Is Arsenic Metabolism more Complex than Expected?

01-07-2004: When "arsenic" comes up in mystery stories, the victim has usually been poisoned with white arsenic, arsenic oxide. Just how toxic an arsenic-containing substance is depends on the exact structure of the compound. Researchers at the Universities of Aberdeen, Scotland, and York, England have found a previously unknown organic arsenic compound in the urine of a rare breed of sheep. The unusual thing about this molecule is that it has a sulfur atom bound to the arsenic atom. This is the first time that a thioorganoarsenate, as this class of compounds is called (Greek theion = sulfur), has been found in a biological sample.

"This is surprising, but not actually unexpected," claims Jörg Feldmann, "because arsenic has a high affinity for sulfur; in the body, arsenic ions bind to hydrogen sulfide groups in proteins, crippling important physiological functions. Bonding between arsenic and sulfur atoms also plays an important role in the breakdown of arsenic-containing compounds in the body." In their search for arsenic-containing metabolic products, Feldmann and his colleagues examined the urine of a British breed of sheep whose preferred food is seaweed. Seaweed accumulates arsenic, which is present in trace amounts in seawater, in the form of arsenosugars, a class of compounds previously considered to be nontoxic. The thioorganoarsenate, whose structure was determined by chromatographic and mass spectroscopic methods, is not very stable, which may be one reason why it has only just been discovered. Treatment of the samples, or allowing them to stand for a long time, causes the compound to be rapidly converted into the corresponding oxoorganoarsenate; the sulfur atom is replaced by an oxygen atom. The oxo compound has been known for some time, it occurs in crustaceans and was thought to be a metabolite of arsenosugars that is excreted in urine.

Perhaps thioorganoarsenates were previously just overlooked in the analysis of biological samples. Says Feldmann: "The standard conditions for the analysis of arsenic compounds seem to be very unfavorable for the detection of thioorganoarsenates." For example, the pH value (acidity) plays an important role in the separation of samples on chromatography columns. In strongly acidic solutions, the thioorganoarsenate from sheep urine decomposes easily, while weakly acidic liquids make it impossible to elute the compound from the column at all. "Now that we know this, we may be able to discover many more thioorganoarsenic compounds," speculates Feldmann. "In any case, the metabolism of arsenic compounds in the body seems to be more complex than previously thought, and more questions about the toxicity of arsenic compounds are raised."