Chromium(III) complexes have been used as nutritional supplements for mammals as well as humans in order to enhance carbohydrate and lipid metabolism. Recently, many studies have been focusing on the benefits of Cr supplements for the prevention and treatment of type 2 diabetes. However, the controversies over the Cr binding to specific biomolecules and the mechanisms of action of Cr(III), and the ambiguous evidence on its anti-diabetic activity, have led to the proposal for withdrawing Cr(III) from the list of essential elements. The possibility of the formation of highly reactive Cr(VI/V/IV) species in the reaction of Cr(III) supplements with biological oxidants in the body in order to exhibit anti-diabetic effects has raised concerns over their potential carcinogenicity and genotoxicity. Because of this controversy, and unclear nutritional status of Cr(III), it is necessary to study the speciation of the Cr-bound biomolecules and to establish biochemical pathways of Cr(III) metabolism.
X-ray absorption spectroscopy (XAS) techniques, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were applied to characterize some Cr(III) nutritional supplements in current use, including: [Cr2(OH)2(nic)4(H2O)2]n (nic = nicotinato (–)), [Cr(his)2]+ (his = L-histidinato(–)), [Cr(phe)3] (phe = L-phenylalaninato(–)), and [Cr(cit)2]3– (cit = citrato(3–)). These techniques have also been used to study their transformations in biological media, including simulated gastrointestinal fluids, blood serum, cell culture medium, mammalian cells, as well as yeast cells.
The results show that [Cr(his)2]+ and especially [Cr(cit)3]3– were more stable in biological media compared to [Cr2(OH)2(nic)4(H2O)2]n and [Cr(phe)3]. The UV-Vis and XANES spectroscopic data suggested that these complexes underwent ligand-exchange reactions with proteins and other low-molecular-weight components in blood serum and cell culture medium, in contrast to the hypothesis of a specific transport process of Cr(III) supplements. While [Cr2(OH)2(nic)4(H2O)2]n and [Cr(phe)3] were decomposed completely in blood serum and tended to form protein cross-links, [Cr(his)2]+ and [Cr(cit)3]3– underwent only partial ligand loss and tended to bind to metal binding sites of proteins. Once the Cr(III) supplements bound to various serum components, these adducts entered the cells via passive diffusion via insulin-independent pathways. The XANES spectra of cells treated with different Cr supplements demonstrated distinct chemical environments of intracellular Cr, and added further evidence for a lack of a specific Cr-containing biomolecule involved in glucose uptake and glucose metabolism, as was postulated previously in the chromodulin hypothesis of Cr activity hypothesis of other groups. The proposed benefits of Cr(III) on metabolism of normal adipocytes or muscle cells were not observed, which is in agreement with many reports of a lack of benefit of Cr(III) on healthy animals or humans. The increase in the level of phosphotyrosine proteins in Cr(III)-treated cells was only observed in the presence of H2O2, a biological oxidant, which supports the hypothesis of anti-diabetic mechanism of Cr(III) caused by its oxidation to Cr(VI). This explains why the positive effects of Cr(III) treatment are only observed in patients with poorly controlled diabetes that is characterised by chronic oxidative stress. Due to the carcinogenicity of Cr(VI), the concerns over the toxicity of using Cr(III) complexes as anti-diabetic drugs, as well as long-term supplements, have been raised.
Uptake and speciation studies of Cr(III) in yeast were also performed since: (i) Cr(III)-treated yeasts are used as nutritional supplements; and (ii) yeasts can be used as model organisms for mammalian Cr-metabolism. The experiments showed that Cr(III) bound extensively to the cell wall of S. cerevisiae, as well as less binding to intracellular targets. This speciation was also dependent on the nature of the supplement.
Advanced techniques of X-ray fluorescence mapping in combination with gel electrophoresis were used to study metal-protein interactions, and preliminary results were obtained. In addition, novel methods of assessing the potential anti-diabetic activity of metal complexes in insulin target cells have been explored, including fatty acid oxidation and glycolytic functions. These techniques could be applied in future research into metal anti-diabetic drugs.