Psammaplin A inhibited topo I catalytic activity. Topo I activity was measured by the relaxation of superhelical plasmid DNA. The assay mixture (20 μl) contained pUC118 (20 μg/ml), topo I (1 unit), and various amounts of the psammaplin A. After 30 min at 30°C, the reactions were stopped by the addition of 5 μl of stop solution. The samples were then loaded onto the agarose gel (0.8%) for electrophoresis followed by photography. (A) The reaction mixtures contain 0.5 mM DTT; (B) Compare the reactions in the presence and absence of 0.5 mM DTT.