Binding Chromium(III) to Form Mixed Cr(III),Fe(III) Serum Transferrins.

Transferrin, Tf, the protein that transports iron as Fe(III) from the blood to the tissues via endocytosis, is believed to also transport Cr(III). Under physiological conditions, Tf binds and releases Cr(III) rapidly from Cr(III)2-Tf; however, the major form of Tf in the bloodstream is mono-ferric Tf (Fe(III)-Tf). Given the low concentration of Cr(III) in the bloodstream, the form of Cr(III)-containing Tf that is transported is probably monochromic, monoferric-Tf (Cr(III),Fe(III)-Tf). Given that Tf has two specific metal-binding sites, one in both its C-terminal and its N-terminal lobe, two forms of Cr(III),Fe(III)-Tf can form. The binding of Cr(III) to mono-ferric Tf to generate both forms of Cr(III),Fe(III)-Tf has been examined in detail for the first time. The addition of Cr(III) to monoferric Tfs in 100 mM HEPES and 25 mM bicarbonate solution, pH 7.4, resulted in a rapid binding of Cr(III) to the open metal-binding site of the Tfs. Titrations of the monoferric Tfs with Cr(III) indicate the tight binding on one Cr(III) in each case. The binding of Cr(III) to monoferric Tfs is accompanied by conformational changes similar to adding two equivalents of Cr(III) to apoTf. Thus, mono-ferric Tfs bind one equivalent of Cr(III) rapidly and tightly to form mixed Cr(III),Fe(III)-Tfs. Cr(III) is probably transported as mixed Cr(III),Fe(III)-Tfs.

Examining the Potential Formation of Ternary DNA Complexes with Chromium­Cysteine, Chromium-Ascorbate, and Chromium-Glutathione and Implications for Their Carcinogenicity.

The mutagenic and carcinogenic properties of chromium(VI) complexes have been ascribed to the formation of ternary Cr(III)-small molecule-DNA complexes. As part of these laboratories' efforts to establish the structure and properties of discrete binary and ternary adducts of Cr(III) and DNA at a molecular level, the properties of Cr(III)-cysteine-DNA, Cr(III)-ascorbate-DNA, and Cr(III)-glutathione-DNA complexes formed from Cr(III) were examined. These studies determined the composition of previously described "pre-reacted" chromium cysteinate and chromium glutathione. Neither of these complexes nor "chromium ascorbate" form ternary complexes with DNA as previously proposed. In fact, these Cr(III) compounds do not measurably bind to DNA and cannot be responsible for the mutagenic and carcinogenic properties ascribed to ternary Cr(III)-cysteine-DNA and Cr(III)-ascorbate-DNA adducts. The results of biological studies where "ternary adducts" of Cr(III), cysteine, glutathione, or ascorbate and DNA were made from "pre-reacted" chromium cysteinate or chromium glutathione or from "chromium ascorbate" must, therefore, be interpreted with caution.