Effects of chromium(VI) and chromium(III) on energy charge and oxygen consumption in rat thymocytes

Acute toxicity of hexavalent chromium in isolated teleost hepatocytes

Acute toxic effects of hexavalent chromium [Cr(VI)], a widely recognised carcinogenic, mutagenic and redox active metal, were investigated in isolated hepatocytes of goldfish (Carassius auratus). Exposure to 250 microM Cr(VI) induced a significant decrease of cell viability from 94% in controls to 88% and 84% after 30 min and 4 h of exposure, respectively. Cr-toxicity was associated with a concentration-dependent stimulation of the formation of reactive oxygen species (ROS). As one potential source of ROS formation we identified the lysosomal Fe(2+) pool, since the ferric ion chelator deferoxamin inhibited ROS formation by approximately 15%. Lysosomal membranes remained nevertheless intact during Cr-exposure, as determined from neutral red retention in this compartment. Another significant source of ROS appear to be the mitochondria, where a presumably uncoupled increase of respiration by 20-30% was triggered by the metal. Inhibition of mitochondrial respiration by cyanide caused an approximately 40% decrease of Cr-induced ROS-formation, whereas the uncoupling agent carbonyl cyanide m-chlorophenyl hydrazine was without effect. Cellular Ca(2+) homeostasis was not disturbed by Cr(VI) and thus played no role in this scenario. Overall, our data show that Cr(VI) is acutely toxic to goldfish hepatocytes, and its toxicity is associated with the induction of radical stress, presumably involving lysosomes and mitochondria as important sources of ROS formation.

Chromium(VI) interaction with plant and animal mitochondrial bioenergetics: a comparative study

The mechanism of Cr(VI)-induced toxicity in plants and animals has been assessed for mitochondrial bioenergetics and membrane damage in turnip root and rat liver mitochondria. By using succinate as the respiratory substrate, ADP/O and respiratory control ratio (RCR) were depressed as a function of Cr(VI) concentration. State 3 and uncoupled respiration were also depressed by Cr(VI). Rat mitochondria revealed a higher sensitivity to Cr(VI), as compared to turnip mitochondria. Rat mitochondrial state 4 respiration rate triplicated in contrast to negligible stimulation of turnip state 4 respiration. Chromium(VI) inhibited the activity of the NADH-ubiquinone oxidoreductase (complex I) from rat liver mitochondria and succinate-dehydrogenases (complex II) from plant and animal mitochondria. In rat liver mitochondria, complex I was more sensitive to Cr(VI) than complex II. The activity of cytochrome c oxidase (complex IV) was not sensitive to Cr(VI). Unique for plant mitochondria, exogenous NADH uncoupled respiration was unaffected by Cr(VI), indicating that the NADH dehydrogenase of the outer leaflet of the plant inner membrane, in addition to complexes III and IV, were insensitive to Cr(VI). The ATPase activity (complex V) was stimulated in rat liver mitochondria, but inhibited in turnip root mitochondria. In both, turnip and rat mitochondria, Cr(VI) depressed mitochondrial succinate-dependent transmembrane potential (Deltapsi) and phosphorylation efficiency, but it neither affected mitochondrial membrane permeabilization to protons (H+) nor induced membrane lipid peroxidation. However, Cr(VI) induced mitochondrial membrane permeabilization to K+, an effect that was more pronounced in turnip root than in rat liver mitochondria. In conclusion, Cr(VI)-induced perturbations of mitochondrial bioenergetics compromises energy-dependent biochemical processes and, therefore, may contribute to the basal mechanism underlying its toxic effects in plant and animal cells.

Mechanisms of chromium carcinogenicity and toxicity

Chromium, like many transition metal elements, is essential to life at low concentrations yet toxic to many systems at higher concentrations. In addition to the overt symptoms of acute chromium toxicity, delayed manifestations of chromium exposure become apparent by subsequent increases in the incidence of various human cancers. Chromium is widely used in numerous industrial processes, and as a result is a contaminant of many environmental systems. Chromium, in its myriad chemical forms and oxidation states, has been well studied in terms of its general chemistry and its interactions with biological molecules. However, the precise mechanisms by which chromium is both an essential metal and a carcinogen are not yet fully clear. The following review does not seek to embellish upon the proposed mechanisms of the toxic and carcinogenic actions of chromium, but rather provides a comprehensive review of these theories. The chemical nature of chromium compounds and how these properties impact upon the interactions of chromium with cellular and genetic targets, including animal and human hosts, are discussed.

Mechanisms of chromium toxicity in mitochondria

The oxygen consumption of isolated rat heart mitochondria was potently depressed in presence of 10-50 microM Na2CrO4 when NAD-linked substrates were oxidized. The succinate stimulated respiration and the oxidation of exogeneous NADH in sonicated mitochondria were not affected by chromate at this concentration range. A rapid and persistent drop (40% in 2 min) in the mitochondrial NADH level was observed after chromate addition (30 microM) under conditions which generally should promote regeneration of NADH. Experiments with bis-(2-ethyl-2-hydroxybutyrato)oxochromate(V) and vanadyl induced reduction of Cr(VI) in presence of excess NADH were performed. These experiments indicated that NADH may be directly oxidized by Cr(V) at physiological pH. The activity of 10 different enzymes were measured after lysis of intact mitochondria pretreated with chromate (1-100 microM). Na2CrO4 at a very low level (3-5 microM) was sufficient for 50% inhibition of alpha-ketoglutarate dehydrogenase. Higher concentrations (20-70 microM) was necessary for similar effect on beta-hydroxybutyrate and pyruvate dehydrogenase. The other enzymes tested were unaffected. Thus, the chromate toxicity in mitochondria may be due to NADH depletion as a result of direct oxidation by Cr(V) as well as reduced formation of NADH due to specific enzyme inhibition.

Microsomal metabolism of hexavalent chromium. Inhibitory effect of oxygen and involvement of cytochrome P-450

The reduction of hexavalent chromium (Cr(VI] by rat liver microsomes was studied. With 15-120 microM Na2CrO4 microsomes (0.5 mg protein/ml) effectively reduced Cr(VI) in the presence of NADPH provided anaerobic conditions. Phenobarbital (PB) and Aroclor 1254 (PCB) pretreatment increased microsomal Cr(VI) reduction while CoCl2 reduced the rate. The rates with 30 microM Na2CrO4 were: 6.4 +/- 0.1, 7.8 +/- 0.7, 13.4 +/- 0.5, 2.95 +/- 0.09 nmol Cr.mg prot.-1 min-1 for control, PB, PCB and cobalt pretreated microsomes respectively. Kinetic studies gave a Michaeli-Menten like first-order kinetics with increases both in Km and Vmax values after pretreatment with PB or PCB. CO partly inhibited the microsomal Cr(VI) reduction. The CO-sensitive reduction rate was directly correlated to the cyt. P-450 content of the different microsomal preparations. Substituting NADH for NADPH gave approximately 27% lower activity with 30 microM Na2CrO4. This activity was neither inducible by cyt. P-450 inducers nor influenced by CO. Oxygen 1.0% and 0.10% gave approximately 100% and 30% inhibition of Cr(VI) reduction (30 microM Na2CrO4) respectively, and an uncompetitive like inhibitory pattern was found. No redox cycling of Cr(VI) was seen. 51Cr binding to the microsomes was approximately 10% after complete reduction of 30 microM Na2CrO4. Externally added FMN, Fe3+-ADP and nitrobenzen stimulated microsomal Cr(VI) reduction. A 60% higher reduction rate of Cr(VI) by isolated hepatocytes was found during anaerobic in comparison with aerobic conditions.

Effects of hexavalent chromium on trout mitochondria

Trout liver mitochondria were incubated in the presence of micromolar concentration of potassium dichromate (Cr(VI)) under several experimental conditions. Cr(VI) strongly inhibited both state 3 and state 4 of respiration supplemented by NAD-linked substrates; it also slightly affected the respiration of FAD-linked substrates. Evidence is provided that the respiratory inhibition induced by dichromate is partially coupled to the Cr(VI) reduction mechanism occurring in mitochondria.

Uptake of chromate by rat thymocytes and role of glutathione in its cytoplasmic reduction

1. Chromate is taken up by rat thymocytes over a wide range of extracellular chromate concentrations (0.074 +/- 5.0 mM). 2. Chromate uptake followed Michaelis-Menten kinetics and was inhibited by 4-acetamido-4'-isothiocyano-stilbene-2,2'-disulphonic acid, suggesting that the anion carrier was responsible for the uptake by rat thymocytes. 3. The decrease in chromate uptake by the thiol(SH)-blocking agent diethyl maleate, the increased chromate uptake caused by the thiol-protecting agent dithiothreitol, and the reduction of glutathione/glutathione disulphide ratio indicated that the maintenance of the integrity of SH-groups greatly influenced the uptake and reduction of Cr(VI), and that glutathione may be responsible for the intracellular reduction of Cr(VI) to Cr(III).

Intracellular chromium reduction

Two steps are involved in the uptake of Cr(VI): (1) the diffusion of the anion CrO4(2-) through a facilitated transport system, presumably the non-specific anion carrier and (2) the intracellular reduction of Cr(VI) to Cr(III). The intracellular reduction of Cr(VI), keeping the cytoplasmic concentration of Cr(VI) low, facilitates accumulation of chromate from extracellular medium into the cell. In the present paper, a direct demonstration of intracellular chromium reduction is provided by means of electron paramagnetic (spin) resonance (EPR) spectroscopy. Incubation of metabolically active rat thymocytes with chromate originates a signal which can be attributed to a paramagnetic species of chromium, Cr(V) or Cr(III). The EPR signal is originated by intracellular reduction of chromium since: (1) it is observed only when cells are incubated with chromate, (2) it is present even after extensive washings of the cells in a chromium-free medium; (3) it is abolished when cells are incubated with drugs able to reduce the glutathione pool, i.e., diethylmaleate or phorone; and (4) it is abolished when cells are incubated in the presence of a specific inhibitor of the anion carrier, 4-acetamido-4'-isothiocyanatostilbene-2-2'-disulfonic acid.