Biophysical analysis of natural, double-helical DNA modified by anticancer heterocyclic complexes of ruthenium(III) in cell-free media.

Modifications of natural DNA by three anticancer heterocyclic ruthenium(III) compounds were studied by methods of molecular biophysics. These methods included DNA binding studies using atomic absorption spectrophotometry, inhibition of restriction endonucleases, mapping of DNA adducts by transcription assay, interstrand cross-linking employing gel electrophoresis under denaturing conditions, DNA unwinding studied by gel electrophoresis, circular dichroism analysis of the B-->Z transition in DNA, and DNA melting curves measured by absorption spectrophotometry. The results indicate that the complexes HIm[trans-Cl4Im2RuIII], HInd[trans-Cl4Ind2RuIII], and Na[trans-Cl4Im(Me2SO)RuIII] (Im and Ind stand for imidazole and indazole, respectively) coordinate irreversibly to DNA. Their DNA binding mode is, however, different from that of cisplatin. Interestingly, Na[trans-Cl4Im(Me2SO)RuIII] binds to DNA considerably faster than the other two ruthenium compounds and cisplatin. In addition, when Na[trans-Cl4Im(Me2SO)RuIII] binds to DNA it exhibits an enhanced base sequence specificity in comparison with the other two ruthenium complexes. Na[trans-Cl4Im(Me2SO)RuIII] also forms bifunctional intrastrand adducts on double-helical DNA which are capable of terminating RNA synthesis in vitro, while the capability of the other two ruthenium compounds to form such adducts is markedly lower. This observation has been interpreted to mean that the bifunctional adducts of HInd[trans-Cl4Ind2RuIII] and Na[trans-Cl4Im2RuIII] formed on rigid double-helical DNA are sterically more crowded by their octahedral geometry than those of Na[trans-Cl4Im(Me2SO)RuIII]. In addition, the adducts of all three ruthenium compounds affect the conformation of DNA, Na[trans-Cl4Im(Me2SO)RuIII] being most effective. It has been suggested that the altered DNA binding mode of ruthenium compounds in comparison with cisplatin might be an important factor responsible for the altered cytostatic activity of this class of ruthenium compounds in tumor cells.