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Arsenic and antimony biomethylation by Scopulariopsis brevicaulis Andrewes, Paul

Abstract

Many organisms have been isolated that transform inorganic arsenic compounds to organoarsenic species. In the case of antimony, simple alkylantimony species have been detected in the environment, but only a few organisms are known that produce these species. The fungus Scopulariopsis brevicaulis was selected as a model microorganism to study antimony biomethylation, because it is known for its ability to biomethylate the chemically similar element, arsenic. Cultures of S. brevicaulis were incubated with inorganic antimony compounds. Volatile antimony biomethylation products, in headspace gases, were determined by using gas chromatography inductively coupled plasma mass spectrometry, and nonvolatile species, in media samples, were determined by using hydride generation gas chromatography atomic absorption spectroscopy. Potassium antimony tartrate and antimony trioxide were biomethylated. The major biomethylation products (~95%) were nonvolatile dimethylantimony and trimethylantimony species, found in the medium. Volatile trimethylstibine (Me3Sb) was produced as a minor biomethylation product (~5%). The methyl source for antimony biomethylation was investigated, by incubating S. brevicaulis with potassium antimony tartrate and CD3-D-methionine, or 13CD3-L-methionine. There was significant, and comparable, incorporation of the methyl group from L-methionine into alkylantimony and alkylarsenic species. There was incorporation of the methyl group from D-methionine into alkylarsenic and alkylantimony species, but less than for L-methionine. These results suggest that antimony biomethylation proceeds via a mechanism very similar to that of arsenic biomethylation. Mixtures of antimony and arsenic compounds were incubated with S. brevicaulis and the biomethylation products determined. In these interaction studies, it was found that inorganic antimony(III) compounds inhibited arsenic biomethylation whereas inorganic arsenic(III) compounds enhanced antimony biomethylation. Microorganisms, resistant to the germicide, 10, 10'-oxybisphenoxarsine (OBPA), were isolated. The OBPA-resistant microorganisms, and S. brevicaulis, were incubated with OBPA for one month and the medium analyzed. There was no evidence, in the form of OBPA break-down products or intermediates on the pathway to trimethylarsine, to suggest that any of these microorganisms might produce volatile arsines from OBPA. Furthermore, when headspace gases from S. brevicaulis cultures, grown in medium containing OBPA, were analyzed, no significant amounts of trimethylarsine were detected.

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