Expression of the cassava nitrate transporter NRT2.1 enables Arabidopsis low nitrate tolerance

The cassava grows well on low-nutrient soils because of its high-affinity to absorb nitrate. However, the molecular mechanisms by which cassava adapts itself to this environment remain elusive, although we have cloned a putative gene named MeNRT2.1 which has a crucial role in high-affinity nitrate transporter from cassava seeding. Here, the expression pattern of MeNRT2.1 was further assessed using the GUS activity driven by MeNRT2.1 promoter in Arabidopsis transformation plants. The GUS activity was monitored over time following the reduction of nitrate supply. The GUS gene expression not only peaked in roots after 12 h in 0.2mM nitrate media, but also stained stems and leaves. Arabidopsis plants with overexpression of MeNRT2.1 increased the biomass compared to the wild type on rich nitrogen (N-full) media. However, chlorate sensitivity analysis showed that Arabidopsis plants expressing MeNRT2.1 were more susceptable to chlorate than wild type. Significantly, after growing for 15 days on media containing 0.2mM nitrate concentration, wild-type plants became yellowor died, while the transgenic MeNRT2.1 Arabidopsis plants maintained normal growth. With significant increases in the amount of ¹⁵NO^- ₃ uptake in roots, the MeNRT2.1 plants also increased the contents of chlorophyll and nitrate reductase. Taken together, these results demonstrate that MeNRT2.1 has an important role in adaptation to low nitrate concentration as a nitrate transporter.

Acute renal toxicity of sodium chlorate: Redox imbalance, enhanced DNA damage, metabolic alterations and inhibition of brush border membrane enzymes in rats

Sodium chlorate (NaClO₃ ) is widely used in paper and pulp industries and as a non-selective herbicide. Humans can be exposed to NaClO₃ through contaminated drinking water due to its improper and unchecked usage in industries and as herbicide. NaClO₃ is also present as a major stable by-product in drinking water that has been disinfected with chlorine dioxide. In this study, we have investigated the effect of a single acute oral dose of NaClO₃ on rat kidney. Adult male Wistar rats were divided into one control and four NaClO₃ treated groups that were orally given different doses of NaClO₃ and euthanized 24 hr after the treatment. Oral administration of NaClO₃ resulted in increased hydrogen peroxide levels, lipid, and protein oxidation while thiol and glutathione content and activities of brush border membrane enzymes were decreased in kidney in a NaClO₃ dose-dependent manner. Significant alterations in the activities of enzymes involved in carbohydrate metabolism and antioxidant defense were also observed. Administration of NaClO₃ induced DNA fragmentation and increased DNA-protein cross-linking. Histological studies showed marked damage in kidney from NaClO₃ treated animals. These results strongly suggest that NaClO₃ induces nephrotoxicity via redox imbalance that results in DNA and membrane damage, metabolic alterations and brush border membrane enzyme dysfunction.

Sodium chlorate induces DNA damage and DNA-protein cross-linking in rat intestine: A dose dependent study

Sodium chlorate (NaClO₃) is widely used in paper and pulp industries and as a non-selective herbicide. It is also a major by-product generated upon disinfection of drinking water by chlorine dioxide. In this study, we have investigated the genotoxicity of NaClO₃ on the small intestine of rats. Adult male rats were divided into 5 groups: one control and four NaClO₃ treated groups. The NaClO₃ treated groups were given a single acute oral dose of NaClO₃ (100, 250, 500 and 750 mg/kg body weight) and sacrificed 24 h later. Administration of NaClO₃ caused significant DNA damage in a dose dependent manner in the rat intestine. This was evident from the comet assay which showed DNA strand breaks and was further confirmed by agarose gel electrophoresis and release of free nucleotides. Increased DNA protein cross-linking in NaClO₃ administered groups showed formation of a critical lesion which hampers activities of proteins/enzymes involved in DNA repair, transcription and replication. Thus, oral administration of NaClO₃ induces DNA damage in the rat intestine, probably through chlorate induced production of reactive oxygen species.

Sodium chlorate, a major water disinfection byproduct, alters brush border membrane enzymes, carbohydrate metabolism and impairs antioxidant system of Wistar rat intestine

Sodium chlorate (NaClO₃ ) is a widely used nonselective herbicide. It is also generated as a by-product during disinfection of drinking water by chlorine dioxide. The purpose of this study was to evaluate the effect of NaClO₃ on rat intestine. Adult male rats were randomly divided into five groups: control and remaining four groups were administered orally different doses of NaClO₃ and sacrificed 24 h after the treatment. The administration of NaClO₃ produced acute oxidative stress in the intestine, which manifested in the form of markedly enhanced malondialdehyde levels and carbonyl content and lowered total sulfhydryl groups and glutathione levels. The activities of several brush border membrane (BBM) enzymes were greatly reduced as compared to control. There were alterations in the activities of various enzymes of carbohydrate metabolism and those involved in maintaining the antioxidant defense system. Histological studies support the biochemical results showing NaClO₃ dose-dependent increase in tissue damage. Thus, the present study shows that oral administration of NaClO₃ decreases the activities of BBM enzymes, induces oxidative stress, alters metabolic pathways, and impairs the antioxidant system of rat intestine. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 1607-1616, 2017.

[Effect of conditioned medium from mesenchymal stem cells on regeneration of endothelium at HCl-induced damage trachea in rats]

The purpose. Respiratory epithelium regeneration is studied in rats with tracheal damage induced by inhaling hydrochloric acid vapor. Method. Regeneration process after the chemical burn was activated by intratracheal administration of preparations obtained from the same-species mesenchymal stem cells (MSC). Results. Tracheal epithelium is shown to recover almost completely on day 3-7 after applying MSC compositions (MSCs). Closed structures containing ciliated cells similar to ciliated cells of the respiratory epithelium lining the trachea are formed in the submucosal epithelium during regeneration. These structures migrate towards epithelium and get incorporated into the damaged epithelium. This phenomenon is apparently indicative of the special mechanism of respiratory epithelium regeneration after HCl-induced injury. Conclusion. It is demonstrated in this study that cell-free MSCs instilled intratracheally promote the recovery of normal submucosal epithelium by either preventing or reducing necrosis and inflammation. Such topical MSCs administration significantly accelerates migration of ciliated cell towards the surface and de novo formation of the ciliary epithelium.

Sodium chlorate, a herbicide and major water disinfectant byproduct, generates reactive oxygen species and induces oxidative damage in human erythrocytes

Sodium chlorate (NaClO₃) is a widely used non-selective herbicide. It is also generated as a byproduct during disinfection of drinking water by chlorine dioxide. In the present work, the effects of NaClO₃ on human erythrocytes were studied under in vitro conditions. Incubation of erythrocytes with different concentrations of NaClO₃ at 37 °C for 90 min resulted in significant hemolysis. Cell lysates were prepared from NaClO₃-treated and untreated (control) erythrocytes and assayed for various biochemical parameters. Methemoglobin levels were significantly increased and methemoglobin reductase activity was reduced upon NaClO₃ treatment. There was a significant increase in protein oxidation and lipid peroxidation with a decrease in reduced glutathione and total sulfhydryl content. This suggests the induction of oxidative stress in erythrocytes upon exposure to NaClO₃. The occurrence of oxidative stress was confirmed by significantly increased generation of reactive oxygen species and lowered antioxidant response of the cells. NaClO₃ treatment also increased nitric oxide levels showing induction of nitrosative stress. The activities of major antioxidant and membrane-bound and metabolic enzymes were significantly altered upon incubation of erythrocytes with NaClO₃. The erythrocytes became more osmotically fragile while electron microscopic images showed gross morphological alterations in NaClO₃-treated cells. These results show that NaClO₃ induces oxidative stress in human erythrocytes, which results in extensive membrane damage and lowers the antioxidant response.

A Mixture of Ammonium Perchlorate and Sodium Chlorate Enhances Alterations of the Pitutary-Thyroid Axis Caused by the Individual Chemicals in Adult Male F344 Rats

Ammonium perchlorate (AP) and sodium chlorate (SC) have been detected in public drinking water supplies in many parts of the United States. These chemicals cause perturbations in pituitary-thyroid homeostasis in animals by competitively inhibiting iodide uptake, thus hindering the synthesis of thyroglobulin and reducing circulating T4 (thyroxine). Little is known about the short-term exposure effects of mixtures of perchlorate and chlorate. The present study investigated the potential for the response to a mixture of these chemicals on the pituitary-thyroid axis in rats to be greater than that induced by the individual chemicals. Adult male F-344 rats were exposed, via their drinking water, to the nominal concentrations of 0.1, 1.0, 10 mg/L AP or 10, 100, 1000 mg/L SC and their mixtures for 7 days. Serum T4 levels were significantly ( p < 0.05) reduced in rats following exposure to the mixtures, but not after exposure to the individual chemicals. Serum T3 (triiodothyronine) was not altered by treatment and TSH (thyroid stimulating hormone) was only increased after the high-dose chlorate treatment. Histological examination of the thyroid gland showed colloid depletion and hypertrophy of follicular epithelial cells in high-dose single chemical and all mixture-treated rats, while hyperplasia was observed only in some of the rats treated with mixtures (AP 10 + SC 100, AP 0.1 + SC 1000, and AP 10 + SC 1000 mg/L). These data suggest that short-term exposure to the mixture of AP and SC enhances the effect of either chemical alone on the pituitary-thyroid axis in rats.

Continuous, low-dose oral exposure to sodium chlorate reduces fecal generic Escherichia coli in sheep feces without inducing clinical chlorate toxicosis

Our objectives were to determine an effective, yet safe, daily dose of sodium chlorate for reducing fecal shedding of generic Escherichia coli in mature ewes. In a completely randomized experimental design, 25 Targhee ewes (age ∼ 18 mo; BW = 62.5 ± 7.3 kg, mean ± SD) were assigned randomly to 1 of 5 sodium chlorate treatments, which were administered in the drinking water for 5 consecutive days. Treatments were control group (no sodium chlorate) and 4 targeted levels of daily sodium chlorate intake: 30, 60, 90, and 120 mg · kg(-1) BW · d(-1) for 5 d. Individual ewe ad libitum intake of water (with treatments) was measured daily, and BW was measured at the beginning of and 15 and 51 d after the 5-d treatment period. Serum chlorate, whole blood methemoglobin and packed-cell volume (PCV), and fecal generic E. coli and general Enterobacteriaceae coliforms were measured from corresponding samples collected at the end of the 5-d treatment period. Average daily intakes of sodium chlorate from drinking water treatments were 95%, 91%, 90%, and 83% of the target treatment intakes of 30, 60, 90, and 120 mg · kg(-1) BW · d(-1), respectively. Daily sodium chlorate intake remained constant for all treatment groups except for ewes offered 120 mg NaClO3 · kg(-1) BW · d(-1), which decreased (quadratic; P = 0.04) over the course of the 5-d treatment period. This decrease in sodium chlorate intake indicated that the 120-mg NaClO3 level may have induced either toxicity and/or an aversion to the drinking water treatment. Serum chlorate concentrations increased (quadratic; P < 0.001) with increasing sodium chlorate intake. At the end of the 5-d treatment period, mean (least squares ± SEM) serum chlorate concentrations for ewes offered 30, 60, 90, and 120 mg NaClO3 · kg(-1) BW · d(-1) were 15.6 ± 14.1, 32.8 ± 15.8, 52.9 ± 14.1, and 90.3 ± 14.1 μg/mL, respectively. Whole blood methemoglobin and PCV were similar (P = 0.31 to 0.81) among the control group and ewes offered sodium chlorate. Likewise, BW was not affected by sodium chlorate (P > 0.27). Ewes consuming approximately 55 mg NaClO3 · kg(-1) BW · d(-1) or more (i.e., ewes offered 60, 90, and 120 mg) had a >1.4 log unit reduction in fecal E. coli and Enterobacteriaceae coliforms compared with control ewes. We suggest that for a short-term, 5-d dosing strategy, 55 to 81 mg NaClO3 · kg(-1) BW · d(-1) is an effective, yet safe, daily oral dose range for mature ewes to achieve a 97% to 99% reduction in fecal shedding of generic E. coli.

Do subtoxic levels of chlorate influence the desiccation tolerance of Egeria densa?

Among the different factors hypothesized to be responsible for the virtual disappearance of Egeria densa, once a dominant aquatic macrophyte in a southern Chile wetland ecosystem, are the negative effects of certain chemical compounds (mainly chlorate) and harsh environmental conditions (desiccation caused by prolonged atmospheric exposure). The authors performed an integrated experiment in which E. densa plants were first exposed for four weeks inside a mesocosm system to levels of chlorate that existed in the wetland at the time of the plant's demise and then exposed to desiccation conditions that also resembled those that the system had experienced. Hence, the authors tested the hypothesis that E. densa plants exposed to sublethal levels of chlorate are more susceptible to the deleterious effect of desiccation compared with plants that had not been exposed to chlorate. This hypothesis was tested by means of quantifying physiologically related parameters in plants right after the four weeks under water and then after the desiccation period of 6 h. Their results rejected this hypothesis, because all plants, regardless of their history, are equally affected by desiccation.

Trihalomethanes, chlorite, chlorate in drinking water and risk of congenital anomalies: a population-based case-control study in Northern Italy

BACKGROUND

Epidemiological evidence of an association between disinfection by-products (DBPs) exposure via drinking water and reproductive outcomes is still inconclusive.

OBJECTIVE

The aim of this study was to investigate the association between trihalomethanes (THMs), chlorite and chlorate exposure and congenital anomalies.

METHODS

A case-control study was carried out in Emilia-Romagna Region (Italy). Data on 1917 different congenital anomalies (neural tube, cardiac, diaphragm and abdominal wall, oesophagus, cleft lip and palate, respiratory, urinary tract and chromosomal anomalies) observed in the period 2002-2005 were extracted from the Regional Malformation Registry. Four controls (newborns without anomalies) were randomly selected form the Regional Birth Register and frequency matched to cases according to pregnancy period. The network supplying water during the first trimester of pregnancy was identified on the basis of mother's address: DBPs data, technical and structural information were linked to each subject.

RESULTS

Overall, THMs exposure was very low (mean: 3.8±3.6 μg/l), and no risk excess was observed. Chlorite and chlorate values were fairly high (mean: 427±184 μg/l and 283±79 μg/l, respectively). Women exposed to chlorite level >700 μg/l were at higher risk of newborns with renal defects (OR: 3.30; 95% IC: 1.35-8.09), abdominal wall defects (OR: 6.88; 95% IC: 1.67-28.33) and cleft palate (OR: 4.1; 95% IC: 0.98-16.8); women exposed to chlorate level >200 μg/l were at higher risk of newborns with obstructive urinary defects (OR: 2.88; 95% IC: 1.09-7.63), cleft palate (OR: 9.60; 95% IC:1.04-88.9) and spina bifida (OR: 4.94; 95% IC:1.10-22).

CONCLUSIONS

This was the first study showing an excess risk of different congenital anomalies related to chlorite and chlorate exposure via drinking water: further research is needed to confirm the observed relationships in large datasets, specifically for chlorate, an unregulated DBP.

Toxic effects of chlorate on three plant species inoculated with arbuscular mycorrhizal fungi

Pot experiments were conducted to examine the toxic effects of chlorate on bermudagrass, bahiagrass, and longan seedling with a focus on arbuscular mycorrhizal fungi-plant associations. The results show that application of chlorate could cause slight soil acidification, but the resulting pH was still around 5.5, which is unlikely to adversely affect plant growth. Increase in the application rate of chlorate resulted in a decrease in colonization rate of arbuscular mycorrhizal fungi in plant roots, P uptake by the plants and plant biomass. This appears to suggest that the reduction in plant growth may be related to impeded uptake of P by the plants due to the failure of the plants to form sufficient mycorrhizal associations when chlorate is in sufficient amounts to cause toxicity to arbuscular mycorrhizal fungi. Under the experimental conditions set for this study, bermudagrass suffered from stronger chlorate stress than bahiagrass and longan seedling did in terms of plant-arbuscular mycorrhizal fungi (AMF) symbiosis development.

Evaluation of chlorite and chlorate genotoxicity using plant bioassays and in vitro DNA damage tests

In the last few years chlorine dioxide has been increasingly used for disinfecting drinking water in many countries. Although it does not react with humic substances, chlorine dioxide added to water is reduced primarily to chlorite and chlorate ions, compounds that are under investigation for their potential adverse effects on human health. The aim of this research was to study the genotoxicity of chlorite and chlorate and their mixtures. The end-points included two plant tests (chromosomal aberration test in Allium cepa and micronucleus assay in Tradescantia, carried out at different times of exposure) and two genotoxicity tests in human HepG2 cells (comet assay and cytokinesis-blocked micronucleus test). Preliminary toxicity tests were carried out for both plant and HepG2 assays. The results showed that chlorite and chlorate are able to induce chromosomal damage to plant systems, particularly chromosomal aberrations in A. cepa root tip cells, even at concentrations lower than the limit established by Italian normative law and WHO guidelines. In HepG2 cells increased DNA damage was only observed for chlorate at the lowest concentration. No increase in micronuclei frequency was detected in any of the samples tested in human HepG2 cells.

Study on subchronic toxicity of chlorine dioxide and by-products in water

Subchronic toxicity of the mixture of ClO2, ClO2- and ClO3- in water on rat was studied through feeding test for 90 days. Statistical analyses of variance on weight gained, food utilization efficiency, indexes of blood and serum, liver/bodyweight and kidney/bodyweight ratios, and histopathological examination on liver and kidney were carried out. The results showed that solution of ClO2 and its by-products ClO2- and ClO3- at a concentration of 553 mg/L was not toxic.

Toxicology and carcinogenesis studies of sodium chlorate (Cas No. 7775-09-9) in F344/N rats and B6C3F1 mice (drinking water studies)

BACKGROUND

Sodium chlorate occurs when drinking water is disinfected by chlorine dioxide. We studied the effects of sodium chlorate in rats and mice to identify potential toxic or carcinogenic hazards to humans.

METHODS

We gave groups of male and female rats drinking water containing 125, 1,000, or 2,000 milligrams (mg) of sodium chlorate per liter (L) of water for two years. Male and female mice received 500, 1,000, or 2,000 mg/L. Other groups of animals received plain tap water and served as the control groups. At the end of the study, tissues from more than 40 sites were examined for every animal.

RESULTS

Male and female rats receiving sodium chlorate had higher rates of follicular cell hypertrophy of the thyroid gland, and the groups receiving 2,000 mg/L had higher rates of thyroid gland cancer, compared with the control groups. Female mice exposed to sodium chlorate had a few pancreatic islet cell tumors.

CONCLUSIONS

We conclude that sodium chlorate caused some thyroid gland neoplasms in male and female rats. The pancreatic islet cell tumors in female mice may have been related to sodium chlorate exposure.

[Adrenal cortex response to prolonged administration of low doses of magnesium chlorate in rats]

Rat adrenals were studied using histological and histochemical methods during prolonged intoxication with pesticide magnesium chlorate which was administered in a dose equal to 1/100 of LD50 (41 mg/kg of body weight). Animals that received distilled water and were kept in similar conditions were used as control. It was demonstrated that intoxication of rats for 3-7 days results in increased secretory activity of all the zones of adrenal cortex. Later (after 14-90 days) the zonal response to pesticide administration was variable. Magnesium chlorate treatment results in the disturbances of hormonal synthesis in adrenocorticocytes. Compensatory-adaptive capacities of zona fasciculata were found to be greater than those in zona glomerulosa and zona reticularis.

[A 90-day repeated dose oral toxicity study of magnesium chloride in F344 rats]

In order to examine the toxicity of magnesium chloride hexahydrate, four groups of 10 male and 10 female F344 rats received the compound by dietary supplementation at 2.5, 0.5, 0.1 or 0% for 90 days. No treatment-related death was observed during the study. Transient soft stool and sustained increase in water consumption were observed both in males and females of the 2.5% group and slight reduction in body weight gain was noted in the high-dose males. There were no toxic changes in food consumption, organ weights, hematology and biochemistry, and histopathological examinations in any treated-groups. Based on these results, the no-observed-adverse-effect-level was estimated to be 0.5%, and 2.5% is considered to be appropriate as highest dose for a 2-year carcinogenicity study.

Subchronic sodium chlorate exposure in drinking water results in a concentration-dependent increase in rat thyroid follicular cell hyperplasia

Chlorine dioxide (ClO2) is an effective drinking water disinfectant, but sodium chlorate (NaClO3) has been identified as a potentially harmful disinfection by-product. Studies were performed to describe the development of thyroid lesions in animals exposed to NaClO3 in the drinking water. Male and female F344 rats and B6C3F1 mice were exposed to 0, 0.125, 0.25, 0.5, 1.0, or 2.0 g/L NaClO3 for 21 days. Additional male F344 rats were exposed to 0, 0.001. 0.01. 0.1, 1.0. or 2.0 g/L NaClO3 for 90 days. Female F344 rats were exposed to 0, 0.5, 1.0, 2.0, 4.0, or 6.0 g/L of NaClO3 for 105 days. Thyroid tissues were processed by routine methods for light microscopic examination, and follicular cell hyperplasia was diagnosed using a novel method. Thyroid hormone levels were altered significantly after 4 and 21 days. NaClO, treatment induced a concentration-dependent increase in the incidence and severity of thyroid follicular cell hyperplasia. Male rats are more sensitive to the effects of NaClO3 treatment than females. Follicular cell hyperplasia was not present in male or female B6C3F1 mice. These data can be used to estimate the human health risk that would be associated with using ClO2, rather than chlorine, to disinfect drinking water.

Toxicity of chlorate and chlorite to selected species of algae, bacteria, and fungi

The present study confirms that chlorate is toxic only to brown algae and not to species of other ecologically relevant taxa. The brown alga Ectocarpus variabilis exhibited a LOEC of 0.005 mM (0.4 mg ClO3-/liter) and an LC50 of 0.012 mM, when cultured with nitrate as a sole source of nitrogen. The toxicity to species other than brown algae as measured in growth inhibition tests ranged from 0.75 mM (96-h NOEC) for Selenastrum capricornutum to > or = 7.48 mM (48-h NOEC) for the fungus Trichoderma hamatum. The nitrogen source, nitrate or ammonium, did not significantly influence the toxicity to the nonsensitive species. The tests on brown algae found that as compared with ammonium, the toxicity to nitrate-grown cultures is higher by a factor of about 10. This confirms the hypothesis that nitrate reductase is involved in the toxic effects of chlorate on brown algae. Chlorite, tested as a potential toxic metabolite of chlorate, demonstrated high toxicity to many of the taxa tested and only low toxicity to E. variabilis. It may be concluded that brown algae are exceptionally sensitive to chlorate. It may also be concluded that various nitrogen sources could not induce toxicity in nonsensitive species. From these experiments no conclusions could be drawn as to the potential role of chlorite in chlorate toxicity. Furthermore it may be concluded that E. variabilis is a suitable laboratory test species for further investigations into the mechanism of chlorate toxicity to brown algae.

The ecotoxicity of chlorate to aquatic organisms: a critical review

In order to assess the risk posed by chlorate in aquatic ecosystems, data on the effects of chlorate on aquatic organisms (microorganisms, algae, invertebrates, and fish) and mesocosm studies have been collated and critically reviewed. The geometric mean E(L)C50 values for both freshwater and marine species were (as ClO3-): microorganisms, 38,583 mg.liter-1; microalgae, 563 mg.liter-1; invertebrates, 2442 mg.liter-1; fish, 3815 mg.liter-1. Marine macro red algae were insensitive to chlorate, whereas marine macro brown algae (e.g., Fucus sp.) appeared to be exceptionally sensitive to chlorate, adverse long-term effects having been reported at concentrations as low as 0.015 mg ClO3-.liter-1. Evidence for the mechanism by which chlorate is thought to be particularly toxic to these species is also reviewed. It is concluded that, based on the species reported, chlorate is nontoxic (acute toxicity > 100 mg.liter-1) to most of the freshwater and marine species examined. However, chlorate is highly toxic (acute toxicity < 0.1 mg.liter-1) to certain macro brown algal species. For macro brown algae, the NOEC after 6 months was reported to be approximately 0.005 mg ClO3-.liter-1. It is also concluded that an improved understanding of the actual mode of action of chlorate in sensitive species is desirable. Together with further information on the environmental fate of chlorate, this will improve the risk assessment for chlorate in the aquatic environment.

Potency ranking of methemoglobin-forming agents

This study represents the first systematic attempt to rank methemoglobin-forming agents. It is a quantitative potency ranking study utilizing linear regression analysis of dose-response data for comparative purposes. Six agents that are direct-acting and eight that require bioactivation were tested for their ability to induce methemoglobin formation in Dorset sheep erythrocytes under defined in vitro conditions. The agents were then ranked according to three complementary methods based on the slope of the linear regression, the calculated dose expected to induce a given amount of methemoglobin formation and the calculated percentage methemoglobin response induced by 1 mmol l-1 of the agent. The direct-acting agents, ranked from most to least potent inducers of methemoglobin formation, are: p-dinitrobenzene > o-dinitrobenzene > copper = nitrite > chlorite > chlorate. The ranking from most to least potent inducers of the bioactivated agents are: alpha-naphthol > p-nitroaniline > m-nitroaniline, o-nitroaniline > p-nitrotoluene = aniline > m-nitrotoluene = o-nitrotoluene. The ranking procedures are discussed and issues of interindividual variation and agent-specific sensitivities are addressed.

The effects of subchronic chlorate exposure in Sprague-Dawley rats

Male and female Sprague-Dawley rats were exposed to drinking water containing 3.0, 12.0 or 48.0 mM sodium chlorate. The mean drinking water consumption varied between exposure groups from 100-200 ml/kg/day. Female exposure groups consistently drank more water (23-42%) than male exposure groups thereby receiving more chlorate/kg/day at every exposure level. There were no compound related deaths; however, both males and females in the high exposure groups had significant weight loss during the 90-day exposure period. Also, in these same groups females had mild but significant decreases in the following relative organ weights; adrenals, thymus and spleen, while the relative brain weight was increased. In males, the heart, kidneys and liver were mildly decreased while the brain and testes were mildly increased. Red blood cell counts and percent hematocrit were decreased in both sexes in the high dose group. Pituitary gland (pars distalis) vacuolization and thyroid gland colloid depletion were prominent in both sexes in mid and/or high dose animals. A NOAEL of 0.36 mM chlorate/kg b.w./day in males and 0.50 mM chlorate/kg b.w./day in females were established.

A chimeric Lhcb::Nia gene: an inducible counter selection system for mutants in the phytochrome signal transduction pathway

One approach towards understanding the transduction pathways of phytochromes is the selection of mutants impaired in various steps. We report here the construction of an inducible counter-selection system for such mutants employing the enzyme nitrate reductase. This enzyme can convert the benign substrate analogue chlorate to the toxic product chlorite, resulting in severe growth inhibition. An Arabidopsis thaliana nitrate reductase gene (Nial*2) was placed under the regulation of an Arabidopsis thaliana light-harvesting chlorophyll a/b protein (Lhcb1*3) promoter that is phytochrome-responsive. The chimeric Lhcb::Nia gene was transformed into A. thaliana. Homozygous transformant lines were selected and grown in the absence of nitrate and the presence of L-glutamine, conditions that substantially inhibited the expression of the endogenous nitrate reductase genes. In darkness seedlings of the transformed lines were resistant to chlorate; however, when seedlings were grown with intermittent red light, increased sensitivity to chlorate was observed. This sensitivity was correlated with an increase in both Nia1*2 RNA levels and nitrate reductase activity. The resistant seedlings were clearly distinguishable from the sensitive ones based on hypocotyl length, with no overlap in this parameter between the two populations. Thus, this system should allow for the selection of mutants that are impaired in phytochrome regulation of the transcription of Lhcb genes.

Strain improvement of chymosin-producing strains of Aspergillus niger var. awamori using parasexual recombination

Parasexual recombination was used to obtain improved chymosin-producing strains and to perform genetic analysis on existing strains. Chlorate resistance was used to select for a variety of spontaneous nitrate assimilation pathway mutations in strains previously improved for chymosin production using classical strain improvement methods including mutation and screening, and selection for 2-deoxyglucose resistance (dgr). Diploids of these improved strains were generated via parasexual recombination and were isolated on selective media by complementation of nitrate assimilation mutations. A preliminary genetic analysis of diploid and haploid segregants indicated that the dgr trait, resulting in overexpression of chymosin, was recessive. Also, mutations in two different dgr genes resulted in an increased level of chymosin production. When these mutations were combined via parasexual recombination, the resulting haploid segregants produced about 15% more chymosin than either parental strain. CHEF gel electrophoresis was used to determine the chromosomal location of the integrated chymosin DNA sequences, and to verify diploidy in one case where the chromosome composition of two haploid parents differed.

Chlorate poisoning: mechanism of toxicity

Intoxications with chlorate salts are characterized by methaemoglobin formation, haemolysis and renal insufficiency. The toxic effects on the erythrocyte can be reproduced in vitro. Incubation of human and rabbit erythrocytes with chlorates induces a concentration-dependent oxidation of haemoglobin. This methaemoglobin formation is followed by denaturation of the globin, a cross-linking of erythrocyte membrane proteins and an inactivation of membrane enzymes. The high sensitivity of glucose-6-phosphate dehydrogenase to denaturation by chlorate explains the inefficacy of methylene blue to reduce methaemoglobin formed, as the antidotal effect of methylene blue depends on NADPH formed mainly by the oxidation of glucose-6-phosphate. The observed changes occur only in the presence of methaemoglobin which forms a destabilising complex with chlorate. Methaemoglobin thus autocatalytically increases methaemoglobin formation and destruction of the erythrocyte. As the rabbit is known to have a high methaemoglobin-reduction capacity, human and rabbit erythrocytes were compared. In vitro, the rabbit erythrocyte is less sensitive to oxidative attack than the human red cell. In vivo, an oral dose of sodium chlorate (1 g/kg body wt.) resulted in high serum (16 +/- 4 mM) and urine concentrations (246 +/- 99 mM) in the rabbit. Methaemoglobin was not formed nor could a nephrotoxic effect be observed. These experiments also indicate that the nephrotoxicity of chlorate is mediated by methaemoglobin catalysis.

Toxicity of and mutagenesis by chlorate are independent of nitrate reductase activity in Chlamydomonas reinhardtii

Spontaneous chlorate-resistant (CR) mutants have been isolated from Chlamydomonas reinhardtii wild-type strains. Most of them, 244, were able to grow on nitrate minimal medium, but 23 were not. Genetic and in vivo complementation analyses of this latter group of mutants indicated that they were defective either at the regulatory locus nit-2, or at the nitrate reductase (NR) locus nit-1, or at very closely linked loci. Some of these nit-1 or nit-2 mutants were also defective in pathways not directly related to nitrate assimilation, such as those of amino acids and purines. Chlorate treatment of wild-type cells resulted in both a decrease in cell survival and an increase in mutant cells resistant to a number of different chemicals (chlorate, methylammonium, sulphanilamide, arsenate, and streptomycin). The toxic and mutagenic effects of chlorate in minimal medium were not found when cells were grown either in darkness or in the presence of ammonium, conditions under which nitrate uptake is drastically inhibited. Chlorate was also able to induce reversion of nit- mutants of C. reinhardtii, but failed to produce His+ revertants or Arar mutants in the BA-13 strain of Salmonella typhimurium. In contrast, chlorate treatment induced mutagenesis in strain E1F1 of the phototrophic bacterium Rhodobacter capsulatus. Genetic analyses of nitrate reductase-deficient CR mutants of C. reinhardtii revealed two types of CR, to low (1.5 mM) and high (15 mM) chlorate concentrations. These two traits were recessive in heterozygous diploids and segregated in genetic crosses independently of each other and of the nit-1 and nit-2 loci. Three hcr loci and four lcr loci mediating resistance to high (HC) and low (LC) concentrations of chlorate were identified. Mutations at the nit-2 locus, and deletions of a putative locus for nitrate transport were always epistatic to mutations responsible for resistance to either LC or HC. In both nit+ and nit- chlorate-sensitive (CS) strains, nitrate and nitrite gave protection from the toxic effect of chlorate. Our data indicate that in C. reinhardtii chlorate toxicity is primarily dependent on the nitrate transport system and independent of the existence of an active NR enzyme. At least seven loci unrelated to the nitrate assimilation pathway and mediating CR are thought to control indirectly the efficiency of the nitrate transporter for chlorate transport. In addition, chlorate appears to be a mutagen capable of inducing a wide range of mutations unrelated to the nitrate assimilation pathway.

The effects of chronic administration of chlorine dioxide, chlorite and chlorate to normal healthy adult male volunteers

The physiological impact of chronic 12 week ingestion of chlorine dioxide and its byproducts, chlorite and chlorate, was compared to the effects of chlorine, chloramine and untreated water. The water disinfectant solutions were administered daily (500 ml, 5 ppm) to normal healthy adult male volunteers. An extensive battery of tests was used to evaluate the physiological impact of the ingested water disinfectants. Upon analysis of both quantitative and qualitative parameters it was concluded that the 12 week chronic administration of chlorine dioxide and its byproducts was accompanied by no clinically important physiological effects.

Effects of the acute rising dose administration of chlorine dioxide, chlorate and chlorite to normal healthy adult male volunteers

Chlorine dioxide is under consideration for use as a water disinfectant alternative to chlorination in the United States. A rising dose tolerance study was undertaken to assess the relative safety and tolerance of acute administration of chlorine dioxide and its byproducts, chlorite and chlorate, to normal healthy adult male volunteers. In evaluation of an extensive battery of laboratory tests and vital signs, no adverse physiological effects were identified. This provided a data base upon which a controlled 5-month study trial of these substances in normal healthy volunteer subjects was designed.

Erythrocyte membrane alterations as the basis of chlorate toxicity

The effects of sodium chlorate and of sodium nitrite on human erythrocytes were studied in vitro. Nitrite rapidly oxidised haemoglobin and glutathione; reduction of methaemoglobin (Hbi) by methylene blue was complete during 3 h of incubation with nitrite. With chlorate, a concentration-dependent lag phase was seen before Hbi was formed. After prolonged incubation, Hbi could no longer be reduced with methylene blue. Several other effects were observed that explain the clinical picture of chlorate poisoning which involves haemolysis followed by disseminated intravascular coagulation and renal failure: increased permeability to cations, increased resistance to hypotonic haemolysis and prolonged filtration time through polycarbonate membranes with cylindrical pores of 5 micron diameter. This suggests an increased membrane rigidity due to membrane protein polymerisation, as demonstrated by SDS polyacrylamide gel electrophoresis. Simultaneously, erythrocyte enzymes were inactivated, primarily glucose-6-phosphate dehydrogenase which is necessary for the therapeutic effect of methylene blue. This explains the inefficacy of methylene blue in the treatment of a case of chlorate poisoning that we observed (Arch. Toxicol., 48 (1981) 281).