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NF-kappa B binding to DNA in the presence of thiol-reactive metals has been explored in vitro. Gel mobility shift assays using total nuclear extracts isolated from tumor necrosis factor alpha-treated A549 cells demonstrated dose-dependent inhibition of NF-kappa B binding by chromium, cadmium, mercury, zinc, and arsenite. Maximum inhibition of binding occurred when these metals were preincubated with the nuclear proteins prior to addition of radiolabeled oligonucleotide. The potency of mercury, cadmium, and zinc for inhibiting binding closely correlated to the affinity of these metals for protein thiols. Further addition of dithiothreitol competitively blocked the effects of all of the metals, except chromium(III), on NF-kappa B binding. This study demonstrates mechanisms for metals to inhibit NF-kappa B-DNA binding through interactions with critical protein sulfhydryls.

Ascorbate (vitamin C) is a biological reductant of the human carcinogen chromium(VI). The product of this reaction is presumed to be dehydroascorbate. However, we have found that chromium(VI) can also react with dehydroascorbate. This reaction was monitored by UV/ visible and electron paramagnetic resonance (EPR) spectroscopies. In sodium acetate buffer at pH 3.8, the reaction of chromium(VI) and excess dehydroascorbate produced chromium(V) and chromium(IV) intermediates. At high reaction concentration, the chromium(V) intermediate formed an EPR silent dimer, which dissociated upon dilution to lower concentration. UV/ visible experiments at pH 3.8 demonstrated that manganese(II) catalyzed the disproportionation of chromium(IV) to chromium(V) and chromium(III). The ability of the reaction intermediates to induce strand breaks in pBR322 DNA was determined at pH 3.8 and pH 5.8. At pH 3.8, chromium(IV) appeared to be the major species responsible for induction of strand breaks because the time course for formation of strand breaks did not parallel that of chromium(V), and strand breaks were decreased in the presence of the chromium(IV) scavenger manganese(II). At pH 5.8, fewer strand breaks were observed; however, the time course for their formation followed that of chromium(V). There has been much effort devoted to identification of the intermediate responsible for the induction of strand breaks during reactions of chromium(VI) with biological reductants. The current results suggest that it is not a single type of species that universally produces the DNA strand breaks observed in different chromium(VI) systems and that the reactivity of intermediates will depend on the chosen experimental conditions. Understanding this variability in chromium(VI) reactions may help to resolve the conflicting results from in vitro studies that are aimed at deciphering mechanisms of chromium(VI)-induced cancers.

Chronic exposure to low levels of environmentally derived arsenite are associated with vascular diseases, such as arteriosclerosis. However, the cellular and molecular mechanisms for vascular disease in response to arsenic are not known. These studies investigated the hypothesis that nonlethal levels of arsenic increase intracellular oxidant levels, promote nuclear translocation of trans-acting factors, and are mitogenic. Incubation of second passage vascular endothelial cells with less than 5 microM arsenite for 4 h increased incorporation of [3H]thymidine into genomic DNA, while higher concentrations failed to stimulate or inhibit DNA synthesis. Within 1 h following addition of noncytotoxic concentrations of arsenite, oxidants accumulated and thiol status increased. During this time period, there was increased nuclear retention of NF-kappa B binding proteins and nuclear translocation of NF-kappa B also occurred in response to 100 microM H2O2. Supershift analysis demonstrated that p65/p50 heterodimers accounted for the majority of proteins binding consensus kappa B sequences in cells treated with arsenite or oxidants. The antioxidants, N-acetylcysteine or dimethylfumaric acid, increased intracellular thiol status and prevented both oxidant formation and translocation of NF-kappa B binding proteins in response to arsenite. These data suggest that arsenite initiates vascular dysfunction by activating oxidant-sensitive endothelial cell signaling.

Chromium(VI) is a known human carcinogen which requires intracellular reduction for activation. Ascorbate (vitamin C) has been reported to function as a major reductant of Cr(VI) in animals and cell culture systems. The reaction of Cr(VI) with varying concentrations of ascorbate was studied under physiological conditions in vitro in order to determine the types of reactive intermediates produced and to evaluate the reactivity of these intermediates with DNA. Reactions of 1.8 mM Cr(VI) with 0-18 mM ascorbate at pH 7.0 in N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (HEPES; 0.10 M) and tris(hydroxymethyl)aminomethane hydrochloride (Tris.HCl; 0.050 M) buffers were studied by electron paramagnetic resonance and UV/visible spectroscopy. Cr(V) and carbon-based free radical adducts of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) were observed at 0.5 to 1 and 1 to 1 reactions of ascorbate to Cr(VI). Levels of Cr(V) were higher for reactions in HEPES buffer, and levels of carbon-based radicals were higher in Tris.HCl buffer. Levels of Cr(IV) and Cr(III) increased with increasing concentration of ascorbate in both buffers. Reaction of Cr(VI) with varying ascorbate in the presence of calf thymus DNA or pBR322 DNA resulted in Cr-DNA adducts and plasmid relaxation, respectively. Maximum binding of Cr to DNA was observed for the 1:1 reaction ratio of Cr(VI) with ascorbate in both HEPES and Tris.HCl buffers, but total Cr bound to DNA was 8-fold lower in Tris.HCl than HEPES buffer. Preincubation of Cr(VI) with ascorbate before reaction with DNA decreased Cr-DNA binding to background levels. Preincubation of Cr(III) with ascorbate resulted in only low Cr-DNA binding. Levels of Cr-DNA binding were higher with single-stranded vs double-stranded DNA. Reactions with 14C-labeled ascorbate produced no cross-linking of ascorbate to DNA. Maximum plasmid relaxation was observed for the 1:1 ascorbate to Cr(VI) ratio in both buffers; however, single-strand breaks were 2-fold higher in Tris.HCl than HEPES buffer. Reactions with plasmid in the presence of DMPO quenched formation of single-strand breaks. Interpretation of these results in light of the spectroscopic studies suggested that Cr(V) and carbon-based radicals were responsible for Cr-DNA adducts and DNA single-strand breaks, respectively.

Previous studies have shown that chromium(VI) induced cell-specific types of DNA damage, i.e. DNA cross-links in liver and DNA strand breaks in red blood cells, in 14 day chick embryos. Direct and indirect pathways for chromium(VI)-mediated DNA damage, in the form of Cr-DNA binding and 8-oxo-2'-deoxyguanosine (8-oxo-dG) respectively, were examined in liver and red blood cells of 14 day chick embryos. Levels of hepatic Cr-DNA binding increased in a Cr(VI) dose-dependent manner. Cr-DNA binding in red blood cells was 10-fold lower than in liver, although the Cr-uptake in red blood cells was only 2-fold lower than in liver. The level of 8-oxo-dG formation in red blood cells increased at all Cr(VI) doses tested but peaked at 0.10 mmol Cr(VI)/kg, whereas no increase in 8-oxo-dG levels over background levels was observed in liver of Cr(VI)-treated embryos. The possible role of glutathione in modulating Cr(VI)-induced DNA damage was examined by using L-buthionine-R,S-sulfoximine (BSO) to deplete glutathione. No changes in glutathione levels were observed in either liver or red blood cells of embryos treated with Cr(VI) in the presence or absence of BSO pretreatment. Ascorbate levels in liver and red blood cells were not affected by treatment of embryos with chromium(VI), BSO or Cr(VI) and BSO. Depletion of glutathione by BSO resulted in a small increase of chromium uptake in liver of embryos treated with 0.050 and 0.10 mmol Cr(VI)/kg, but had no effect on hepatic chromium uptake at 0.20 mmol Cr(VI)/kg. BSO had no effect on chromium uptake in red blood cells. Depletion of glutathione had no effect on hepatic or red blood cells Cr-DNA binding in embryos treated with Cr(VI). However, depletion of glutathione significantly decreased the 8-oxo-dG levels in red blood cells at all Cr(VI) doses tested. Levels of 8-oxo-dG in liver of Cr(VI)-treated embryos remained at background in the presence or absence of BSO pretreatment. These results indicate that Cr(VI)-induced DNA damage in 14 day chick embryos is through a direct interaction of chromium with DNA in liver, but is through an indirect oxidative pathway in red blood cells. It appears that glutathione plays an important role in chromium(VI)-induced formation of 8-oxo-dG in red blood cells.

Lead chromate induces chromosomal damage as a result of extracellular dissolution producing solubilized chromium and lead and we show here that the dissolution process is greatly accelerated by the presence of cells. We have sought to determine which of these ions is involved in lead chromate-induced clastogenicity. Cell-mediated extracellular dissolution of particulate lead chromate resulted in the accumulation of both solubilized chromium and solubilized lead, reaching concentrations in the extracellular medium of 15 and 1.9 microM respectively and reaching concentrations inside the cell of 2700 and 97 microM respectively. Both the extracellular and intracellular accumulation of chromium was time dependent and both the solubilized lead and chromium increased proportionately from a lower dose to a higher dose. Exposing cells to water soluble sodium chromate under conditions which produced similar time-dependent intracellular concentrations of chromium also produced a similar amount and spectrum of chromosome damage as lead chromate. In contrast, exposure to lead glutamate resulted in intracellular lead levels 438-times higher than those produced by lead chromate, but produced no chromosome damage. A higher dose of lead glutamate was weakly clastogenic, but it induced a different spectrum of chromosomal aberrations than lead chromate. Pretreatment of cells with vitamin E had no effect on the uptake of chromium, but reduced both sodium chromate- and lead chromate-induced clastogenesis by 54-93%. Vitamin E pretreatment did not affect lead glutamate-induced clastogenesis. The results of this study indicate that although lead(II) is weakly clastogenic at high doses, hexavalent chromium is the proximate clastogen in lead chromate-induced clastogenesis. Additionally, this is the first report that pretreatment of cells with vitamin E can block clastogenesis induced by particulate chromates.

The ability of the carcinogen chromium(VI) to affect the basal and zinc-inducible expression of liver metallothionein was examined in 14- and 18-day chicken embryos in vivo. Metallothionein expression varied with the stage of embryo development, with basal steady-state mRNA levels being approximately three times lower in livers of 18-day versus 14-day chicken embryos. Chromium(VI) treatment had no effect on the basal steady-state levels of metallothionein mRNA and protein in either 14- or 18-day chicken embryo liver. Treatment of 14-day embryos with zinc(II) resulted in a 3- to 5-fold increase in steady-state levels of metallothionein mRNA in liver. Pre-treatment of 14-day embryos with chromium(VI) inhibited the zinc(II)-induced increase in steady-state levels of metallothionein mRNA and protein in liver by 30-50%. In contrast, chromium(VI) and/or zinc(II) treatments had no effect on steady-state levels of beta-actin mRNA.

Reaction of potassium dichromate with sodium ascorbate was studied by EPR spectroscopy at room temperature, in 0.10 M N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES), phosphate, cacodylate, and tris(hydroxymethyl)aminomethane hydrochloride (Tris.HCl) buffers at pH 7.0, in the presence of 0.10 M spin trap [5,5-dimethyl-1-pyrroline 1-oxide or 2-methyl-N-(4-pyridinylmethylene)-2-propanamine N,N'-dioxide]. Chromium(V), ascorbate radical, CO2-, and other carbon-based spin trap-radical adducts were observed. Chromium(V), CO2-, and the carbon-based radicals were observed at low ratios of ascorbate to chromium, and ascorbate radical was observed at high ratios of ascorbate to chromium. The presence of Cr(IV) was detected indirectly by reaction with Mn(II) and a subsequent decrease in the Mn(II) EPR signal. More Cr(IV) was found for the higher reaction ratios of ascorbate to Cr(VI). The only buffer effect observed was a relative decrease of the Cr(V) signal in Tris.HCl vs HEPES, phosphate, and cacodylate buffers, no change in the radical adducts was observed. There was no evidence for reactive oxygen species an intermediates in this reaction. Addition of the singlet oxygen trap 2,2,6,6-tetramethyl-4-piperidone hydrochloride showed no 2,2,6,6-tetramethyl-1-piperidinyloxy radical formation. The Cr(V) species did not react with dioxygen, and dioxygen did not affect the formation of carbon-based radicals. A mechanism consistent with these observations is discussed.

The binding of CrCl3.6H2O, Cr(NO3)3.9H2O, [Cr(L-His)2] (NO3)3.H2O, [Cr(L-Cys)(L-His)].3.5H2O, [Cr(L-His)(D-Pen)].H2O, Na[Cr(L-Cys)2].2H2O, K2[Cr(GS)2].3H2O, Na2-CrO4.4H2O, and Na2Cr2O7.2H2O to salmon sperm nuclei and nuclear DNA was determined. The Cr(III)-amino acid complexes and Cr(VI) exhibited significantly lower Cr-nuclei and Cr-DNA binding levels relative to the inorganic complexes CrCl3.6H2O and Cr(NO3)3.9H2O. The binding of CrCl3.6H2O, Cr(NO3)3.9H2O and Na2Cr2O7.2H2O to salmon sperm nuclei and nuclear DNA in the presence of rat lung cytosol was determined under the same conditions. For those complexes studied in both buffer and cytosol, the Cr-DNA binding levels for Cr(III) complexes were higher in buffer than in cytosol, while a relatively higher binding level was observed for Cr(VI) in cytosol than in buffer. Slightly lower nuclear protein levels were present in Cr(VI) incubations than in Cr(III) incubations with nuclei both in the presence and the absence of cytosol. The relative binding of CrCl3.6H2O, Cr(NO3)3.9H2O, [Cr(L-His)2](NO)3.H2O, [Cr(L-Cys) (L-His)].3.5H2O, [Cr(L-His)(D-Pen)].H2O, Na[Cr(L-Cys)2].2H2O and Na2CrO4.4H2O to isolated calf thymus DNA in buffer was also determined. Positively charged, labile inorganic Cr(III) complexes, CrCl3.6H2O and Cr(NO3)3.9H2O, exhibited higher binding to DNA than [Cr(L-His) (D-Pen)].H2O, and no binding to DNA was observed with Cr(VI) and the other neutral, positively and negatively charged, inert Cr(III)-amino acid complexes. Although labile aquo chromium(III) complexes are quite reactive with DNA, the reactivity of chromium(III), formed upon intracellular reduction of carcinogenic chromium(VI), toward DNA will be diminished by complexation with cellular proteins, peptides and amino acids.

In order to understand the possible role of reactive intermediates in the formation of tissue-specific DNA damage by chromium(VI), electron paramagnetic resonance spectroscopy was used to study the in vivo formation of chromium(V) in the liver and red blood cells of 14 day chick embryos following treatment with chromium(VI). In vivo administration of sodium dichromate onto the inner shell membrane of 14 day chick embryos resulted in the formation of a persistent chromium(V) species in liver cells (g = 1.987). The intensity of the chromium(V) signal in liver cells plateaued at 70 min and persisted for 240 min after treatment with chromium(VI). The dependence of chromium(V) formation on the dose of sodium dichromate administered to the embryo was clearly different in liver versus red blood cells. Chromium(V) was detected in red blood cells only at high doses of sodium dichromate (0.50-0.60 mmol/kg), whereas chromium(V) was undetectable in red blood cells at lower doses of sodium dichromate (0.10-0.30 mmol/kg) which produced clear evidence for chromium(V) in liver. Uptake studies showed that total chromium levels in red blood cells were 10-fold greater than in liver cells, and that up to 10% of the total chromium existed as chromium(V) in liver and red blood cells in vivo. Depletion of glutathione by pretreatment of embryos with L-buthionine-S,R-sulfoximine (BSO) for 24 h prior to treatment with a high dose of sodium dichromate (0.60 mmol/kg) caused both a decrease in the levels of chromium(V) species produced and a decrease of chromium uptake into red blood cells 50 min after treatment. At this high dose of chromium(VI), BSO pre-treatment had no effect on the level of the chromium(V) or on chromium uptake into liver cells after a 70 min incubation period. Thus, the concentration of chromium(V) inside the cell correlated with the levels of chromium taken up into the cell. Chromium(V) may be the form of chromium which is responsible for induction of DNA damage following in vivo administration of sodium dichromate.

The mechanism by which phenobarbital and similar compounds regulate gene expression has remained elusive for many years despite intense investigation. We had previously reported that the mRNA expression for the phenobarbital-inducible CYP2H1 gene was increased by cycloheximide treatment as rapidly and to a similar extent as by the phenobarbital-type drugs glutethimide and 2-propyl-2-isopropylacetamide (PIA), in primary cultures of chick embryo hepatocytes or in chick embryo liver in vivo (J. W. Hamilton, W. J. Bement, P. R. Sinclair, J. F. Sinclair, and K. E. Wetterhahn, 1988, Biochem. J. 255, 267-275). To examine the mechanism of this induction further, we determined the effects of various structurally related and unrelated inhibitors of protein synthesis on CYP2H1 expression in cultured chick embryo hepatocytes. Cycloheximide increased the transcription rate of the CYP2H1/2 genes to a similar extent as did PIA, and had little or no effect on CYP2H1 mRNA half-life. A number of other protein synthesis inhibitors, including streptovitacin, acetoxycycloheximide, pactamycin, and ricin, all increased CYP2H1 mRNA expression to a similar extent. The dose responses for induction of CYP2H1 mRNA and inhibition of protein synthesis by these agents were closely correlated. There was no relationship between the effectiveness of these agents to induce CYP2H1 mRNA expression and their structures or lipophilicity. Cycloheximide acetate required deesterification to cycloheximide for both inhibition of protein synthesis and induction of CYP2H1 mRNA. These results suggest that a labile negative regulatory protein is involved in CYP2H1/2 gene expression. It is also possible that this factor is involved in regulating the phenobarbital response of CYP2H1/2.

Chromium(VI) reductase activity was measured in ultrafiltrates of rat lung after various pretreatments in vitro at 37 degrees C and pH 7.0. Pretreatment of ultrafiltrates with L-ascorbate oxidase (EC 1.10.3.3), which specifically eliminated ascorbate, blocked approximately 95% of chromium(VI) reductase activity in ultrafiltrates. Preincubation of ultrafiltrates with heat-denatured ascorbate oxidase or the sulfhydryl-blocking agent N-ethylmaleimide (NEM) had no significant effect on Cr(VI) reductase activity. In rat lung cytosols, L-ascorbate oxidase blocked approximately 95% and NEM blocked approximately 15% of Cr(VI) reductase activity. The extent of inhibition of Cr(VI) reductase activity in cytosols by L-ascorbate oxidase was significantly decreased to approximately 75% after addition of 1.0 mM NADPH. When Cr(VI) was incubated with salmon sperm nuclei suspended in rat lung cytosol for 15 min, Cr became bound to nuclear DNA. This Cr-DNA binding was completely inhibited by preincubation of rat lung cytosols with L-ascorbate oxidase and inhibited approximately 60% by preincubation with NEM. Taken together these data suggest that ascorbate and/or ascorbate-dependent factors are the principal reductants of Cr(VI) in both ultrafiltrates and cytosols prepared from rat lung and ascorbate-dependent metabolism of Cr(VI) results in Cr binding to nuclear DNA in vitro. Although sulfhydryl-containing factors and NADPH-dependent factors only make a minor contribution to Cr(VI) reduction in rat lung cytosols, sulfhydryls may be significantly involved in the binding of Cr to nuclear DNA.

Incubation of rat liver cytosolic or microsomal fractions with chromium(VI) led to a dramatic decrease in chromium(VI) mutagenicity, as determined by the Ames Salmonella assay using the TA100 tester strain. The cytosol-dependent decrease in chromium(VI) mutagenicity was found to be counteracted in the presence of dicumarol, an inhibitor of the cytosolic enzyme NAD(P)H:quinone oxidoreductase (DT-diaphorase). In order to determine whether DT-diaphorase is a significant factor in enzymatic reduction of chromium(VI) in rat liver tissue, cytosolic and microsomal fractions were analyzed for NAD(P)H-dependent chromium (VI) reductase activity leading to chromium(V) formation by using electron paramagnetic resonance (EPR) spectroscopy. Reaction of chromium(VI) with NADH or NADPH in the presence of either cytosolic or microsomal fractions led to the formation of stable chromium(V)--NAD(P)H complexes. When glucose 6-phosphate (G6P) was present in the reaction as part of a NADPH-generating system, stable chromium(V)--G6P complexes were formed in addition to the chromium(V)--NAD(P)H complexes. The chromium(V) complexes had g values of 1.980-1.982 and superhyperfine splitting constants of 0.8-0.9 characteristic of bis(diol)oxochromium(V) complexes. Inhibition of 90% of the cytosolic DT-diaphorase activity by dicumarol led to only partial (20-22%) inhibition of chromium(V) formation. Visible and EPR spectroscopic studies showed that purified DT-diaphorase had no detectable chromium(VI) reductase activity and did not catalyze formation of chromium(V). Inhibition of 69% of microsomal aryl hydrocarbon hydroxylase activity by ketoconazole led to partial (10%) inhibition of chromium(V) formation. These results indicate that intracellular NAD(P)H-dependent enzymatic reduction of chromium(VI) in rat liver cannot be attributed to the activity of any one enzyme in the cytosolic or microsomal fractions. DT-diaphorase appears to play an indirect role in decreasing chromium(VI)-induced mutagenicity in Salmonella, possibly through interaction with other redox active cellular components. The involvement of diols such as sugars and pyridine nucleotides in stabilizing intracellularly generated chromium(V) is discussed.

Chromium (VI) reductase activity was measured in ultrafiltrates of rat liver and kidney after various pretreatments in vitro at 37 degrees C and pH 7.0. Preincubation of ultrafiltrates with L-ascorbate oxidase (EC 1.10.3.3), which specifically eliminated ascorbate, blocked approximately 80% of the Cr(VI) reductase activity. Heat-denatured ascorbate oxidase had no effect on Cr(VI) reductase activity in ultrafiltrates. Preincubation of ultrafiltrates with N-ethylmaleimide, which non-specifically blocked sulfhydryls, including reduced glutathione, decreased Cr(VI) reductase activity by only 20%. Treatment of male Sprague-Dawley rats with phorone decreased non-protein sulfhydryl (NPSH) levels in rat liver by greater than 90% and tripled reduced ascorbate levels 2 h after treatment. Ultrafiltrates of liver prepared from phorone-treated rats had twice the Cr(VI) reductase activity of control ultrafiltrates, and greater than 95% of this activity could be blocked by preincubation with ascorbate oxidase. Treatment of rats with sodium dichromate (20 mg/kg) caused a significant decrease in ascorbate levels in kidney but not liver, and no change in NPSH levels in kidney or liver, 15 min after treatment. We conclude that ascorbate is the major reductant of Cr(VI) in rat liver and kidney ultrafiltrates and may well be the major non-enzymatic reductant of Cr(VI) in rat liver and kidney in vivo.

Hepatic 5-aminolevulinate (ALA) synthase, the first and rate-limiting enzyme in the heme biosynthetic pathway, is known to be feedback repressed by the end product of the pathway, heme. We investigated whether heme regulates ALA synthase mRNA expression transcriptionally or post-transcriptionally in primary cultures of chick embryo hepatocytes. 2-Propyl-2-isopropylacetamide increased the rate of transcription of the ALA synthase gene, whereas heme or an inhibitor of heme biosynthesis, desferrioximine, had no effect on the drug-induced transcription rate. Heme decreased the half-life of ALA synthase mRNA from approximately 3.5 h to 1.2 as recently reported by Drew and Ades (1989, Biochem. Biophys. Res. Commun. 162, 102-107). We also found that the heme-mediated decrease in mRNA stability was prevented by cycloheximide treatment, suggesting that the heme effect was mediated by a labile protein. These results support a model for hepatic ALA synthase regulation in which inducing drugs directly stimulate ALA synthase gene transcription, whereas heme regulates ALA synthase expression post-transcriptionally by modulating mRNA stability as well as by blocking translocation of ALA synthase enzyme into the mitochondrion.

The effect of Cr(VI) on liver, kidney, and lung glutathione (GSH) levels and the effect of GSH depletion on Cr(VI)-induced nephrotoxicity were studied in male Sprague-Dawley rats (150-200 g). GSH levels, measured as nonprotein sulfhydryls, were determined between 0.5 and 26 hr after intraperitoneal injection of the maximum non-toxic dose of sodium dichromate (10 mg/kg). While Cr(VI) at this dose did not significantly change hepatic, renal, or pulmonary GSH levels, there appeared to be an initial decrease of hepatic GSH followed by an increase to approximately 120% of control between 5 and 12.5 hr after Cr(VI) treatment. The increase in hepatic GSH levels was significant 5 hr after treatment with 20 mg/kg sodium dichromate, was manifested as an increase in both non-protein sulfhydryls and total glutathione, and was prevented by L-buthionine sulfoximine (BSO) pretreatment. In rats pretreated with 4.0 mmol/kg BSO to deplete GSH, subsequent treatment with Cr(VI) further reduced hepatic GSH levels 2 hr after Cr(VI) treatment and inhibited weight gain in the first 24 hr after treatment. Intraperitoneal injection of Cr(VI) did not inhibit hepatic glutathione reductase activity, even at toxic doses. Depletion of renal GSH to approximately 25% of control with BSO potentiated the acute nephrotoxicity of 30 mg/kg sodium dichromate as measured by serum urea nitrogen levels and relative kidney weight. However, GSH depletion with BSO did not appear to affect the incidence of glucosuria, haematuria, or lysozymuria over a range of Cr(VI) doses, nor did it affect renal uptake of Cr. Taken together, these data show that GSH protects against the acute nephrotoxicity of Cr(VI), although it is not clear whether GSH is directly involved in the intracellular metabolism of Cr(VI) at non-toxic doses.

The ability of the thiols glutathione, cysteine, beta-mercaptoethanol and dithiothreitol to effect chromium(VI)-induced DNA damage in vitro has been investigated. Reaction of pBR322 DNA with chromium(VI) in the presence of the thiols led to formation of chromium(V) and chromium--DNA adducts. The extent of chromium binding to DNA differed by several orders of magnitude among the thiols tested, in the order dithiothreitol greater than beta-mercaptoethanol much greater than cysteine greater than or equal to glutathione. The maximal level of chromium(V) formed also differed among the thiols tested, in the order beta-mercaptoethanol greater than dithiothreitol much greater than glutathione greater than or equal to cysteine. Electronic spectral studies of these reactions indicated that the rate of reduction of chromium(VI) is dependent on the thiol tested, in the order cysteine greater than dithiothreitol greater than glutathione greater than beta-mercaptoethanol. Electron paramagnetic resonance studies of these reactions indicate that a significant level of chromium(III) is detected only with cysteine. Chromium--DNA adducts formed by reaction of chromium(VI) in the presence of glutathione or cysteine did not lead to DNA conformational changes detectable upon agarose gel electrophoretic analysis. Changes in DNA conformation were detected as altered electrophoretic mobility of pBR322 DNA on agarose gels after reaction with chromium(VI) in the presence of dithiothreitol or beta-mercaptoethanol. Effects on DNA electrophoretic mobility, which depended on whether the initial conformation of the plasmid was linear or supercoiled, included altered and heterogeneous mobility, as well as complete inhibition of migration of the plasmid. Transmission electron microscopy of chromium--DNA complexes revealed aggregates of several plasmids, as well as condensation of individual plasmids into compact kinked forms. These effects may be due to cross-linking of DNA induced by chromium metabolites. These studies indicate that the levels of chromium bound to DNA are related to the levels and stabilities of the chromium(V) species formed upon reaction of chromium(VI) with the various thiols. Chromium--thiol interactions may play an important role in chromium(VI) genotoxicity.

L-Buthionine-S,R-sulfoximine (BSO), a potent inhibitor of gamma-glutamylcysteine synthetase, is commonly used as an experimental tool for the specific depletion of glutathione. Since cysteine is a key precursor for glutathione biosynthesis, we investigated the possibility that BSO might also affect the free cysteine pool in rat liver and kidney tissues in vivo. Male CD(SD)BR rats (150-200 g) were injected ip with various doses of BSO (0.25-4.0 mmol/kg), and glutathione and cysteine were measured in liver and kidney using HPLC with electrochemical detection and/or spectroscopic techniques. No hepatotoxicity or nephrotoxicity was observed at the highest BSO dose (4.0 mmol/kg) used. BSO caused the expected decreases of hepatic and renal glutathione at all doses, although glutathione depletion was more rapid, was achieved at a lower BSO dose, and was more sustained in kidney than in liver. Hepatic cysteine levels nearly doubled 20 min after BSO treatment (1.0 mmol/kg, ip), but were not significantly different from control at later time points. In contrast, renal cysteine was significantly depleted from 20 min to 25 hr postinjection with a time course closely paralleling that of renal glutathione depletion. These changes are discussed in the context of models for inter- and intraorgan transport of glutathione and cysteine. We also provide evidence that an artifact, most likely the gamma-glutamyltranspeptidase (GGT)-initiated breakdown of glutathione, leads to a rapid postmortem increase of cysteine levels in liver and particularly in kidney of rats. Simultaneous decreases in GSH levels can be demonstrated in kidney. This artifact needs to be minimized in toxicological studies of glutathione and cysteine in kidney and other GGT-rich organs, as the measured levels of these thiols may not reflect the true concentrations occurring in vivo.

The reaction of chromium(VI) with hydrogen peroxide was studied in the presence of glutathione. In vitro, reaction of chromium(VI) with hydrogen peroxide alone led to production of hydroxyl radical as the significant reactive intermediate, while reaction of chromium(VI) with glutathione led to formation of two chromium(V)-glutathione complexes and the glutathione thiyl radical. Incubation of chromium(VI) with glutathione prior to addition of hydrogen peroxide led to formation of peroxochromium(V) species and a dramatic increase in hydroxyl radical production over that detected in the reaction of chromium(VI) with hydrogen peroxide alone. In contrast, addition of chromium(VI) to a preincubated mixture of glutathione and hydrogen peroxide led to a decrease in hydroxyl radical production over that obtained in the reaction of chromium(VI) with hydrogen peroxide. When pBR322 DNA was added to the above reactions, the extent of chromium(VI)-induced DNA strand breakage correlated with the relative amount of hydroxyl radical formed. Reaction of chromium(VI) with calf thymus DNA in the presence of a preincubated mixture of glutathione and hydrogen peroxide led to detection of the 8-hydroxydeoxyguanosine adduct, whose formation correlated with that of hydroxyl radical production. No significant chromium-DNA adduct formation was detected. The results suggest that, in the cellular metabolism of chromium(VI), preformed chromium(V)-glutathione complexes may react with hydrogen peroxide in a Fenton-type manner to produce hydroxyl radical as the DNA-damaging agent. However, if glutathione reacts with hydrogen peroxide prior to exposure to chromium(VI), the amount of hydroxyl radical generated may not be sufficient to cause significant DNA damage.(ABSTRACT TRUNCATED AT 250 WORDS)