An appetite for arsenic: The seaweed-eating sheep from Orkney

The toxicity of arsenic depends strongly on its species. Cation and anion exchange chromatography has been coupled to ICP-MS in order to separate and detect 15 different arsenic species in seaweed extracts and in urine and blood samples of a seaweed-eating sheep from Northern Scotland. This rare breed of North Ronaldsay sheep live the entire year on the beach and eat up to 3 kg seaweed daily, which is washed ashore on this remote Orkney island. Seaweed, especially brown algae (e.g., Laminaria spp.) contains about 20-100 mg arsenic per kg dry weight, mainly as dimethylarsinoylriboside species commonly called arsenosugars. The daily intake is approximately 50 mg arsenic. In order to investigate the metabolism of the sheep, urine, blood and wool samples were taken from 20 individual sheep, and from another I I sheep samples of liver, kidney and muscle were extracted and analysed for their arsenic content.

The arsenic concentration in the tissue and the wool is at least two orders of magnitude higher than for non-exposed sheep, but does not reach the maximum allowed arsenic level in foodstuffs according to a UK guideline (I mg As per kg fresh weight). The arsenic speciation in the urine reveals that the arsenosugars are taken up by the sheep and excreted mainly as dimethylarsinic acid (DMAA) into the urine. The proposed metabolism includes an enzymatic side chain elimination and a beta -elimination reaction of the non-substituted riboside to give DMAA. In addition to DMAA minor metabolides such as tetramethylarsonium, and monomethylarsonic acid (MMAA) have been identified. The occurrence of MMAA in the urine and the large amount of arsenic bound to the wool suggests that in addition to the arsenosugars intake, the sheep take up non-methylated arsenic species which may occur in other seaweed. This leads to the question of whether this particular sheep is resistant to large quantities of inorganic arsenic, since the tradition of having sheep on the beach is several hundred years old and they have shown no adverse effects.