Arsenic and liver: Chicken feed analysis

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  • Published: May 15, 2017
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Arsenic and liver: Chicken feed analysis

Rox to tox

Poultry Feed with Arsenic More Problematic than Assumed?  New methylated phenylarsenical metabolites identified in chicken livers Credit: Wiley-VCH

An analysis of mass spectrometric data reveals the existence of three toxicologically important methylated phenylarsenical metabolites in chicken liver from fowl given feed containing the additive 3-nitro-4-hydroxyphenylarsonic acid (ROX). This additive and others containing arsenic were banned in the European Union in 1999 and in North America as recently as 2013, but they are still widely used elsewhere in the world.

Arsenic additives in chicken feed have been used to prevent infection with parasites in the fowl and so to promote health and more importantly from the commercial point of view, weight gain. Writing in the international edition of the journal Angewandte Chemie, Hanyong Peng and Bin Hu of Wuhan University, Wuhan, China, and Qingqing Liu, Jinhua Li, Xing-Fang Li, Hongquan Zhang, and Chris Le of the University of Alberta Edmonton, Canada, describe how the danger to human health from these additives for people who eat poultry products, such as chicken liver pâté may be greater than was previously thought. The metabolic breakdown of the additives in chickens takes place through various intermediate compounds that are themselves a lot more toxic than the additives themselves.

Metabolic issue

"Arsenic consistently ranks first on the priority list of environmental contaminants because of the occurrence, persistence, and toxicity of various arsenic compounds," the team reports. "Chronic exposure to high concentrations of arsenic puts more than 100 million people around the world at risk of developing cancer and other adverse health effects." Much of the exposure is through well water contaminated with natural arsenic salts that become solubilised during periods of drought. This problem has been widely known since the early 1990s when I reported on the problem in the UK national press, but is yet to be addressed. Industrial exposure is also a significant issue. The current research shows that despite the banning of chickenfeed arsenicals in many countries, the issue remains acute for others, adding to the burden of arsenic absorption for countless people.

Roxarsone (3-nitro-4-hydroxyphenylarsonic acid, also known more simply as Rox is a well-known feed supplement. The compound itself is only slightly toxic to animals that have so far been tested. However, little was known until now about the metabolic route it takes through animals that ingest the compound. Moreover, there was a dearth of information about whether or not eating poultry products from fowl treated with this additive would therefore represent a health risk to people. After all, the toxicity of arsenic-containing species depends strongly on the type of compound, the oxidation state of the arsenic and ligands to which it is bound and can vary by orders of magnitude.

Hu's team at Wuhan working with Le's team in Edmonton, studied 1600 chickens under controlled feeding conditions. They analysed samples of liver taken from fowl treated with Rox or left untreated. Earlier research had shown that several different arsenic-containing compounds can be present in chicken liver, breast meat, and waste. They used a raft of mass spectrometric and chromatographic methods to obtain data that then allowed them to identify three additional, and rather significant, arsenic-containing metabolites.

Methylation, that's the name of the game

The team explains that the new compounds are Rox derivatives that contain an additional methyl group on their arsenic atom. These three methylated metabolites comprise approximately 42 percent of the total arsenic load found in the chicken liver samples. Methylation can ultimately lead to the release of a toxic element from a compound into the body where it can cause subsequent poisoning that would be precluded in the unmethylated starting compound.

The team wanted to know what causes this methylation process. The most likely suspect would normally be the enzyme arsenic methyltransferase (As3MT), which is also involved in the human metabolism of arsenic. However, As3MT only methylates trivalent arsenic, arsenic(III), whereas Rox and its derivatives contain arsenic in its pentavalent form, arsenic(V). The team carried out tests with reduced versions of Rox and have shown that breakdown of Rox can occur through trivalent intermediates. Tests with cell cultures then demonstrated that these compounds are 300 to 30000 times more toxic than arsenic(V) Rox derivatives. The researchers are yet to determine the concentrations at which these highly toxic intermediates are present in fowl treated with Rox.

Common practice in the poultry industry is to cease Rox supplementation five days prior to slaughter. However, liver samples taken after this interval contain residues of arsenic compounds at a concentration that would be worrying if those chicken livers were consumed.

Related Links

Angew Chem Int Edn 2017, online: "Methylated Phenylarsenical Metabolites Discovered in Chicken Liver"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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