How do the toxicities of arsenate and arsenite compare?
I see now. 

According to the same website with the reaction mechanism, 


  Although the arsenite formed is more toxic than arsenate , it can be extruded from some bacteria by EC 3.6.3.16 , arsenite-transporting ATPase; in other organisms, arsenite can be methylated by EC 2.1.1.137 , arsenite methyltransferase, in a pathway that produces non-toxic organoarsenical compounds. cf EC 1.20.4.4, arsenate reductase (thioredoxin) .