ-lyase, which could be prevented by prior treatment with thiol antioxidant. Thus, cystathionine γ-lyase itself is susceptible to inactivation by oxidative stress and this can potentially exacerbate oxidant-induced damage. Cystathionine γ-lyase is present in neuronal cells in human brain and its activity is several-fold higher compared to mouse brain. It could potentially play an important role in maintaining glutathione and protein thiol homeostasis in brain and hence afford neuroprotection

Oxidative stress induced by administration of the neuroleptic drug haloperidol is attenuated by higher doses of haloperidol

The effect of haloperidol administration on lipid peroxidation and glutathione/protein thiol homeostasis in the brain was examined 4 h following subcutaneous administration of a single dose of haloperidol; 1.0, 1.5, 2.0 or 2.5 mg/kg b.wt. Glutathione (GSH) levels decreased significantly in cortex, striatum and midbrain after haloperidol administration. Maximal decreases of GSH was observed in the striatum. The depleted GSH was recoverable as protein glutathione mixed disulfide (Pr-SSG) with concomitant loss of protein thiols (Pr-SH) in all the regions of the brain examined. Administration of 1.5 mg/kg b.wt of haloperidol resulted in significant depletion of GSH in striatum and midbrain as compared to that after administration of the lower dose of 1.0 mg/kg b.wt. of haloperidol. However, administration of higher doses of haloperidol (2.0 and 2.5 mg/kg b.wt.) did not result in greater depletion of GSH; the GSH levels were not significantly different from that observed following the administration of 1.5 mg/kg b.wt. of haloperidol. However, Pr-SSG levels increased dose-dependently following haloperidol administration. The total GSH recovered as sum of GSH and Pr-SSG was significantly higher than controls in striatum and midbrain following administration of higher doses of haloperidol, namely, 2.0 and 2.5 mg/kg b.wt. The depleted GSH was not recoverable as glutathione disulfide (GSSG), GSSG levels were not significantly different from controls 4 h after administration of 1.5 mg/kg b.wt. of haloperidol. The levels of malondialdehyde (indicative of lipid peroxidation) increased significantly as compared to control levels (280-220%) following administration of 1.0 and 1.5 mg/kg b.wt. of haloperidol. Thereafter, the malondialdehyde levels in brain regions decreased and were only (186-150%) of control levels after administration of 2.0 and 2.5 mg/kg b.wt. of haloperidol, respectively. The present study demonstrates that administration of low doses of haloperidol results in depletion of GSH and increased levels of malondialdehyde. However, administration of higher doses of haloperidol results in attenuation of peroxidative damage with concomitant increase in the total GSH recovered as sum of free GSH and GSH bound to protein thiols (Pr-SSG)

Glutaredoxin is essential for maintenance of brain mitochondrial complex I: studies with MPTP

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L-BOAA induces selective inhibition of brain mitochondrial enzyme, NADH-dehydrogenase

Lathyrism, a human neurological disorder has been linked to the excessive consumption of a plant toxin, β-oxalylamino-L-alanine (L-BOAA) present in Lathyrus sativus. The present study was carried out to elucidate the biochemical mechanisms underlying L-BOAA-induced toxic insult. Incubation of sagittal slices of mouse brain with L-BOAA resulted in dose and time-dependent inhibition of mitochondrial NADH-dehydrogenase (NADH-DH). Significant inhibition of NADH-DH was seen following incubation of brain slices with very low concentration of L-BOAA (0.1 pM). L-BOAA also induced lactate dehydrogenase (LDH) leakage from the slice into the medium in dose-dependent manner. The inhibition of NADH-DH preceded LDH leakage from the slices into the medium. L-BOAA had no effect on other mitochondrial enzymes, namely, isocitrate dehydrogenase or cytochrome c oxidase. Incubation of isolated mouse brain mitochondria with L-BOAA also resulted in inhibition of NADH-DH. L-BOAA-induced inhibition of NADH-DH was prevented by non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonists in general and α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor antagonist (NBQX) in particular. Other glutamate agonists examined namely, N-methyl-D-aspartate, β-N-methylamino-L-alanine (L-BMAA), L-glutamic acid, N-acetylaspartylglutamate (NAAG), quisqualic acid, kainic acid or AMPA did not have any effect on NADH-DH activity in slices although they induced LDH leakage from the slice into the medium. Incubation of brain slices with L-BOAA did not induce lipid peroxidation or changes in glutathione levels. Prior incubation of slices with glutathione (GSH) or GSH-isopropyl ester did not prevent L-BOAA-induced inhibition of NADH-DH. However, incubation of isolated mitochondria with L-BOAA in the presence of GSH-isopropyl ester prevented L-BOAA-induced inhibition of NADH-DH, indicating the protective effect of mitochondrial glutathione in the prevention of L-BOAA-induced toxicity

Protection and potentiation of MPTP-induced toxicity by cytochrome P-450 inhibitors and inducer: in vitro studies with brain slices

Exposure to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes loss of dopaminergic neurons in humans, primates and mice. Exposure of sagittal slices of mouse brain to MPTP (100 pM) caused inhibition of mitochondrial NADH-dehydrogenase activity. Leakage of lactate dehydrogenase from the slice into the medium was observed following incubation of slices with 1 nM MPTP. Neurotoxicity induced by MPTP was prevented by prior exposure of the slices to the dopamine uptake inhibitor GBR 12935. Deprenyl and pargyline (inhibitors of monoamine oxidase), also protected the slices from MPTP-induced toxicity. However, both pargyline and deprenyl also inhibited cytochrome P-450 mediated aminopyrine N-demethylase activity in brain slices. Pargyline, when administered in vivo to mice, decreased brain cytochrome-450 levels significantly. Other cytochrome P-450 inhibitors, namely, piperonyl butoxide and SKF 525A were found to offer protection against MPTP induced neurotoxicity in slices without affecting monoamine oxidase activity. MPTP toxicity was potentiated significantly in brain slices prepared from mice pretreated with phenobarbital, an inducer of cytochrome P-450. The present study suggests the possible involvement of cytochrome P-450 in MPTP-induced neurotoxicity, in vitro, in brain slices

Human brain thioltransferase: constitutive expression and localization by fluorescence in situ hybridization

Thioltransferase (glutaredoxin) is a member of the family of thiol-disulfide oxido-reductases that maintain the sulfhydryl homeostasis in cells by catalyzing thiol-disulfide interchange reactions. One of the major consequences of oxidative stress in brain is formation of protein-glutathione mixed disulfide (through oxidation of protein thiols) which can be reversed by thioltransferase during recovery of brain from oxidative stress. Here we have visualized the location of thioltransferase in brain regions from seven human tissues obtained at autopsy. Constitutively expressed thioltransferase activity was detectable in all human brains examined although inter-individual variations were seen. The enzyme activity was significantly higher in hippocampus and cerebellum as compared to other regions. Constitutive expression of thioltransferase mRNA was detectable by Northern blot analysis. Localization of thioltransferase mRNA by fluorescence in situ hybridization revealed its presence predominantly in neurons in the cerebral cortex, Purkinje and granule cell layers of the cerebellum, granule cell layer of the dentate gyrus and in the pyramidal neurons of CA1, CA2 and CA3 subfields of hippocampus. These discrete neuronal concentrations of thioltransferase would be consistent with an essential role in modulating recovery of protein thiols from mixed disulfides formed during oxidative stress

Rat brain cytochromes P-450: catalytic, immunochemical properties and inducibility of multiple forms

Cytochrome P-450 (P-450) and associated mono-oxygenase activities were estimated in male and female rat brain microsomes. The P-450 concentration in male rat brain was one-tenth the corresponding hepatic levels, which is considerably higher than earlier reports. A distinct sex-related difference was observed in the levels of total P-450 and mono-oxygenase activities known to be mediated by P-450b,e; the female brain levels were 60% of those in the males. Immunoinhibition and immunoblot studies using antisera to P-450b,e and P-450c,d indicated the presence of multiple forms of P-450, immunologically similar to P-450b,e, P-450c and P-450d in the rat brain. Prior treatment with phenobarbital resulted in two-fold increase of total P-450 and selective induction of aminopyrine N-demethylase (APD) and morphine N-demethylase (MND) activities. Administration of 3-methylcholanthrene, selectively induced the levels of ethoxycoumarin O-deethylase (ECD) and arylhydrocarbon hydroxylase, although the levels of total P-450 were not increased. 3-Methylcholanthrene induction was also accompanied by a shift in the absorption maximum of the reduced carbon monoxide difference spectrum from 452 to 448 nm. Immunocytochemical localization using antibodies to P-450b,e indicated the presence of P-450 predominantly in the neuronal cell bodies and to a lesser extent in the fibre tracts in cerebral cortex, cerebellum, thalamus, hypothalamus, hippocampus and brainstem. These studies indicate that the brain contains significant amounts of P-450, which exists in multiple forms and can be selectively induced by prior exposure to phenobarbital or 3-methylcholanthrene

Presence of splice variant forms of cytochrome P4502D1 in rat brain but not in liver

Cytochromes P450 (P450), a family of heme-containing proteins, is involved in the oxidative metabolism of both foreign and endogenous compounds. Although liver is quantitatively the major organ involved in the metabolism of most xenobiotics, there is increasing evidence that these enzymes are present in extrahepatic tissues, such as lung, kidney, brain, etc and they may contribute to the in situ metabolism of xenobiotics in these organs. The possible relationship between genetic polymorphism seen in P4502D6 and incidence of neurodegenerative diseases, such as Parkinson's disease, has prompted the characterization of P4502D enzymes in rat brain. In the present study, we demonstrate that P4502D1 (the rat homologue of human P4502D6) is constitutively expressed in rat brain and the mRNA and protein are localized predominantly in neuronal cell population in the olfactory bulb, cortex, cerebellum, and hippocampus. An alternate spliced transcript of CYP2D1 having exon 3 deletion was detected in rat brain but not in liver. Deletion of exon 3 causes frame shift and generates a stop codon at 391 bp relative to the start codon ATG leading to premature termination of translation. Thus, Northern blotting and in situ hybridization represent contributions from functional transcripts and alternate spliced variants that do not translate into functional protein. Further, the splice variant having partial inclusion of intron 6 detected in human brain was not detected in rat brain indicating that alternate spliced gene products of P450 enzymes are generated in species-specific and tissue-specific manner

Low glutathione levels in brain regions of aged rats

Glutathione (GSH) was measured in 6 regions of brain and liver of young adult, middle-aged and aged rats. GSH levels were significantly lower in cortex, cerebellum, striatum, thalamus and hippocampus of aged rats, while no changes were observed in liver as compared to young adult rats. On the other hand, lipid peroxidation as measured by thiobarbituric acid-reactive products increased significantly in all the regions of brain examined and in the liver of aged rats. Since GSH plays an important role as a cellular protectant against oxygen radical-mediated injury, decreased levels of GSH in aged rat brain are indicative of the vulnerability of the aged cerebral tissue to oxidative injury

Estrogen and neuroprotection: higher constitutive expression of glutaredoxin in female mice offers protection against MPTP-mediated neurodegeneration

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