Toxic species arsenic

Arsenic compounds must be considered extremely poisonous. Dust or fumes irritate mucous membranes and lead to arsenical poisoning. When swallowed they irritate the stomach and affect the heart, Hver, and kidneys. Nervousness, thirst, vomiting, diarrhea, cyanosis, and coUapse are among the symptoms of arsenical poisoning (3). In spite of the toxicity of arsenic compounds, there is evidence that arsenic is an essential nutrient for several animal species (4). 

If species such as sodium, potassium, chloride, and sulfate are tolerable in a fairly large range, other toxic species such as arsenic and lead have narrower limits. For arsenic and lead, the limits in pharmacopeial compendia are generally set between 1 and 10 ppm in fact, these limits are high for such harmful species. 

Mkandawire, M., Lyubun, Y.V., Kosterin, P.V. and Dudel, E.G. (2004) Toxicity of arsenic species to Lemna gibba L. and the influence of phosphate on arsenic bioavailability, Environmental Toxicology 19 (1), 26-34. 

The species that are the most hazardous to humans and other animals as poisons are those that have a toxic effect on the body and that are soluble in water or mild acid. Some species, like realgar, contain a potentially toxic element (arsenic), but have such a low solubility potential that their toxicity as a poison is considered to be low under normal circumstances. 

The redox environment can also determine some of the properties of metallic and non-metallic species. For example, the toxicity of arsenic when present in oxic (oxidizing) environments such as As( V) is very low, whereas its reduced form, As(III), is highly poisonous. The opposite occurs with Cr(VI) that is much more toxic than its reduced counterpart, Cr(III). 

The toxicity of arsenic is well-documented. Trivalent arsenic (arsenite) is almost always more toxic than the pentavalent species (arsenate)The differential toxicity is due to their different modes of action. The primary biological targets of trivalent arsenic are thiol-containing molecules, e.g. in active sites of enzymes (see below). Arsenate, on the other hand, is better tolerated (detoxified) at low concentrations and, at higher levels, competes with phosphate, an ubiquitous biological anion. Indeed the toxicity of pentavalent arsenic in vivo may be partly due to its intracellular reduction to the trivalent form . ... 

A number of different factors can influence the toxicity of arsenicals, including chemical structure, physical properties, mode of administration and species of organism. 

Study on the soil-to-carrot uptake rate of As 34 (hioavailahility). Inorganic arsenic species were prevalent in soil. The ingestion of the potentially toxic inorganic arsenic via consumption of carrots grown in soil contaminated at 30pgAsg was conservatively estimated at 37 pg per week... 

Examples showing that metal speciation is important to metal toxicity include arsenic, copper, selenium, and chromium. While ionic copper (Cu2+) and CuClj are highly toxic, Q1CO3 and Cu-EDTA have low toxicity (Morrison et al, 1989). Toxicity tests show that As(III) is about 50 times more toxic than As(VI). Trivalent chromium is much less toxic than hexavalent chromium, probably because Cr(VI) is much smaller and the chemical structure of chromate is similar to sulfate. A special channel already exists in biomembranes for sulfate transport. While modeling metal speciation is not always possible, and redox equilibrium is not achieved in all natural waters, geochemical modeling of equilibrium species distribution remains one of the methods of discerning metal speciation. 

There are many species and strain differences in the toxicity of arsenical compounds. The purity, physical form, and solubility of the compounds also influence toxicity. In a NRCC study (1978), the range of acute toxic values for fish was similar for arsenite and arsenate. Of seven tested fish species, LC50 values (median lethal dose) ranged from 13.3 mg (rainbow trout) to... 

The reduced species (arsenite) is more toxic than the oxidized species (arsenate) since it may react with sulfhydryl groups of cysteine in structure and enzyme proteins. Therefore oxidation of arsenite in (micro)organisms is an important protective mechanism which is described for various species of the genus Pseudomonas, Xantho-monas and Achromobacter. However, some bacteria are able to reduce arsenate to the more toxic form arsenite. This process is described for some strains of Pseudomonas fluorescens and Anabaena oscillaroides (Cullen and Reimer, 1989). 

Elemental As is not considered poisonous, but many arsenic compounds are extremely toxic. Arsenic differs in toxicity in different oxidation states. Trivalent arsenic (As(III)) is considerably more toxic than pentavalent arsenic (As(V)). The toxicity of arsenic is not only dependent on its chemical form but also on its solubility and mobility in soils. Understanding the speciation and species transformation of arsenic is essential to determining its toxic effect on plants and animals. The chemical form determines the availability to animals and plants. 

Environmental fate and behavior, bioavailability, and toxicity of arsenic vary dramatically with the chemical forms (species) in which arsenic exists. While inorganic arsenite [As(III)] and arsenate [As(V)] are highly toxic, mono-methylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)] are less toxic, and predominant arsenic species present in most crustacean types of seafood are essentially nontoxic (1,5-8). Thus, assessments of environmental impact and human health risk sPictly based on measurements of total element concenfiation are not reliable. It is important to identify and quantify individual chemical species of the element (i.e., chemical speciation). 

Arsenic has found wide use in pesticides, herbicides, wood preservatives and desiccants. Arsenic exposure can thus occur in many circumstances including occupations e.g., coal mines and smelters), air and food. The toxicity of arsenic varies widely with different chemical forms. As(III), As(V), monomethylarsonic acid (MMA) and dimetihylarsenic acid (DMA) are the arsenic species studied most in the literature. Inorganic arsenics are more toxic than organoarsenicals, while trivalent arsenic compounds are more toxic than their pentavalent counterparts. MMA and DMA are the two main metabolites known to be toxic. 

Reproductive system Environmental arsenic exposure can impair male fertility. A case-control study in China concluded that elevated inorganic arsenate (Asi(V)) exposure is associated with arsenic-induced male infertility. The mechanism of toxicity by arsenic species may involve oxidative stress and sexual hormone disruption measured by biomarkers including acylcamitines, aspartic acid and hydroxyestrone, which were negatively associated with infertility, and uridine and methylxanthine, which were positively associated [44 ]. Arsenic exposme is also correlated to a decrease in human semen quality as reported in a reproductive-age Chinese cohort. The study demonstrated significant association of dimethyl arsenic species (DMA) concentration with low sperm concentrations [45 -]. 

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