[Show abstract][Hide abstract] ABSTRACT: We previously developed a screening method to identify proteins that undergo aggregation through S-mercuration by methylmercury (MeHg) and found that rat arginase I is a target protein for MeHg (Kanda et al. in Arch Toxicol 82:803-808, 2008). In the present study, we characterized another S-mercurated protein from a rat hepatic preparation that has a subunit mass of 42 kDa, thereby facilitating its aggregation. Two-dimensional SDS-polyacrylamide gel electrophoresis and subsequent peptide mass fingerprinting using matrix-assisted laser desorption and ionization time-of-flight mass spectrometry revealed that the 42 kDa protein was NAD-dependent sorbitol dehydrogenase (SDH). With recombinant rat SDH, we found that MeHg is covalently bound to SDH through Cys44, Cys119, Cys129 and Cys164, resulting in the inhibition of its catalytic activity, release of zinc ions and facilitates protein aggregation. Mutation analysis indicated that Cys44, which ligates the active site zinc atom, and Cys129 play a crucial role in the MeHg-mediated aggregation of SDH. Pretreatment with the cofactor NAD, but not NADP or FAD, markedly prevented aggregation of SDH. Such a protective effect of NAD on the aggregation of SDH caused by MeHg is discussed.
Full-text · Article · Jun 2012 · Archives of Toxicology
[Show abstract][Hide abstract] ABSTRACT: The toxicity of methylmercury (MeHg) is, in part, thought to be due to its interaction with thiol groups in a variety of enzymes, but the molecular targets of MeHg are poorly understood. Arginase I, an abundant manganese (Mn)-binding protein in the liver, requires Mn as an essential element to exhibit maximal enzyme activity. In the present study, we examined the effect of MeHg on hepatic arginase I in vivo and in vitro. Subcutaneous administration of MeHg (10 mg/kg) for 8 days to rats resulted in marked suppression of arginase I activity. With purified arginase I, we found that interaction of MeHg with arginase I caused the aggregation of arginase I as evaluated by centrifugation and subsequent precipitation, and then the reduction of catalytic activity. Experiments with organomercury column confirmed that arginase I has reactive thiols that are covalently bound to organomercury. While MeHg inhibited arginase I activity, Mn ions were released from this enzyme. These results suggest that MeHg-mediated suppression of hepatic arginase I activity in vivo is, at least in part, attributable to covalent modification of MeHg or substantial leakage of Mn ions from the active site.
No preview · Article · Jun 2008 · Archives of Toxicology
[Show abstract][Hide abstract] ABSTRACT: Inorganic mercury is a toxic metal that accumulates in the proximal tubules of the kidney, causing apoptosis. Arginase II is known to inhibit apoptosis, but its role in the renal apoptosis caused by inorganic mercury is poorly understood. In the present study, we examined the involvement of arginase II in inorganic mercury-dependent apoptosis. A single exposure to mercuric chloride (HgCl(2), 1 mg/kg) in rats resulted in a dramatic time-dependent reduction in the activity of arginase II in the kidney; for example, the activity at 48 h after exposure was 31% of the control level. The decrease in arginase II activity was due to a decrease in the protein level, not to a reduction in gene expression or to direct inhibition of the activity itself. More interestingly, diminished arginase II activity was well correlated with the induction of apoptosis as evaluated by renal DNA fragmentation (r = 0.99). Overexpression of arginase II in LLC-PK(1) cells blocked cell death during exposure to inorganic mercury. These results suggest that inorganic mercury causes a reduction in protein levels of arginase II, and that impaired arginase II activity is, at least in part, associated with the apoptotic cell damage caused by this heavy metal.
Full-text · Article · Mar 2008 · Archive für Toxikologie
[Show abstract][Hide abstract] ABSTRACT: To determine the mechanism of 2,4,6-trinitrotoluene (TNT)-induced oxidative stress involving neuronal nitric oxide synthase (nNOS), we examined alterations in enzyme activity and gene expression of nNOS by TNT, with an enzyme preparation and rat cerebellum primary neuronal cells. TNT inhibited nitric oxide formation (IC(50) = 12.4 microM) as evaluated by citrulline formation in a 20,000 g cerebellar supernatant preparation. A kinetic study revealed that TNT was a competitive inhibitor with respect to NADPH and a noncompetitive inhibitor with respect to L-arginine. It was found that purified nNOS was capable of reducing TNT, with a specific activity of 3900 nmol of NADPH oxidized/mg/min, but this reaction required CaCl(2)/calmodulin (CaM). An electron spin resonance (ESR) study indicated that superoxide (O(2)(.-)) was generated during reduction of TNT by nNOS. Exposure of rat cerebellum primary neuronal cells to TNT (25 microM) caused an intracellular generation of H(2)O(2), accompanied by a significant increase in nNOS mRNA levels. These results indicate that CaM-dependent one-electron reduction of TNT is catalyzed by nNOS, leading to a reduction in NO formation and generation of H(2)O(2) derived from O(2)(.-). Thus, it is suggested that upregulation of nNOS may represent an acute adaptation to an increase in oxidative stress during exposure to TNT.
No preview · Article · Sep 2004 · Free Radical Biology and Medicine
[Show abstract][Hide abstract] ABSTRACT: Phenanthraquinone (PQ) is a component of die- sel exhaust particles which inhibits nitric oxide syn- thase (NOS) activity by shunting electrons away from the normal catalytic pathway of the enzyme and which can oxidize proximal protein sulfhydryls. In the present study, we examined the possibility that PQ may also modify critical thiol residues on endothelial NOS (eNOS), leading to a disruption of catalytic activity. PQ and the thiol-modifying agent N-ethylmaleimide (NEM) suppressed NO formation from L-arginine by the total membrane fraction of bovine aortic endothe- lial cells in a concentration-dependent manner. The dithiol agent dithiothreitol (DTT) completely blocked NEM-mediated inhibition of eNOS activity. In con- trast, PQ-inhibited eNOS activity was reduced by DTT, but not by the monothiol agent glutathione. These re- sults suggest that PQ-mediated suppression of eNOS activity involves not only uncoupling of the electron transport of this enzyme, but also modification of pre- sumably the proximal protein sulfhydryls that play an important role in the maximal catalytic activity.
Full-text · Article · Dec 2001 · Journal of health science
[Show abstract][Hide abstract] ABSTRACT: Diesel exhaust particles cause an impairment of endothelium-dependent vasorelaxation and are associated with cardiopulmonary-related diseases and mortality, but the mechanistic details are poorly understood. Since we reported previously that phenanthraquinone, an environmental chemical contained in diesel exhaust particles, suppresses neuronal nitric oxide synthase (nNOS) activity by shunting electrons away from the normal catalytic pathway, it was hypothesized that phenanthraquinone inhibits endothelial NOS (eNOS) activity and affects vascular tone. Therefore, the effects of phenanthraquinone on eNOS activity, endothelium-dependent relaxation, and blood pressure were examined in the present study. Phenanthraquinone inhibited NO formation evaluated by citrulline formed by total membrane fraction of bovine aortic endothelial cells with an IC(50) value of 0.6 microM. A kinetic study revealed that phenanthraquinone is a competitive inhibitor with respect to NADPH and a noncompetitive inhibitor with respect to L-arginine. Endothelium-dependent relaxation of rat aortic rings by ACh was significantly inhibited by phenanthraquinone (5 microM), whereas the endothelium-independent relaxation by nitroglycerin was not. Furthermore, an intraperitoneal injection of phenanthraquinone (0.36 mmol/kg) to rats resulted in an elevation of blood pressure (1.4-fold, P < 0.01); under this condition, plasma levels of stable NO metabolites, nitrite/nitrate, in phenanthraquinone-treated rats was reduced to 68% of control levels. The present findings suggest that phenanthraquinone has a potent inhibitory action on eNOS activity via a similar mechanism reported for nNOS, thereby causing the suppression of NO-mediated vasorelaxation and elevation of blood pressure.
No preview · Article · Jul 2001 · AJP Regulatory Integrative and Comparative Physiology
[Show abstract][Hide abstract] ABSTRACT: Alterations in renal superoxide dismutase (SOD) isozymes were examined in cancerous tissues of human renal cell carcinoma and the corresponding non-cancerous renal tissues. Cu,Zn-SOD activities in cancerous tissues were lower than those in normal tissues. Mn-SOD activities were varied in the cases examined, whereas no significant difference between cancerous and normal tissues was observed for Mn- or total-SOD activities. Immunoblot analysis showed that the loss in enzyme activity in cancerous tissue was greater than the decrease in protein content for either isozyme. The selective decrease in Cu,Zn-SOD activities in cancerous tissue observed in this study suggests that the cytoplasmic defense against free radical damage appears to be reduced in renal cell carcinoma.
No preview · Article · Jul 2000 · Research communications in molecular pathology and pharmacology