Has NADPH-dependent FMN reductase activity and high NADPH-dependent ferric reductase activity with highest activity for Fe(3+) as substrate. No activity with NADH, iron trichloride, Cu(2+) or Ag(+). May be involved in cytosolic ferric iron assimilation as an NADPH-dependent ferric reductase in vivo. Optimum pH is 6.5 for the ferric reductase activity. Optimum temperature is 35 degrees Celsius for the ferric reductase activity. Homotetramer. Belongs to the ArsH family.