Nuclear magnetic resonance studies of chromium(III) pyridinecarboxylate complexes

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

The need for combined inorganic, biochemical, and nutritional studies of chromium(III)

The history of biochemical and nutritional studies of the element is unfortunately full of twists and turns, most leading to dead ends. Chromium (Cr), as the trivalent ion, has been proposed to be an essential element, a body mass and muscle development agent, and, in the form of the most popular Cr-containing nutritional supplement, to be toxic when given orally to mammals. None of these proposals, despite significant attention in the popular media, has proven to be correct. Trivalent chromium has also been proposed as a therapeutic agent to increase insulin sensitivity and affect lipid metabolism, although a molecular mechanism for such actions has not been elucidated. Greater cooperative research interactions between nutritionists, biochemists, and chemists might have avoided the earlier issues in nutritional and biochemical Cr research and is necessary to establish the potential role of Cr as a therapeutic agent at a molecular level.

Chromium: is it essential, pharmacologically relevant, or toxic?

Over fifty years ago, the element chromium (as the trivalent ion) was proposed to be an essential element for mammals with a role in maintaining proper carbohydrate and lipid metabolism. Evidence for an essential role came from dietary studies with rodents, studies on the effects of chromium on subjects on total parenteral nutrition, and studies of the absorption and transport of chromium. Over the next several decades, chromium-containing nutritional supplements became so popular for weight loss and muscle development that sales were second only to calcium among mineral supplements. However, the failure to identify the responsible biomolecules(s) that bind chromium(III) and their mode of action, particularly a postulated species named glucose tolerance factor or GTF, resulted in the status of chromium being questioned in recent years, such that the question of its being essential needs to be formally readdressed. At the same time as chromium(III)'s popularity as a nutritional supplement was growing, concerns over its safety appeared. While chromium has been conclusively shown not to have beneficial effects on body mass or composition and should be removed from the list of essential trace elements, chromium(III) compounds are generally nontoxic and have beneficial pharmacological effects in rodents models of insulin insensitivity, although human studies have not conclusively shown any beneficial effects. Mechanisms have been proposed for these pharmacological effects, but all suffer from a lack of consistent supporting evidence.

Mass spectrometric and spectroscopic studies of the nutritional supplement chromium(III) nicotinate

Despite chromium nicotinate's popular use as a chromium nutritional supplement, the structure and composition of chromium nicotinate have only been poorly described. As solid chromium nicotinate is intractable, being insoluble or unstable in common solvents, studies on the solid have been limited, and studies of the solution from which the "compound" precipitates have additionally provided little additional data. The results of mass spectrometric and spectroscopic investigations designed to further elucidate the structure and composition of chromium nicotinate are described. The results demonstrated that the three common methods for producing "chromium nicotinate" all yield different compounds, all of which are polymers of Cr(III), oxygen-bound nicotinate, hydroxide, and water. Implications for interpreting results of nutritional studies of "chromium nicotinate" are discussed.

Chemical properties and toxicity of chromium(III) nutritional supplements

The status of Cr(III) as an essential micronutrient for humans is currently under question. No functional Cr(III)-containing biomolecules have been definitively described as yet, and accumulated experience in the use of Cr(III) nutritional supplements (such as [Cr(pic) 3], where pic = 2-pyridinecarboxylato) has shown no measurable benefits for nondiabetic people. Although the use of large doses of Cr(III) supplements may lead to improvements in glucose metabolism for type 2 diabetics, there is a growing concern over the possible genotoxicity of these compounds, particularly of [Cr(pic) 3]. The current perspective discusses chemical transformations of Cr(III) nutritional supplements in biological media, with implications for both beneficial and toxic actions of Cr(III) complexes, which are likely to arise from the same biochemical mechanisms, dependent on concentrations of the reactive species. These species include: (i) partial hydrolysis products of Cr(III) nutritional supplements, which are capable of binding to biological macromolecules and altering their functions; and (ii) highly reactive Cr(VI/V/IV) species and organic radicals, formed in reactions of Cr(III) with biological oxidants. Low concentrations of these species are likely to cause alterations in cell signaling (including enhancement of insulin signaling) through interactions with the active centers of regulatory enzymes in the cell membrane or in the cytoplasm, while higher concentrations are likely to produce genotoxic DNA lesions in the cell nucleus. These data suggest that the potential for genotoxic side-effects of Cr(III) complexes may outweigh their possible benefits as insulin enhancers, and that recommendations for their use as either nutritional supplements or antidiabetic drugs need to be reconsidered in light of these recent findings.

Nutritional supplement chromium picolinate generates chromosomal aberrations and impedes progeny development in Drosophila melanogaster

Chromium picolinate, [Cr(pic)(3)], is a popular nutritional supplement found in a variety of consumer products. Despite its popularity, safety concerns over its use have arisen. The supplement has been shown to generate clastogenic damage, mitochondrial damage, oxidative damage, and mutagenic effects in cultured cells and oxidative DNA damage and lipid peroxidation in rats. Recently [Cr(pic)(3)] has been demonstrated to generate heritable genetic change and delays in progeny development in Drosophila melanogaster. Based on the damage to chromosomes of cultured cells and of animal models, similar chromosome damage appeared to be a likely source of the mutagenic effects of the supplement in Drosophila. The current three-part study examines the effects of several chromium-containing supplements and their components on hatching and eclosion rates and success of development of first generation progeny of adult Drosophila fed food containing these compounds. It further examines the effects of the compounds on longevity of virgin male and female adults. Finally, the chromosomes in the salivary glands of Drosophila late in the third instar larval stage, which were the progeny of Drosophila whose diets were supplemented with nutritional levels of [Cr(pic)(3)], are shown to contain on average over one chromosomal aberration per two identifiable chromosomal arms. No aberrations were observed in chromosomes of progeny of untreated flies. The results suggest that human consumption of the supplement should be a matter of concern and continued investigation to provide insight into the requirements of chromium-containing supplements to give rise to genotoxic effects.

Lack of mutagenicity of chromium picolinate in the hypoxanthine phosphoribosyltransferase gene mutation assay in Chinese hamster ovary cells

Chromium picolinate (CrPic, Chromax) is a dietary supplement that is stable and more bioavailable than other commercially available forms of chromium. Chromium supplementation is known to enhance the action of insulin, particularly in insulin resistance and type 2 diabetes mellitus. A previous study reported that CrPic produced increases in mutations of the hypoxanthine phosphoribosyltransferase (Hprt) gene in Chinese hamster ovary (CHO) cell mutation tests. This study, however, evaluated CrPic produced by the testing laboratory and used an atypical 48 h exposure period for this test system. The current study evaluated the mutagenic potential of the most widely utilized commercial form of CrPic in CHO/Hprt mutation tests following International Conference on Harmonisation (ICH) Guidelines (+/-S9 metabolic activation with a 5h exposure) in addition to repeating the test with a 48 h exposure period -S9 activation. CrPic was suspended in dimethyl sulfoxide (DMSO) up to a concentration of 50 mg/mL; exposures were conducted under conditions in which precipitate was not evident and under conditions in which some precipitate of CrPic was visually evident in the cell culture medium at the highest concentrations (500 microg/mL). The concentrations evaluated for mutagenicity ranged from 15.6 to 500 microg/mL (+S9 and -S9) for the 5 h exposure and 31.3-500 microg/mL for the 48 h exposure (-S9). Only a slight degree of cytotoxicity was seen in the standard tests up to the limit of solubility in the medium. Toxicity, i.e., cloning efficiency < or =50% of the solvent control, but no mutagenic increases were observed at 500 microg/mL following a 48 h exposure period. The results of these studies showed that CrPic was non-mutagenic in two independent CHO/Hprt assays and in an assay using a 48 h exposure period.

A validated method for analysis of chromium picolinate in nutraceuticals by reversed phase high performance liquid chromatography

A validated high performance liquid chromatographic method was developed for the determination of chromium picolinate in pharmaceutical dosage forms. The analysis was performed at room temperature using a reversed-phase Supelcosil LC-18 (250 x 4.6 mm, 5 microm) column. The mobile phase consisted of acetonitrile:water (40:60 v/v) at a fl ow rate of 0.8 mL/min. The UV-detector was set at 264 nm. The developed method showed a good linear relationship in the concentration range from 0.125 to 12.5 microg/mL with a correlation coefficient from 0.999. The limit of detection and limit of quanti fi cation were 0.091 and 0.181 microg/mL, respectively.

Nutritional supplement chromium picolinate causes sterility and lethal mutations in Drosophila melanogaster

The nutritional dietary supplement chromium picolinate, [Cr(pic)(3)], has gained much notoriety as a safe supplement that supposedly promotes fat loss and muscle enhancement in humans. Thus, a significant industry has materialized around the incorporation of [Cr(pic)(3)] in many sports foods and drinks and a variety of weight loss products. However, in vitro studies have suggested that low levels of [Cr(pic)(3)] in the presence of biological reducing agents can catalytically generate reactive oxygen species, and recent in vivo studies have detected oxidative damage in rats receiving the supplement. The potential deleterious in vivo effects of this activity were examined by using Drosophila melanogaster. [Cr(pic)(3)], but not CrCl(3), at levels of 260 microg Crkg food or less were found to lower the success rate of pupation and eclosion and to arrest development of pupae in a concentration dependent fashion. X-linked lethal analysis indicates that the supplement greatly enhances the rate of appearance of lethal mutations and dominant female sterility.

Structure and reactivity of a chromium(v) glutathione complex

Chromium(V) glutathione complexes are among the likely reactive intermediates in Cr(VI)-induced genotoxicity and carcinogenicity. The first definitive structure of one such complex, [Cr(V)O(LH(2))(2)](3)(-) (I; LH(5) = glutathione = GSH), isolated from the reaction of Cr(VI) with excess GSH at pH 7.0 (O'Brien, P.; Pratt, J.; Swanson, F. J.; Thornton, P.; Wang, G. Inorg. Chim. Acta 1990, 169, 265-269), has been determined by a combination of electrospray mass spectrometry (ESMS), X-ray absorption spectroscopy (XAS), EPR spectroscopy, and analytical techniques. In addition, Cr(V) complexes of GSH ethyl ester (gamma-Glu-Cys-GlyOEt) have been isolated and characterized by ESMS, and Cr(III) products of the Cr(VI) + GSH reaction have been isolated and characterized by ESMS and XAS. The thiolato and amido groups of the Cys residue in GSH are responsible for the Cr(V) binding in I. The Cr-ligand bond lengths, determined from multiple-scattering XAFS analysis, are as follows: 1.61 A for the oxo donor; 1.99 A for the amido donors; and 2.31 A for the thiolato donors. A significant electron withdrawal from the thiolato groups to Cr(V) in I was evident from the XANES spectra. Rapid decomposition of I in aqueous solutions (pH = 1-13) occurs predominantly by ligand oxidation with the formation of Cr(III) complexes of GSH and GSSG. Maximal half-lives of the Cr(V) species (40-50 s at [Cr] = 1.0 mM and 25 degrees C) are observed at pH 7.5-8.0. The experimental data are in conflict with a recent communication (Gaggelli, E.; Berti, F.; Gaggelli, N.; Maccotta, A.; Valensin, G. J. Am. Chem. Soc. 2001, 123, 8858-8859) on the formation of a Cr(V) dimer as a major product of the Cr(VI) + GSH reaction, which may have resulted from misinterpretation of the ESMS and NMR spectroscopic data.

In vivo distribution of chromium from chromium picolinate in rats and implications for the safety of the dietary supplement

Chromium picolinate, [Cr(pic)(3)], is the second most popular nutritional supplement after calcium supplements. However, the supplement, unlike simple inorganic Cr(III) salts, has been shown in the presence of biological reducing agents in vitro to catalytically generate appreciable quantities of hydroxyl radicals, resulting in DNA damage. The complex has also been shown to be remarkably stable in vitro at neutral, basic, or weakly acidic pHs. Thus, the significance of this ability to generate hydroxyl radicals depends on whether the complex is absorbed by cells intact along with the stability and concentration of the complex in cells. Consequently, male Sprague Dawley rats have been injected with (51)Cr- and (3)H-labeled [Cr(pic)(3)]. The tissue distribution, urinary and fecal loss, and subcellular hepatocyte distribution and concentration of the labels suggest that [Cr(pic)(3)] has a lifetime of less than 1 day in vivo, minimizing the potential threat from the supplement itself.

Elucidating a biological role for chromium at a molecular level

Chromium is an essential trace element for mammals and is required for maintenance of proper carbohydrate and lipid metabolism. However, elucidating its function at a molecular level has proved to be problematic. Recent research has revealed that the chromium-binding oligopeptide chromodulin may play a unique role in the autoamplification of insulin signaling. Attempts to develop chromium-containing nutritional supplements and therapeutics are described.

The nutritional supplement chromium(III) tris(picolinate) cleaves DNA

Chromium(III) tris(picolinate) [Cr(pic)3] is currently a very popular nutritional supplement; however, its safety has recently been questioned, especially with regard to its ability to act as a clastogen. At physiologically relevant concentrations, Cr(pic)3 is reduced by biological reductants, including ascorbate and thiols, to Cr(II)-containing species. These species are susceptible to air oxidation, resulting in the catalytic generation of the potent DNA-damaging agent hydroxyl radical. In the absence of reductants, H2O2 can interact with Cr(pic)3 to produce hydroxyl radicals by a second, less efficient mechanism. Cr(pic)3 is extremely stable, which allows the complex to be readily absorbed but also to potentially be incorporated into cells intact. In this form, Cr(pic)3 is primed by its redox potential to enter into the generation of hydroxyl radicals. This study suggests that investigation of the long-term effects of supplementation of the diet with Cr(pic)3 are needed to assess the safety of this material.