DNA Interactions and Bacterial Mutagenicity of Some Chromium(III) Imine Complexes and their Chromium(V) Analogues. Evidence for Chromium(V) Intermediates in the Genotoxicity of Chromium(III)

Abstract

The in vitro DNA interactions and bacterial mutagenicities of cis-[Cr^III(phen)₂(OH₂)₂]³⁺ and trans-[Cr^III(salen)(OH₂)₂]⁺ and their Cr^V analogues are reported. At pH 3.3, cis-[Cr(phen)₂(OH₂)₂]³⁺ (0.02–2.0 mM) causes negatively supercoiled pUC9 DNA to smear on agarose gels, with substantial precipitation in the well at ≥1.0 mM. Much weaker interactions between Cr^III and DNA were apparent at pH 7.4. The interactions between DNA and Cr^V phen complexes (0.5 mM total Cr, pH 3.3) generated by oxidation of cis-[Cr(phen)₂(OH₂)₂]³⁺ (for 10–30 min) resulted in almost complete nicking of form I DNA to forms II and III DNA. Nicking of form I DNA (≥80%) was also apparent at pH 7.4 following reaction of DNA with PbO₂-oxidized [Cr(phen)₂(OH₂)₂]³⁺ (2 mM Cr). Interactions between trans-[Cr^III(salen)(OH₂)₂]⁺ and DNA were weaker than those of the Cr^III phen complex at both pH 3.3 and 7.4. The Cr^V salen derivative (0.5 mM total Cr) caused the disappearance of form I DNA at oxidation times of ≥10 min and at pH 3.3 with substantial cleavage. While oxidation of [Cr(salen)(OH₂)₂]⁺ by PbO₂ was not observed at pH 7.4, the complex was oxidized by iodosobenzene to produce short-lived [CrO(salen)]⁺ that caused DNA smearing on the agarose gel. In bacterial mutagenicity assays, the Cr^III imine complexes and their Cr^V analogues produced similar mutagenic responses, which were believed to be due to the instabilities of the Cr^V species in the bacterial growth medium. While the spectrum of the mutagenic activities differed between the chromium phen and salen complexes, both exhibited greatest mutagenicity in Salmonella typhimurium TA102. These data suggest that Cr^V species, generated in vivo by cellular oxidative enzymes, may be responsible for Cr^III-induced mutagenesis.