Investigation of arsenosugar metabolism in sheep
This thesis describes part of an on-going study of the metabolism of arsenosugars in sheep living on a diet of seaweed. Two major analytical techniques have been combined in a three-pronged approach. ICP-MS has served to quantify total arsenic in seaweed, urine and faeces and has enabled a mass balance to be drawn up indicating that around 86% of ingested arsenic (average 35 mg/d) is excreted in the urine, 13% in the faeces and less than 1% is accumulated in the animal. HPLC-ICP-MS has served to quantify both known and unknown metabolites at down to low ng/g levels in urine and in faeces, though they are often present in mg/g levels in the undiluted samples. Seaweed fed to the sheep, Laminaria digitata and L. hyperborea, contained arsenic mainly (70%) as arsenosugars, a little (2%) as dimethylarsinic acid, DMA(V), and the remainder was inextractable. The principal metabolite seen in the urine and in the faeces was DMA(V). A major problem in these studies has been the low recovery of the chromatography, which has the consequence that the speciation must be considered thus far as incomplete. The chromatographic recoveries, form urine samples alone, have ranged from 4 to 100%. Coupling of ICP-MS and electrospray MS (ES-MS) simultaneously to the outlet of the same HPLC column has permitted the identities of several new compounds to be established. DMAA (dimethylarsinoyl acetic acid) has been found as an arsenosugar metabolite in urine for the first time. The simultaneous coupling was crucial because DMAA co-elutes with methylarsonic acid (MA(V)) from the commonly used PRP-X100 strong anion-exchange column and without the use of a molecular detector it would have been misidentified. A new class of compounds has been identified in the urine samples by ES-MS, in which arsenic in its pentavalent state is bound to sulfur. Dimethylarsinothioic acid (DMAS) has been positively identified by retention time and by molecular mass. It turns out to have apparently the same retention time as a DMA(III) standard.