Arsenic competition with other anions

The purple acid phosphatases are typically assayed by their ability to hydrolyze p-nitrophenylphosphate. The K values for this substrate are in the millimolar range (171b, 174). The product phosphate and its analogue arsenate are weak competitive inhibitors (A mM) in addition, they potentiate the conversion of the diiron enzymes under aerobic conditions to an irreversibly inactivated, oxidized form (45, 185-187). Other tetraoxo anions such as molybdate and tungstate, on the other hand, are tightly bound inhibitors K, xM) which do not potentiate the oxidation of the diiron enzymes (185, 186). Interestingly, the FeZn forms are similarly inhibited by these anions with comparable K, values, but are not susceptible to the oxidative inactivation observed for the diiron derivatives (185). 

The equilibrium distribution of arsenic between the surface and solution can be destabilized by changes in environmental conditions. Changes in pH or in the concentration of competing ions could shift a pre-existing sorption equilibrium to release arsenic into the contacting fluid. Consistent with competitive desorption, correlations between dissolved arsenic and other (competing) anions... 

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