1.13.11.50 - 1.13.11.50 affinity column chromatography 1.13.11.50 purified, concentrated enzyme is dialyzed twice against 2 mM EDTA in 20 mM Tris/HCl (pH 7.5) to strip off iron, incubation in 2 mM metal ion as sulfate salt plus ascorbate to prevent Fe2+ oxidation, unbound metal ions are removed by 3 cycles of gel filtration using nucleic acid purification column 1.13.11.50 recombinant Strep-tagged wild-type and mutant enzymes from Escherichia coli strain BL21(DE3) by affinity chromatography 1.13.11.50 Sepharose Q fast flow column chromatography Superdex 200 gel filtration, Resource Q column chromatography, Nap-5 column gel filtration, and Phenyl Sepharose HP chromatography