Influence of arsenic on antimony methylation by the aerobic yeast Cryptococcus humicolus

The anamorphic basidomycetous yeast Cryptococcus humicolus was shown by hydride generation-gas chromatography-atomic absorption spectrometry to methylate inorganic antimony compounds to mono-, di-, and trimethylantimony species under oxic growth conditions. Methylantimony levels were positively correlated with initial substrate concentrations up to 300 mg Sb l(-1) as potassium antimony tartrate (K-Sb-tartrate). Increasing concentrations of K-Sb-tartrate increased the ratio of di- to trimethylantimony species, indicating that methylation of dimethylantimony was rate limiting. Antimony methylation capability in C. humicolus was developed after the exponential growth phase and was dependent upon protein synthesis in the early stationary phase. Inclusion of inorganic arsenic (III) or (V) species alongside antimony in culture incubations enhanced antimony methylation. Pre-incubation of cells with inorganic arsenic (III) further induced antimony methylation capability, whereas pre-incubation with inorganic antimony (III) did not. Exposure of cells to inorganic arsenic-either through pre-incubation or provision during cultivation-influenced the antimony speciation; involatile trimethylantimony species was the sole methylated antimony species detected, i.e. mono- and dimethylantimony species were not detected. Competitive inhibition of antimony methylation was observed at high arsenic loadings. These data indicate that antimony methylation is a fortuitous process, catalysed at least in part by enzymes responsible for arsenic methylation.