Only arsenite leads to allosteric inhibition of pyruvate dehydrogenase which is the most important mechanism for arsenite poisoning. Citric acid cycle is interrupted. As(V) must be reduced in vivo to As(III). Also organic As-compounds must be metabolized into As(III). For this reason these species are less toxic.
As (III) can easily bind with specific proteins and transported into the cell. On the other hand, As(V) cannot reach inside the cell by this mechanism. As(V) get to reduced to +3 state for this transport to occur. Also, As(III) can bind with -SH group present in many proteins, enzymes, thus inhibiting their activity. Most forms of organo aresnicals such as arsenobetaine are metabolized by our body and eliminated via urine. Fyrther, inorganic forms are better absorbed by body than organic arsenic.
These factors contribute towards the different toxicity of As(III) and As(V).
THogh, it is believed in general, that organic arsenic are non-toxic, recent research has shown that some organic arsenic compounds show simillar toxicity as that of inorganic arsenic.
Also refer the following article.
Molin, M., Ulven, S. M., Meltzer, H. M., & Alexander, J. (2015). Arsenic in the human food chain, biotransformation and toxicology--Review focusing on seafood arsenic. Journal of trace elements in medicine and biology: organ of the Society for Minerals and Trace Elements (GMS), 31, 249.
Inorganic arsenic is generally more toxic than organic arsenic. Forms of arsenic that are more rapidly absorbed are more toxic, while those most rapidly eliminated tend to be less toxic. Arsenite (III) and arsenate (V) forms are highly soluble in water. Arsenobetaine and arsenocholine are the organic forms known as “fish arsenic” and are relatively nontoxic to humans.
In agriculture:
As(III) is usually more prevalent under reduced environmental conditions (e. g., flooded soils). As(V) is usually more prevalent under oxidized conditions (e. g., drained soils). However, both oxidation states of arsenic are often observed together in soils, because of the sometimes slow rates of transformation between oxidation states.
Oxidation state influences many properties of arsenic, including:
- Adsorption to soil minerals
- Solubility in soil
- Uptake by plants
- Toxicity to plants and animals
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