Effect of lead toxicity on the organ concentration of glutathione and glutathione-related free amino acids in the chick

Thiolation and Carboxylation of Glutathione Synergistically Enhance Its Lead-Detoxification Capabilities

The natural tripeptide glutathione (GSH) is a ubiquitous compound harboring various biological tasks, among them interacting with essential and toxic metal ions. Yet, although weakly binding the poisonous metal lead (Pb), GSH poorly detoxifies it. β-Mercaptoaspartic acid is a new-to-nature novel amino acid that was found to enhance the Pb-detoxification capability of a synthetic cyclic tetrapeptide. Aiming to explore the advantages of noncanonical amino acids (ncAAs) of this nature, we studied the detoxification capabilities of GSH and three analogue peptides, each of which contains at least one ncAA that harbors both free carboxylate and thiolate groups. A thorough investigation that includes in vitro detoxification and mechanistic evaluations, metal-binding affinity, metal selectivity, and computational studies shows that these ncAAs are highly beneficial in additively enhancing Pb binding and reveals the importance of both high affinity and metal selectivity in synergistically reducing Pb toxicity in cells. Hence, such ncAAs join the chemical toolbox against Pb poisoning and pollution, enabling peptides to strongly and selectively bind the toxic metal ion.

Heavy Metals and Human Health: Mechanistic Insight into Toxicity and Counter Defense System of Antioxidants

Heavy metals, which have widespread environmental distribution and originate from natural and anthropogenic sources, are common environmental pollutants. In recent decades, their contamination has increased dramatically because of continuous discharge in sewage and untreated industrial effluents. Because they are non-degradable, they persist in the environment; accordingly, they have received a great deal of attention owing to their potential health and environmental risks. Although the toxic effects of metals depend on the forms and routes of exposure, interruptions of intracellular homeostasis include damage to lipids, proteins, enzymes and DNA via the production of free radicals. Following exposure to heavy metals, their metabolism and subsequent excretion from the body depends on the presence of antioxidants (glutathione, α-tocopherol, ascorbate, etc.) associated with the quenching of free radicals by suspending the activity of enzymes (catalase, peroxidase, and superoxide dismutase). Therefore, this review was written to provide a deep understanding of the mechanisms involved in eliciting their toxicity in order to highlight the necessity for development of strategies to decrease exposure to these metals, as well as to identify substances that contribute significantly to overcome their hazardous effects within the body of living organisms.

Phosphorus amendment reduces hepatic and renal oxidative stress in mallards ingesting lead-contaminated sediments

Lead poisoning of waterfowl has been reported for decades in the Coeur d'Alene River Basin (CDARB) in Idaho as a result of the ingestion of lead-contaminated sediments. This study was conducted to determine whether the addition of phosphoric acid to CDARB sediments would reduce the bioavailability and toxicity of lead to the liver and kidney of mallards (Anas platyrhynchos). Mallards received diets containing 12% clean sediment (controls) or 12% sediment from three different CDARB sites containing 4520, 5390, or 6990 microg/g lead (dry weight) with or without phosphoric acid amendment. Liver and kidney lead concentrations were significantly higher in all CDARB treatment groups and ranged from geometric mean values of 18.2 (liver) and 28.7 (kidney) for the first 2 sites to 22.5 (liver) and 45.6 (kidney) microg/g (wet weight) for the third site. With amendments all liver lead concentrations were reduced 36 to 55%, and all kidney lead concentrations were lowered 54 to 73%. Unamended CDARB sediment from the third site resulted in the following hepatic effects: over 1.6-fold elevation of liver glutathione (reduced form; GSH) concentration, higher GSH S-transferase and oxidized glutathione (GSSG) reductase activities, and lower protein-bound thiols (PBSH) concentration. Renal effects included higher kidney GSH concentrations for all CDARB sites, with over 2.1-fold higher for the third site. Resulting kidney GSSG to GSH ratios were lower at two sites. At the third site, gamma-glutamyl transferase (GGT) activity was elevated, and lipid peroxidation as thiobarbituric acid-reactive substances (TBARS) was 1.7-fold greater. Amendment restored all hepatic variables as well as the renal variables TBARS and GGT so they did not differ from controls. Although amendments of phosphorus substantially reduced the bioavailability of lead and some of the adverse effects, lead concentrations in the tissues of mallards fed the amended sediments were still above those considered to be harmful to waterfowl under the present conditions.

Changes in expression of neuronal and glial glutamate transporters in lead-exposed adult rat brain

Excitatory amino acid transporters (EAATs) are membrane-bound proteins localized in glial and neuronal cells which transport glutamate (Glu) in a process essential for terminating its action and protecting neurons from excitotoxic damage. Since Pb-induced neurotoxicity has a glutamatergic component and astrocytes serve as a cellular Pb deposition site, it was of interest to investigate the response of main glutamate transporters to short-term lead exposure in the adult rat brain (25mg/kg b.w. of lead acetate, i.p. for 3 days). We examined the expression of mRNA and protein of GLAST, GLT-1 and EAAC1 in homogenates obtained from cerebellum, hippocampus and forebrain. Molecular evidence is provided which indicates that, of the two glial transporters, GLT-1 is more susceptible than GLAST to the neurotoxic effect arising from Pb. RT-PCR analysis revealed highly decreased expression of GLT-1 mRNA in forebrain and hippocampus. In contrast, GLAST was overexpressed in forebrain and in cerebellum. In the case of EAAC1, the enhanced expression of mRNA and protein of transporter was observed only in forebrain. The results demonstrate regional differences in the expression of glutamate transporters after short-term exposure to Pb. In forebrain, downregulation of GLT-1 is compensated by enhanced expression of GLAST, while in hippocampus, the expression of both is lowered. This observation suggests that under conditions of Pb toxicity in adult rat brain, the hippocampus is most vulnerable to the excitotoxic cell damage arising from impaired clearance of the released glutamate.

The role of astroglia in Pb-exposed adult rat brain with respect to glutamate toxicity

Astrocytes maintain neuronal homeostasis in brain and controlling of the released glutamate is one of the most important functions. Since it is suggested that glutamatergic component underlies lead-induced neurotoxic effects and simultaneously, astrocytes serve as a cellular lead (Pb) deposition site, it was of interest to investigate the functioning of astroglia in adult rat brain after short-term exposure to Pb. We examined the expression of main astrocytic glutamate/aspartate transporters--GLAST and GLT-1, which regulate extracellular glutamate concentration. Molecular evidence is provided which indicates overexpression of GLAST mRNA and protein. Simultaneously, decreased expression of GLT-1 mRNA and protein was observed, indicating that of the two glial transporters, GLT-1 is more susceptible to the toxic Pb effect. Protein expression of glutamine synthetase (GS), which converts toxic glutamate to non-toxic glutamine, was doubly enhanced. Moreover, Na+-dependent transport of radioactive glutamine to astroglia-derived fraction was affected in Pb-exposed rats. Both the rate of accumulation and the efflux of amino acid were diminished. Additionally, we observed enhanced expression of glutathione-protein complexes after Pb treatment what suggests activation of S-glutathionylation processes. The results of current studies indicate that lead toxicity in adult rat brain activates astrocytic processes connected with the controlling of glutamate homeostasis. The response of astroglia is rather of neuroprotective character however, downexpression of GLT-1 glutamate transporter and activation of S-glutathionylation processes lead to the question about their significance in Pb-induced neurotoxicity.

Effects of lead-contaminated sediment and nutrition on mallard duckling brain growth and biochemistry

Day-old mallard (Anas platyryhnchos) ducklings received either a clean sediment (24%) supplemented control diet, Coeur d'Alene River Basin, Idaho (CDARB) sediment (3449 microg/g lead) supplemented diets at 12% or 24%, or a positive control diet (24% clean sediment with equivalent lead acetate to the 24% CDARB diet) for 6 weeks. The 12% CDARB diet resulted in a geometric mean concentration of 396 ppb (WW) brain lead with decreased brain protein and ATP concentrations but increased oxidized glutathione (GSSG) relative to the control diet. The 24% CDARB diet resulted in a concentration of 485 ppb brain lead with lower brain weight and ATP concentration than controls but higher concentrations of reduced glutathione (GSH) and calcium. Lead acetate accumulated twice as well as CDARB derived lead and resulted in histopathological lesions of the brain. With a combination of a suboptimal diet and 24% CDARB, brain lead concentration was higher (594 ppb) than with 24% CDARB in the standard diet, histopathological lesions became apparent and GSH was higher than suboptimal diet controls.

Astroglial reaction during the early phase of acute lead toxicity in the adult rat brain

The developing nervous system is susceptible to lead (Pb) exposure but less is known about the effect of this toxic agent in adult rat brain. Since astrocytes serve as a cellular Pb deposition site, it is of importance to investigate the response of astroglial cells in the adult rat brain in a model of acute lead exposure (25 mg/kg b.w. of lead acetate, i.p. for 3 days). An increased immunoreactivity of glial fibrillary acidic protein (GFAP) on Western blots was noticeable in fractions of astroglial origin-glial plasmalemmal vesicles (GPV) and in homogenates from the hippocampus and cerebral cortex but not in the cerebellum. The features of enhanced astrocytic reactivity (i.e. large accumulation of mitochondria, activated Golgi apparatus and increment of gliofilaments) were observed in electron microscopy studies in the same tissues. Total glutathione levels increased both in GPV fractions and in brain homogenates-in the cerebellum (120% above control) and in hippocampus (30% above control). The results of current studies indicate that acute lead exposure is accompanied by astrocyte activation connected with the presence of the enhanced expression of GFAP. It may indicate lead-induced neuronal injury. At the same time, a regional enhancement of detoxicative mechanisms (GSH) was noticed, suggesting activation of astrocyte-mediated neuroprotection against toxic Pb action.

Lead reduces the nephrotoxicity of mercuric chloride

The nephrotoxic effect of a single intraperitoneal dose of mercuric chloride (HgCl(2); 6 mg/kg) on adult CD-1 female mice was reduced at 24 and 48 h after injection, by a 48-h pretreatment nontoxic dose of lead acetate (Pb; 5 mg/kg) delivered by intravenous tail-vein injection (intravenous). While protection is temporally associated with lead-induced mitosis, occurring about 39 h after intracardiac lead injection (D. D Choie and G. W. Richter, 1974, Lab. Invest. 30, 447-451), the mechanism of the observed protection remains to be established.

The effects of exogenous glutathione and cysteine on oxidative stress induced by ferric nitrilotriacetate

The effects of the antioxidants, glutathione (GSH) and its precursor cysteine (Cys) on oxidative damage induced by ferric nitrilotriacetate (Fe-NTA) were examined. Fe-NTA-associated oxidative stress caused the depletion of renal cellular GSH content. Administration of exogenous GSH and Cys suppressed 8-hydroxydeoxyguanosine (8-OH-dG) formation, an indicator of oxidative DNA damage and nephrotoxicity following Fe-NTA treatment. This suggests that generation of free radicals may be causally involved in oxidative lesion generation. Since lipid peroxidation was found to be inhibited only by GSH and not Cys treatment, this suggests that this effect and the DNA damage might be mediated by different pathways. Fe-NTA-associated oxidative stress in renal tubular cells might thus operate via both intracellular and external space modes.

Decreased glutathione S-transferase activity in mice livers by acute treatment with lead, independent of alteration in glutathione content

Glutathione S-transferase (GST) activity and glutathione content in livers of male mice were assayed after acute administration of lead acetate (100 mg/kg i.p.). Fall in GST activity of lead-treated mice followed the decrease in glutathione content with a delay of more than 1 day. In addition, L-methionine (250 mg/kg i.p.) pretreatment did not antagonize the fall in GST activity induced by lead. In contrast, diethyl maleate, a potent glutathione depletor, increased GST activity. Thus, lead administration reduced the ability of the phase II reaction of drug metabolism, although glutathione depletion was not necessarily a critical factor for impairmen of GST per se.