Transient-state kinetic time courses of strand incision activity of APE1 acting on AP (blue), AP-Red (green) or THF (red) DNA in the presence of 5 mM (A) Mg2+, (B) Mn2+, (C) or Ni2+. Inset depicts the full time course. Experimental conditions were 50 nM DNA, 500 nM APE1, 50 mM HEPES-KOH, 100 mM KCl, 0.12 mg/mL BSA, 0.25 mM EDTA, and 5 mM MgCl2, MnCl2, or NiCl2, pH 7.5. Reactions were performed at 37 °C using a RQF instrument and quenched with 100 mM EDTA.