Summary - As

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Arsenic is described as a metalloid because it displays properties intermediate of those typical for metals and non-metals [1-Link]. The chemistry of arsenic is similar in many respects to that of nitrogen and phosphorus, two essential elements. These chemical similarities may be the reason that arsenic occurs at high levels in many marine organisms, and hence in many seafoods (Francesconi and Edmonds, 1997)[2-Link]. For example, the inorganic ion arsenate occurs in seawater together with the structurally similar phosphate. Marine algae appear unable to distinguish between these two oxoanions; in their efforts to take up essential phosphate they inadvertently take up the potentially toxic arsenate. The process of detoxification begins by methylation leading to methylated organoarsenic compounds. Arsenobetaine is structurally similar to glycine betaine, which is used by aquatic organisms to maintain osmotic balance under conditions of changing salinity, i.e. when ambient salinity is high, an organisms glycine betaine level is high. The coincidental structural similarity between arsenobetaine and glycine betaine might explain why arsenobetaine levels are much higher in marine animals than they are in freshwater animals.

 

Although arsenic forms species under reducing conditions with the arsenic atom in oxidation state -3 and +3, the most stable arsenic species found under normal environmental conditions contain the arsenic atom in oxidation state +5. Consequently, the vast majority of arsenic species found in

organisms and in foods also contain arsenic in oxidation state +5 (e.g. arsenate, dimethylarsinate, arsenobetaine, arsenosugars). The table below summarises some arsenic species found in foods, and some relevant human metabolites.

 

Names, abbreviations, and chemical structures for some relevant arsenic species in food.

Name

Abbreviation

Chemical structure (a)

Relevance/comment

 

Inorganic arsenic

iAs

 

Sum of As(III) and As(V).

Arsenite

As(III)

As(O -) 3

 

Trace to low levels in most foods; highly toxic.

Arsenate

As(V)

O=As(O -) 3

 

Trace to low levels in most foods; a major form in water; highly toxic.

Arsenobetaine

AB

(CH 3) 3As +CH 2COO -

Major arsenic species in most seafoods; non-toxic.

Arsenosugars b

 

 

Major (edible algae) or significant (molluscs) arsenic species in many seafoods.

Dimethylarsinate

DMA

(CH 3) 2AsO(O -)

Minor arsenic species in seafoods and some terrestrial foods; the major human urine metabolite of iAs, arsenosugars and arsenolipids.

(a)       The simpler arsenic species are also often referred to in their protonated forms such as As(III) arsenous acid, H3AsO3; As(V) arsenic acid, H3AsO4; DMA dimethylarsinic acid (CH3)2AsO(OH).

(b)      Over 20 arsenosugars have been reported as natural products.

 

The inorganic arsenic forms are more toxic as compared to the organic arsenic occurring in food. The need for speciation data is evident because especially in seafood most of the arsenic is present as organic arsenic and thus is in the less toxic form. Consequently, a risk assessment not taking into account the different species but considering the total arsenic as being present exclusively as inorganic arsenic would lead to a considerable overestimation of the health risk related to dietary arsenic exposure.

 

In food samples inorganic arsenic is often reported as arsenite and arsenate even though it is likely bound to thio groups in peptides or proteins in the food itself. Because food products of terrestrial origin generally contain low concentrations of total arsenic their inorganic arsenic content is also low. Rice, however, appears to be an exception because it contains significant amounts of inorganic arsenic (between 0.1 to 0.4 mg arsenic/kg dry mass) and sometimes considerably higher  (Sun et al., 2008)[3-Link]. Although fish and other seafood have a high total arsenic content (typically 2-60 mg arsenic/kg dry mass), their levels of inorganic arsenic are typically <0.2 mg arsenic/kg dry mass (Sloth et al., 2005)[4-Link]. There are, however, some notable exceptions.

 

Arsenobetaine is the major form of arsenic in marine fish and most other seafoods. Arsenobetaine has also been found in some terrestrial foods, in particular in some mushroom species, although generally as a minor compound  (Francesconi and Kuehnelt, 2002)[5-Link]. Arsenosugars are usually the major arsenical constituents of marine algae (typically 2-50 mg arsenic/kg dry mass), and they also are found at significant concentrations in animals feeding on algae (e.g. mussels and oysters; typically 0.5-5 mg/kg dry mass) (Francesconi and Kuehnelt, 2002)[5-Link]. They occur in many other marine organisms as well, albeit at lower concentrations. In terrestrial organisms, arsenosugars occur generally at trace levels only, although interesting exceptions have been reported.

The provisional tolerable weekly intake (PTWI) of 15 µg inorganic arsenic /kg b.w. established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) is no longer appropriate according to the EFSA Panel on Contaminants in the Food Chain (CONTAM). The CONTAM Panel modelled the dose-response data from key epidemiological studies and selected a benchmark response of 1 % extra risk. A range of benchmark dose lower confidence limit (BMDL01) values between 0.3 and 8 µg /kg b.w. per day was identified for cancers of the lung, skin and bladder, as well as skin lesions. The estimated dietary exposures to inorganic arsenic for average and high level consumers in Europe are within the range of the BMDL01 values identified, and therefore there is little or no margin of exposure and the possibility of a risk to some consumers cannot be excluded.

Of the organic forms of arsenic, arsenobetaine, which is the major form in fish and most seafood, is widely assumed to be of no toxicological concern. Arsenosugars and arsenolipids are mainly metabolised in humans to dimethylarsinate, but no specific information is available regarding their toxicity. For other organoarsenic compounds no human toxicity data are available. Because of the lack of data, arsenosugars, arsenolipids, methylarsonate and dimethylarsinate were not considered in the CONTAM risk characterisation.

The CONTAM Panel recommended that dietary exposure to inorganic arsenic should be reduced. In order to refine risk assessment of inorganic arsenic there is a need to produce speciation data for different food commodities to support dietary exposure assessment and dose-response data for the possible health effects.