Structural preferences for the binding of chromium nucleotides by beef heart mitochondrial ATPase

What Are the Implications of Cr(III) Serving as an Inhibitor of the Beta Subunit of Mitochondrial ATP Synthase?

A recent report has shown the active site of the beta subunit of mitochondrial ATP synthase is probably the site of action of Cr(III) action, independent of the insulin signaling pathway. This works appears to answer an important question about the mode of action of Cr(III) at a molecular level when supplied in supra-nutritional levels to rodents. However, as with any good research, the research also raises several questions. The relationship between this study and the results of rodent studies of chromium supplementation and between this study and the current understanding the chromium(III) transport and detoxification system are put into perspective.

Mitochondrial ATP synthase as a direct molecular target of chromium(III) to ameliorate hyperglycaemia stress

Chromium(III) is extensively used as a supplement for muscle development and the treatment of diabetes mellitus. However, its mode of action, essentiality, and physiological/pharmacological effects have been a subject of scientific debate for over half a century owing to the failure in identifying the molecular targets of Cr(III). Herein, by integrating fluorescence imaging with a proteomic approach, we visualized the Cr(III) proteome being mainly localized in the mitochondria, and subsequently identified and validated eight Cr(III)-binding proteins, which are predominately associated with ATP synthesis. We show that Cr(III) binds to ATP synthase at its beta subunit via the catalytic residues of Thr213/Glu242 and the nucleotide in the active site. Such a binding suppresses ATP synthase activity, leading to the activation of AMPK, improving glucose metabolism, and rescuing mitochondria from hyperglycaemia-induced fragmentation. The mode of action of Cr(III) in cells also holds true in type II diabetic male mice. Through this study, we resolve the long-standing question of how Cr(III) ameliorates hyperglycaemia stress at the molecular level, opening a new horizon for further exploration of the pharmacological effects of Cr(III).

ATP synthase and the actions of inhibitors utilized to study its roles in human health, disease, and other scientific areas

ATP synthase, a double-motor enzyme, plays various roles in the cell, participating not only in ATP synthesis but in ATP hydrolysis-dependent processes and in the regulation of a proton gradient across some membrane-dependent systems. Recent studies of ATP synthase as a potential molecular target for the treatment of some human diseases have displayed promising results, and this enzyme is now emerging as an attractive molecular target for the development of new therapies for a variety of diseases. Significantly, ATP synthase, because of its complex structure, is inhibited by a number of different inhibitors and provides diverse possibilities in the development of new ATP synthase-directed agents. In this review, we classify over 250 natural and synthetic inhibitors of ATP synthase reported to date and present their inhibitory sites and their known or proposed modes of action. The rich source of ATP synthase inhibitors and their known or purported sites of action presented in this review should provide valuable insights into their applications as potential scaffolds for new therapeutics for human and animal diseases as well as for the discovery of new pesticides and herbicides to help protect the world's food supply. Finally, as ATP synthase is now known to consist of two unique nanomotors involved in making ATP from ADP and P(i), the information provided in this review may greatly assist those investigators entering the emerging field of nanotechnology.

The effects of exchange-inert metal-nucleotide complexes on the kinetics of beef heart mitochondrial F1-ATPase

The inhibition of beef heart mitochondrial F1 by exchange-inert metal-nucleotide complexes was examined. Mono- and bidentate Cr(NH3)4ATP were found to be mixed noncompetitive inhibitors of F1-catalyzed ATP hydrolysis (values of Ki = 0.5 and 0.1 mM; values of alpha = 0.2 and 24, respectively). Rh(H2O)nATP was also found to be a mixed noncompetitive inhibitor of F1-catalyzed ATP hydrolysis (Ki = 0.3 mM, alpha = 0.7). These compounds were used in a series of dual inhibition experiments, along with mono- and bidentate CrATP and Co(NH3)4ATP. All the exchange-inert metal-nucleotides examined were found to be mutually exclusive inhibitors of F1, indicating that they all bind to the same site(s). It is postulated that the pKa of the metal-coordinated ligands is related to the potency of inhibition by these compounds. It appears probable that the exchange-inert nucleotide complexes are binding to site(s) in addition to the catalytic site(s) of F1.

Chromium-associated 32P-labeling of proteins

The incubation of proteins with chromium (Cr3+ or Cr6+) in the presence of 32P ([gamma-32P]ATP or H3(32)PO4) at room temperature for 10-30 min resulted in the labeling of these proteins with 32P. The 32P-labeled proteins could be separated by SDS-polyacrylamide gel electrophoresis and identified by exposure to X-ray film. The characteristics of this procedure included: the optimal chromium concentration was 100 microM; the minimum requirement of each protein was 1 microgram; the optimal pH value was between 6 and 8; metal ions such as V5+, Mn2+ and Fe3+ strongly inhibited the effect of chromium, whereas Ca2+ and Mg2+ had little effect. It was concluded that chromium binds to the proteins and forms a complex with 32P to achieve the 32P-labeling of the proteins. This technique can be applied for the rapid preparation of 32P labels on protein markers for gel electrophoresis and for the identification of unknown protein species.

Synthesis of gamma-monodentate and beta, gamma-bidentate CrITP. Isolation and characterization of the two chelation isomers and the individual diastereomers of beta, gamma-bidentate CrITP

The two chelation isomers of CrITP, gamma-monodentate and beta, gamma-bidentate CrITP, as well as the diastereomers of beta, gamma-bidentate CrITP were synthesized, isolated, and characterized. Synthesis of these complexes was done using pH titration methods similar to that described by Cleland [W.W. Cleland, Methods Enzymol. 87, 159 (1982)], and separation of the two chelation isomers was accomplished with DEAE-sephadex A-25 using 0-0.3 N linear HCl gradient. Diastereomer separation (analytical and preparative scales) of beta, gamma-bidentate CrITP using reverse-phase high-performance liquid chromatography, and then analysis of the diastereomers with circular dichroism spectroscopy, shows four diastereomers that exist as two pairs of mirror-image isomers, similar to the four diastereomers of beta, gamma-bidentate CrATP as presented by Dunaway-Mariano and Cleland [D. Dunaway-Mariano and W.W. Cleland, Biochemistry 19, 1496 (1980)]. Reverse-phase high-performance liquid chromatography analysis of gamma-monodentate CrITP shows the presence of two major peaks, both of which convert to beta, gamma-bidentate CrITP upon incubation at pH 6.0 for 1 hr.

The effect of Co(III)(NH3)4ATP on the kinetics of beef heart mitochondrial ATPase

Bidentate cobalt(III)tetraamine adenosine triphosphate [Co(NH3)4ATP] was investigated as an inhibitor of the beef heart mitochondrial F1-ATPase. The compound was found to have a mixed noncompetitive mechanism with a Ki of 0.4 mM and an alpha of 1.4 during ATP hydrolysis. Co(NH3)4ATP also noncompetitively inhibited ATP hydrolysis in the presence of bicarbonate. ITP hydrolysis was similarly affected. Co(NH3)4ATP was also used in dual inhibitor studies with adenylylimidodiphosphate (AMP-PNP) and azide; it was found to be mutually exclusive with AMP-PNP and azide. The compound also protected the F1 from modification by 4-chloro-7-nitrobenzofurazan. These results are discussed in terms of the regulation of the ATP hydrolysis reaction.

alpha, beta-Bidentate CrADP abolishes the negative cooperativity of yeast mitochondrial F1-ATPase

The hydrolysis of MgATP and MgITP by mitochondrial F1-ATPase from Saccharomyces cerevisiae is competitively inhibited by alpha, beta-CrADP, alpha, beta, gamma-CrATP and beta, gamma-CrATP. The apparent K1 values of the three complexes are in the range of the half-saturating MgATP concentration. The negative cooperativity (nH = 0.7) of MgATP hydrolysis is totally abolished by alpha, beta-CrADP (nH = 1.0), while it is not affected by the CrATP. It is concluded that alpha, beta-CrADP binds exclusively at the regulatory site and that CrATP binds exclusively to the catalytic site.

Inhibitory action of hexavalent chromium (Cr(VI)) on the mitochondrial respiration and a possible coupling to the reduction of Cr(VI)

The reduction of hexavalent chromium (Cr(VI] in isolated liver mitochondria was studied under different redox conditions in the respiratory chain. With 25 microM Na2CrO4 the rates were 1.6 +/- 0.7, 13.9 +/- 0.6 and 12.7 +/- 0.7 nmole Cr(VI) reduced 15 min/mg protein with the electron transport chain oxidized, reduced and only complex 1 reduced respectively. Electrons from succinate, bypassing complex 1, were apparently unavailable for Cr(VI)-reduction. The kinetics of chromate reduction was studied with only complex 1 in a reduced state. A rapid and a slow phase were found, probably corresponding to different electron donors in the mitochondria. Blocking the free thiols with N-ethylmaleimide lead to less than 10% decrease in the rapid initial Cr(VI)-reduction and to about 20% decrease during the whole incubation period (15 min). The amounts of free thiols were moderately decreased (15%) in chromate treated mitochondria during the slow reduction of Cr(VI) only. SH-groups may thus participate as reductants during the slow reduction phase. The respiration rate was inhibited about 50% by 25 microM Na2CrO4 when the mitochondria oxidized NAD-linked substrates. In contrast, succinate stimulated respiration was inhibited 50% by 3.6 mM Na2CrO4. The observed inhibition was Na2CrO4 in the micromolar range was therefore probably localized at complex 1 and may be coupled to the reduction of Cr(VI) at the same place. The respiration of isolated hepatocytes was also affected by Na2CrO4. Five micromolar chromate caused 5-10% inhibition. The inhibitory action of chromate on the mitochondrial respiration may thus constitute an important cytotoxic mechanism.

Effects of potassium dichromate on ATP content of mammalian cells cultured in vitro

In order to elucidate the mechanism of the cytotoxic activity of hexavalent chromium (Cr(VI)), the alterations of intracellular ATP levels induced by potassium dichromate in cultured hamster fibroblasts (BHK line) have been studied. Two kinds of treatment procedures were adopted: (1) BHK cell suspensions were exposed to 0.05--1.00 mM K2Cr2O7 in Hanks' balanced salt solution (BSS) for up to 180 min and ATP concentrations were determined immediately after the exposure to Cr(VI). A decrease of ATP content was observed with 0.25--12.00 mM K2Cr2O7 but only in the case of the highest dose was it related in a linear fashion to the duration of the treatment. (2) Cells were preincubated in BSS for 30 min with 0.05--1.00 mM dichromate. They were then reincubated in Eagle's minimal essential medium (MEM) for up to 180 min and ATP was measured at different time points. Immediately after the exposure to chromium all the treated cultures showed a depletion of ATP content. However while the cells treated with 0.25--0.25 mM dichromate rapidly resumed ATP levels very similar to that of the control, no recovery was detected in cells treated with 0.50 and 1.0 mM K2Cr2O7, even after 180 min. The observed effects have been attributed to the oxidizing activity of Cr(VI), which subtracts electrons from electron donors involved in metabolic pathways producing ATP, and to the ability of Cr(III), deriving from Cr(VI) reduction, to form stable coordination complexes with ATP precursors and enzymes involved in ATP synthesis.