Induction of DNA damage by dimethylarsine, a metabolite of inorganic arsenics, is for the major part likely due to its peroxyl radical

Induction of micronuclei in haemocytes and gill cells of zebra mussels, Dreissena polymorpha, exposed to clastogens

Zebra mussels, Dreissena polymorpha, were exposed to four directly acting reference clastogens (mitomycin C, bleomycin, dimethylarsinic acid and potassium chromate) under laboratory conditions. The aim was to examine the inducibility of micronuclei (MN) in haemocytes and gill cells. Positive responses were observed in both tissues for all four substances used under the given test conditions. The mean MN frequencies in treated mussels ranged between 3.2 and 6.9/1000 in haemocytes and between 5.4 and 6.7/1000 in gill cells. The spontaneous MN levels averaged 1.2 and 2.8/1000 in haemocytes and gill cells, respectively. The MN induction capacity of the different chemicals was equivalent in both tissues, except for the treatment with dimethylarsinic acid which generated a significantly higher MN rate in gill cells than in haemocytes. Several characteristics suggest that haemolymph is the more appropriate test tissue for environmental genotoxicity assessment: (1) a shorter preparation time of slides, (2) a more accurate identification of unambiguous MN, (3) a lower baseline MN frequency and a higher induction factor.

Induction of chromosomal aberrations in cultured human fibroblasts by inorganic and organic arsenic compounds and the different roles of glutathione in such induction

Clastogenic effects of a variety of arsenic compounds were examined on cultured human fibroblasts. The following compounds were tested: inorganic arsenicals (arsenite and arsenate), the major metabolites of inorganic arsenicals in human and experimental animals [methylarsonic acid (MAA), dimethylarsinic acid (DMAA) and trimethylarsine oxide (TMAO)], and water-soluble organoarsenic derivatives [2', 3'-dihydroxypropyl-5-deoxy-5-dimethylarsinoyl-beta-D-riboside (arsenosugar), arsenocholine, arsenobetaine and tetramethylarsonium iodide] found in marine organisms. Arsenic compounds induced mainly chromatid gaps and chromatid breaks. The rank order of compounds in terms of clastogenic potency was arsenite > arsenate > DMAA > MAA > TMAO. DMAA was very potent and caused chromosome pulverizations in most metaphases when present at doses higher than 7 x 10(-3) M. Arsenosugar, arsenocholine, arsenobetaine and tetramethylarsonium iodide were less effective. Depletion of cellular glutathione (GSH) with L-buthionine-SR-sulfoximine (BSO), increased the incidence of chromosomal aberrations induced by arsenite, arsenate and MAA, and markedly suppressed the clastogenic effects of DMAA. DMAA was highly clastogenic even in GSH-depleted cells when the cells were incubated with DMAA in the presence of GSH (5 and 10 mM). These results suggest that GSH might play a role in protecting cells against the clastogenic effects of arsenite, arsenate and MAA. GSH might be involved in the expression of clastogenic actions of DMAA.

Time dependence of accumulation and binding of inorganic and organic arsenic species in rabbit erythrocytes

The uptake by rabbit erythrocytes of 0.4 mM arsenate, As(V), monomethylarsinate, MMA(V) and dimethylarsonate, DMA(V) were compared over 24 h. In membrane-free hemolysate, the distribution of As between proteins (10 kDa) and ultrafiltrate was determined by ultrafiltration and arsenic species in the ultrafiltrate were identified by thin layer chromatography methods. 1H spin-echo Fourier transform NMR was used to follow the binding of these arsenic species to glutathione (GSH). 31P-NMR was used to observe their effects on high-energy adenine nucleotide levels (ATP, ADP). These results demonstrate that As(III) readily accumulates in cells, reaches a quasi-plateau at 78% of the total As in the incubation after 1 h and 88% of the total As after 24 h. On average, 20% of the total erythrocyte As(III) burden is associated with the protein fraction, particularly with hemoglobin (Hb). About 68% of the erythrocyte As(III) burden is bound to GSH. As(III) has no effect on ATP levels during a 5-h incubation. By comparison, As(V) enters erythrocytes more slowly (53% of the total As after 5 h). Erythrocytes take up 81% of the As(V) in the reaction system after a 24 h incubation. Of the total As burden in As(V)-exposed erythrocytes, 22% was associated with the proteins (10 kDa) and possibly reduced to As(III) and 59% was in the ultrafiltrate (8% as As(III) and 51% as As(V)). This finding indicates that, over a 24 h incubation period, the reduction of As(V) to As(III) may account for 30% of the total As in rabbit erythrocytes. As(V) present in the erythrocytes enters the phosphate pool and depletes ATP. In comparison, about 65% of the total MMA(V) or about 44% of the total DMA(V) in the incubation system is taken up by rabbit erythrocytes during a 24 h incubation. Neither organoAs species perturbed the Hb signals observed by spin-echo Fourier transform NMR and the binding to GSH was minimal. Unlike As(V), MMA(V) and DMA(V) do not perturb phosphate metabolism, showing that, despite their pentavalent oxidation state, these arsenic species are not analogs for phosphate.

Modulation of cellular antioxidant defense activities by sodium arsenite in human fibroblasts

Many studies have shown that oxygen radicals can be produced during arsenic metabolism. We report here that in human fibroblasts (HFW cells) sodium arsenite exposure caused increased formation of fluorescent dichlorofluorescein (DCF) by oxidation of the nonfluorescent form. The enhanced DCF fluorescence was inhibited by a radical scavenger, butylated hydroxytoluene. The effects of sodium arsenite treatment on cellular antioxidant activities were then examined. Treatment of HFW cells with sodium arsenite resulted in a significant increase in heme oxygenase activity and ferritin level. Sodium arsenite-enhanced heme oxygenase synthesis was inhibited by co-treatment of cells with the antioxidants sodium azide and dimethyl sulfoxide. Furthermore, sodium arsenite treatment did not apparently affect glucose-6-phosphate dehydrogenase activity, but resulted in significantly increased glutathione levels and superoxide dismutase activity, slightly decreased glutathione peroxidase activity, and significantly decreased catalase activity. Sodium arsenite toxicity was partly reduced by addition of catalase to the culture medium. These results imply that arsenite can enhance oxidative stress in HFW cells.

Arsenic toxicity in humans: Research problems and prospects

The disposition and toxicity of the metalloid, arsenic, is affected by its oxidation state and on the extent to which it is converted to methylated species. Given that these chemical modifications influence the fate and action of arsenic, new research efforts should be directed both towards elucidating the molecular processes involved in the metabolism of arsenic and in characterising interindividual variation in capacity for processes such as the methylation of arsenic. This information will contribute to a better understanding of the mechanisms of arsenic toxicity and carcinogenicity and to a better assessment of the hazards associated with chronic exposure to this agent.

Glutathione plays different roles in the induction of the cytotoxic effects of inorganic and organic arsenic compounds in cultured BALB/c 3T3 cells

The cytotoxicity of arsenic compounds towards BALB/c 3T3 cells in culture was investigated, together with the role of glutathione (GSH) in the induction of the cytotoxic effects. The rank order of cytotoxicity was as follows: arsenite (As3+) > arsenate (As5+) > dimethylarsinic acid (DMAA) > methylarsonic acid (MAA) > trimethylarsine oxide (TMAO). Arsenobetaine, arsenocholine and the tetramethylarsonium ion were less toxic. Depletion of GSH enhanced the cytotoxic effects of As3+, As5+, MAA and TMAO, while the cytotoxicity of DMAA was markedly reduced by depletion of GSH. These results suggest that GSH plays a role in protecting the cells against the toxic effects of As3+, As5+, MAA and TMAO while it is involved in the induction of the cytotoxic effects of DMAA.

Cellular response to oxidative damage in lung induced by the administration of dimethylarsinic acid, a major metabolite of inorganic arsenics, in mice

Oral administration of dimethylarsinic acid (DMAA), a major metabolite of inorganic arsenics, induces DNA damage in the mouse and rat lung due to both active oxygens and dimethylarsenic peroxyl radical produced in the metabolism of DMAA. Our paper describes the cellular response to DMAA in the mouse lung. In male ICR mice given a single po dose (1500 mg/kg) of DMAA-Na, the activities of mitochondrial superoxide dismutase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase significantly increased at 6 hr or longer after dosing, while cytosolic superoxide dismutase and catalase did not. With regard to cellular sulfhydryls after DMAA dosing, levels of reduced glutathione and nonprotein sulfhydryl decreased, while mixed disulfides significantly increased. Further, NADPH markedly decreased at 6-9 hr after DMAA dosing. These cellular variations suggest that the mouse pulmonary cell produced active oxygens, i.e., superoxide anion radical, hydrogen peroxide, and subsequent radicals in the metabolism of DMAA and that these and also the dimethylarsenic peroxyl radical were responsible for pulmonary DNA damage.