Methylation of arsenic in vitro by cell extracts from bentgrass (Agrostis tenuis): effect of acute exposure of plants to arsenate

Compared with microorganisms and mammalian tissues, information is scant on the enzymes responsible for arsenic metabolism in plants. This study investigated the arsenic methylation activities extractable from leaves and roots of Agrostis tenuis Sibth. plants grown in complete nutrient media and exposed to arsenate (135-538 muM) for 3 d before harvesting. Methylation activity was determined in leaf and root extracts using an in vitro assay based on S-[H-3-methyl]adenosyl-L-methionine (H-3-SAM) with either arsenite or arsenate as substrate. Arsenite methylation activity was low in leaf extracts from plants not exposed to arsenate, but was greatly enhanced after acute exposure, with the induced methylation activity greatest in extracts from plants exposed to 269 muM arsenate. Monomethylarsonate (MMA) was the predominant early product, but over longer assay times dimethylarsinate (DMA) accumulated at the rate of 660 amol mg protein(-1) min(-1) to levels exceeding MMA. With arsenate as substrata methylation activity was much lower than with arsenite, implying that arsenite is the preferred substrate for methylation. Root extract assays exhibited no DMA, however small amounts of MMA were formed with arsenite as substrate. In contrast to leaves, the methylation activity did not increase in root extracts from plants exposed to arsenate. These findings suggest that arsenate in the plant growth medium was taken up by the roots and converted to arsenite before methylation proceeded in the leaves, accompanied by induction of arsenic methyltransferase activities.