Effect of intercalating and groove-binding ligands on formation of covalent complexes between nitracrine (Ledakrin, C-283) or 8-methoxypsoralen and DNA.

The effect of ethidium bromide, actinomycin D, distamycin A and netropsin on covalent binding of nitracrine (1-nitro-9-(3,3-N,N-dimethylaminopropylamino)acridine, Ledakrin, C-283) and 8-methoxypsoralen to DNA was examined. The competition was assayed either directly with [3H]- and [14C]nitracrine or indirectly by estimation of transcriptional template activity of nitracrine-DNA and 8-methoxypsoralen-DNA complexes formed in the presence of the ligands. A higher protective effect of ethidium bromide and distamycin on the photo-binding of 8-methoxypsoralen than on the dithiothreitol-dependent attachement of nitracrine to DNA assayed at 0.15 M KCl or NaCl was observed. The non-intercalating antibiotics showed lower competitive effect on 8-methoxypsoralen binding than ethidium bromide. Actinomycin D showed relatively low competition for both drugs with DNA. In contrast to the reaction of 8-methoxypsoralen, the decrease of nitracrine binding in the presence of competing ligands considerably depends on ionic strength. Particularly high inhibition of the adduct formation in the presence of ethidium at 1 M KCl was shown, while the amount of nitracrine bound in the presence of distamycin increases at elevated ionic strength. The results may indicate steric demands of the reaction between nitracrine and DNA.