Catalytic inhibition of topo II and abrogation of etoposide-stabilized, enzyme–DNA complexes by hydroquinone,

p-benzoquinone, and catechol. The pBR322 plasmid DNA (300 ng) was combined with topo II (6 U) following a 10-minute incubation with compound at indicated concentrations in the absence or presence of 100 μM etoposide. The dose-dependent loss of the relaxed band is indicative of inhibition of overall topo II catalytic activity. Although etoposide alone stabilizes enzyme-linked DNA complexes (indicated by the linear band), coincubation with increasing hydroquinone (A) and p-benzoquinone (B) concentrations antagonizes this effect in a dose-dependent fashion, whereas catechol (C) does not.