Has NADPH-dependent FMN reductase activity. No activity with NADH. May play a role in resistance to heavy metal toxicity. Concentration of FMN required for half-maximal activation is 7.5 uM. Optimum pH is between 6.5-7.5. Homotetramer. Sensitive to low levels of AsO4(3-) and to a lesser degree to As(5+). Belongs to the ArsH family.