Crystalline and water soluble Cr(4+) and Cr(5+) model compounds for chromium toxicity studies

DNA mismatch repair protein Mlh1 is required for tetravalent chromium intermediate-induced DNA damage

Hexavalent chromium (Cr[VI]) is associated with occupational lung cancer and poses a significant public health concern. When exposed to Cr[VI], cells rapidly internalize this compound and metabolize it to Cr[III]. Byproducts of Cr[VI] metabolism include unstable Cr[V] and Cr[IV] intermediates that are believed to be directly responsible for the genotoxicity and carcinogenicity caused by Cr[VI] exposure; however, the carcinogenic potential of the Cr intermediates and the mechanisms of Cr-induced carcinogenesis remain to be further defined. Utilizing synthetic Cr[IV] and Cr[V] compounds, we demonstrate here that Cr[IV] or Cr[V] exposure induces DNA double-strand breaks; however, of the two compounds, mammalian cells only respond to Cr[V]-induced DNA damage. Exposure to Cr[V], but not Cr[IV], results in initiation of cell cycle checkpoints and activates the ATM kinase, a critical regulator of the DNA damage response. Furthermore, cells exposed to Cr[IV] have significantly increased mutation frequencies in the HPRT gene compared to cells exposed to Cr[V], indicating that Cr[IV] possesses a higher mutagenic potential than Cr[V]. We also find that MLH1, a critical mismatch repair (MMR) protein, is required for activation of the G2/M cell cycle checkpoint in response to Cr[VI] exposure and to limit Cr-induced mutagenesis. Our results provide evidence for Cr[IV] as the ultimate mutagenic intermediate produced during Cr[VI] metabolism and indicate that functional MMR is crucial in the cellular response to chromium exposure.

Hexavalent chromium induces reactive oxygen species and impairs the antioxidant power of human erythrocytes and lymphocytes: Decreased metal reducing and free radical quenching ability of the cells

The toxicity of hexavalent chromium [Cr(VI)] in biological systems is thought to be closely associated with the generation of free radicals and reactive oxygen species. These species are produced when Cr(VI) is reduced to its trivalent form in the cell. This process results in oxidative stress due to an imbalance between the detoxifying ability of the cell and the production of free radicals. We have studied the effect of potassium dichromate (K2Cr2O7), a [Cr(VI)] compound, on the antioxidant power of human erythrocytes and lymphocytes under in vitro conditions. Incubation of erythrocytes and lymphocytes with different concentrations of K2Cr2O7 resulted in a marked dose-dependent decrease in reduced glutathione and an increase in oxidized glutathione and reactive oxygen species levels. The antioxidant power of the cells was decreased, as determined by metal reducing and free radical quenching assays. These results show that [Cr(VI)] upregulates the generation of reactive oxygen species and, as a consequence, the cellular antioxidant defences are compromised. The resulting oxidative stress may contribute to Cr(VI)-induced cellular damage.

Review of chromium (VI) apoptosis, cell-cycle-arrest, and carcinogenesis

Hexavalent chromium combines with glutathione in chloride intracellular channel carrier to form tetravalent and pentavalent chromium in plasma and organelle membranes. It also combines with NADH/NADPH to form pentavalent chromium in mitochondria. Tetravalent- and pentavalent- chromium (directly and indirectly) mediated DNA double strand breaks activate DNA damage signaling sensors: DNA-dependent-protein-kinase signals p53-dependent intrinsic mitochondrial apoptosis, and ataxia-telangiectasia-mutated and ataxia-telangiectasia-Rad3-related signal cell-arrest for DNA repair. Tetravalent chromium may be the most potent species since it causes DNA breaks and somatic recombination, but not apoptosis. Upon further failure of apoptosis and senescence/DNA-repair, damaged cells may become immortal with loss-of-heterozygosity and genetic plasticity.

Catalytic aerobic oxidation by a trianionic pincer Cr(III)/Cr(V) couple

Aerobic oxidation that incorporates both O atoms into a substrate (PPh(3)) is achieved by employing a Cr(III)/Cr(V)[triple bond]O catalytic couple. A terphenyl trianionic pincer ligand stabilizes a high oxidation state Cr(V)[triple bond]O complex, and both the reduced (Cr(III), IR/X-ray) and oxidized (Cr(V)[triple bond]O, electron paramagnetic resonance/IR/X-ray) participants in the catalytic cycle are characterized.