Video uit de film Erin Brockovich over Chroom-VI in ons drinkwater
Registratierichtlijnen van door chroom veroorzaakte beroepsziekten

Onderwerpen (beknopt)

6 juli 2000

Introduction

Chromium (VI), which was added to water cooling towers to prevent corrosion and scaling of the tower walls, was reported to be present in groundwater at levels 1,000-5,000 times the safe limit for drinking and 50,000 times the safe level for inhalation. Not only was the company aware of the potential danger of the chromium (VI), but officials met with residents and distributed flyers advertising the beneficial health effects of chromium and stressing that their water was completely safe for drinking, cooking, and swimming (LawBuzz.com 2000).

According to a speech given by Erin Brockovich in October, 2000, at least 50 of the original 648 plaintiffs have died from causes attributed to chromium (VI) poisoning, and PG&E has been ordered to stop using hexavalent chromium. In recent years much research has been done in order to discern the pathogenesis of cancers and other health problems related to toxicities of certain forms of metals, such as chromium (VI), chromium (V), and iron (III). The results of this research suggest that certain transition metals are capable of initiating a free radical chain reaction that results in the production of reactive oxygen species and reactive carbon species that can attack DNA, leading to cancer (Sugden and Stearns 2000;Toyokuni 1996) .

Chromium (V) and (VI) can be reduced by physiological substrates such as ascorbate (vitamin C), glutathione (gamma-glutamylcysteinylglycine or GSH), and hydrogen peroxide (H2O2) to produce the carboxy radical (CO2-o), the hydroxy radical (OH-o), and glutathione thiyl species (GS-o), respectively, all of which can attack DNA (Sugden and Stearns 2000). The reduction of iron (III) results in the production of OH-o and other reactive oxygen species, which can attack DNA and form DNA adducts such as 8-hydroxyguanine, which has been implicated in carcinogenesis (Toyokuni 1996). While a certain amount of oxidative stress is necessary for normal body function, excessive amounts of DNA oxidation can lead to harmful mutations and strand breakages (Sugden and Stearns 2000).

Metal-Induced Oxidative Stress

Chromium

Chromium occurs naturally in its trivalent state and is an essential nutrient in human and animal diets, with a recommended daily allowance (RDA) of 50-200 mcg/day for humans. It can be found in high levels in organ meats, mushrooms, wheat germ, and broccoli. Chromium (III) is so essential because it is a cofactor for proper insulin function and because it aids in the metabolism of fats, proteins, and energy (Miguel 1999). Oxidation states of (V) and (VI) for chromium are not naturally occuring and result from industrial processes such as the production of stainless and hard alloy steels and pigments (Miguel 1999). This high valence chromium is often found in the form of chromate (CrO42-), hydrogen chromate (HCrO4-) or dichromate (Cr2O72-) (Sugden and Stearns 2000).

While the difference in valences between the forms of chromium is small, the reactivities of these forms vary drastically. Unlike chromium (VI) and chromium (V), chromium (III) is very stable and thus is not harmful to the body, even in amounts greatly exceeding the RDA (Miguel 1999). Another important factor in the genotoxicty of chromium is its location. Chromium (V) or (VI) that crosses a cell membrane into a cell via the sulphate-anion channel can be reduced by one of many substrates to chromium (III). It is this reduction that results in the formation of intermediate valence states of chromium (V and IV) and radicals of oxygen, carbon, or even sulfur that proceed to attack DNA molecules, producing legions and/or strand breakages that interfere with normal transcription (Sugden and Stearns 2000; Wetterhahn et al. 1989).

Chromium that enters a cell as chromium (III), however, is not harmful. The reduction of chromium from its high valence state to its stable state must take place inside the cell for mutagenic products to be formed (Blasiak 2000). The following reaction summarizes the total reduction of chromium VI, in the from of chromate, to chromium III (Sugden and Stearns 2000). CrO42- + 4H20 + 3e- Cr(OH)3 + 5OH-

(1) Chromium can be reduced intracellularly both nonenzymatically by ascorbate, glutathione, hydrogen peroxide and cysteine, and enzymatically by microsomal cytochrome P450 reductase, the endoplasmic reticulum, mitochondria, and heme-containing proteins and flavoproteins (Sugden and Stearns 2000). The focus of most research has been the mechanisms by which chromium is reduced nonenzymatically. Of the small reducing molecules mentioned above, glutathione is present in the highest concentrations intracellularly but acts the most slowly on chromium. In the absence of molecular oxygen (O2), the product of the reaction between glutathione and chromium is the glutathione thiyl species (GS-o), while in the presence of molecular oxygen the main products are the glutathione sulfinyl radical and a Cr(IV)-GSH complex (the proposed composition of this complex is Na4Cr(GSH)2GSSGo8H2O), both of which are capable of attacking DNA to form apurinic/apyrimidinic (AP) sites and strand breakages (Sugden and Stearns 2000; Aiyar et al. 1991).

Although intracellular concentrations of hydrogen peroxide are considerably lower than those for GSH (micrmolar compared to millimolar), some researchers estimate that 1-5% of all oxygen consumed during mitochondrial respiration is converted to H2O2. Therefore reduction of chromium by H2O2 may be a significant source of DNA damage to mitochondrial DNA (Sugden and Stearns 2000). The reaction involves the production of hydroxyl radicals through the Fenton mechanism (2). Cr (VI) + e- Cr (V) + HOOH OH-o + OH- + Cr (IV)

(2) Another possible product of Chromium (VI) reduction is tetraperoxychromate (V), or Cr (V) (O2)43-, which can be converted back to Cr (VI) with the formation of reactive oxygen species such as singlet oxygen (1O2), superoxide anion (O2-o), and the hydroxyl radical (Sugden and Stearns 2000). All of these species have the power to oxidize DNA and produce extensive damage, including the formation of the 8-hydroxydeoxyguanosine adduct and strand breakage (Aiyar et al. 1991).

Ascorbate can react with Cr (VI) to produce a variety of metal and radical species, depending upon the relative concentrations of the two compounds. At high ascorbate:Cr(VI) ratios (greater than 3:1), the predominant species is the ascorbyl radical. Below this point, a mixture of Cr (V), Cr (IV) and carbon-based radicals, including CO2-o are observed. Unlike the cases for GSH and H2O2, molecular oxygen plays no role in the reduction involving ascorbate, but oxygen is required for the acutal attack of DNA. Both the carbon based radicals and the high valence chromium compounds are capable of producing DNA strand breakages and AP sites in equal amounts (Sugden and Stearns 2000).

Reduction of chromium (VI) and (V) produce intermediates that are harmful to DNA in a number of ways. The reactive products of chromium reduction by glutatione, hydrogen peroxide, and ascorbate can result of DNA-adducts such as 8-hydroxydeoxyguanosine that cause misreading of DNA templates (Aiyar et al. 1991). This type of DNA modification is significant because guanine modifications have been shown to be the major source of mutations that lead to p53-linked cancers (Sugden and Stearns 2000). Stable Cr-GSH complexes may crosslink with DNA, producing irreparable lesions. Many of the radials described above have the power to abstract a hydrogen atom from the deoxyribose sugar moiety of DNA through a homolytic bond cleavage, which results in the production of a carbocation intermediate that can react with nucleophiles such as water to form a hydroxylated deoxyribose sugar product (Sugden and Stearns 2000).

Chromium-induced DNA damage can lead to a variety of cancers. Studies have shown that workers exposed to large amounts of chromium have a higher risk for respiratory, dermal, renal, and hepatic cancers. Tumors, nephrotoxicity, and hepatotoxicity have been observed in experimental animals. In bacteria, chromium damage results in mutations, chromosomal aberrations, sister chromatid exchanges, DNA-interstrand crosslinks, and single-strand breaks (Sugiyama 1992). Each of these chromosomal abnormalities may lead to further disease.

Like chromium, iron is a transition metal that is essential in the human and animal diets. Iron is a part of hemoglobin, which transports oxygen in the blood of mammals, catalase, which converts hydrogen peroxide to water and oxygen, and cytochromes, that perform a variety of functions within the body. Iron has even been implicated in helping to advance primitive life forms by producing reducing equivalents in the presence of UV light. Iron also has the capability, however, to produce reactive oxygen species through the Fenton reaction (3). Fe (II) + H2O2 Fe (III) + OH-o + OH-

(3) In order for iron to participate in this kind of reaction, it must be "free," or redox-active and diffusible. Fe (II) is not as stable as Fe (III) at neutral pH, but Fe (III) is not very soluble at neutral pH. This problem is overcome by chelating iron with citrate, ADP, ATP, or GTP. This chelated iron (III) can be reduced to iron (II) by a number of substrates, including ascorbate, glutathione and adult T-cell leukemia-derived factor (ADF). Iron can also be released from lactoferrin, saturated transferrin, ferritin, and hemosiderin by the highly reactive superoxide anion, which is produced in higher amounts during processes such as inflammation. The reactive oxygen species produced by iron can attack DNA and lead to the formation of 8-hydroxydeoxyguanosine, just as with chromium (Toyokuni 1996). Iron (II) can also react with molecular oxygen to aid in the formation of hydrogen peroxide (4). 2Fe (II) + O2 + 2H+ 2Fe(III) + H2O2

(4) Furthermore, iron (III) ions that are bound to DNA can undergo cyclic reduction and oxidation, resulting in the production of free radical species at the site where they can cause significant damage. This is referred to as the site-specific mechanism for iron genotoxicity (Toyokuni 1996). Iron can also work in concert with other metals such as nickel and copper to induce carcinogenesis. In humans and animals, the combination of iron and nickel in high amounts results in renal sarcomas. Rats with copper accumulation in their organs experience higher iron levels in their livers and a greater occurrence of hepatocellular carcinomas. Researchers postulate that these effects may be due to a tumor-enhancing property of iron in combination with these other carcinogenic metals.

Many different forms of cancer can be induced by excess iron stores in the body or by excess dietary intake of iron. Prolonged exposure to unabsorbed dietary iron in the colon can lead to colo-rectal cancer. In addition, high body iron stores have been associated with neuroblastomas, childhood Hodgkin's disease, and acute lymphocytic anemia. Genetic hemochromatosis is an iron overload disorder that makes patients more likely to suffer from hepatocellular carcinoma. Absetosis is a disease caused by excessive exposure to asbestos, which is 30% iron by weight. This disorder significantly increases the risk of lung cancers such as diffuse malignant mesothelioma and bronchogenic carcinoma. In animals, repeated injections of iron-dextran have been shown to cause spindle-cell sarcoma and pleomorphic sarcoma, while intraperitoneal injections of ferric nitrilotriacetate resulted in diffuse mesotheliomas (Toyokuni 1996).

In addition to attack of DNA molecules by reactive oxygen species, both chromium and iron have the ability to induce lipid peroxidation, or the attack of polyunsaturated fatty acids by reactive oxygen species. The lipids normally targeted are the phospholipids of cell membranes. The same reactive oxygen species (hydroxyl radical, superoxide anion, and others) form lipid hydroperoxides and alkoxy radicals, thus initiating a radical chain reaction. Two of the most harmful products of lipid peroxidation are 4-hydroxy-2-nonenal (4-HNE) and malondialdehyde (MDA). These molecules are extremely carcinogenic because of their ability to attack DNA molecules. Therefore metal-induced lipid peroxidation provides an indirect mechanism for carcinogenesis (Toyokuni 1996).

To combat the action of reactive oxygen species and other harmful radicals, the body uses a system of antioxidants that include vitamin E, vitamin C, catalase, glutatione peroxidase and superoxide dismutase. These enzyme or coenzymes are capable of inhibiting oxidative stress or of removing harmful reactive oxygen species to prevent further oxidative damage. As mentioned previously, catalase works by converting hydrogen peroxide to water and oxygen, thus removing one possible reducer for metals such as chromium and iron. Superoxide dismutase is another oxygen species scavenger that removes superoxide radicals from the body, which can reduce the amount of iron that is extracted from proteins. Glutathione peroxidase reduces lipid hyroperoxides and hydroxynonenals to less harmful products such as alcohols, ending the lipid peroxidation chain reaction and preventing further oxidative damage to both membranes and DNA (Sugiyama 1992).

Vitamin E also has anticancer and antioxidant properties and has been shown to prevent chromium (VI)-induced DNA damage (Sugiyama 1992). While vitamin C reduces chromium (VI) to stable and less harmful chromium (III) outside of the cell without harm, this reduction inside the cell results in oxidative damage (Blasiak 2000). Therefore vitamin C fulfills two roles, depending on its location. Mannitol and manganese (II) have both been implicated as chromium scavengers which are able to remove harmful chromium compounds from cells (Sugden and Stearns 2000; Kortenkamp et al. 1996). Mannitol is also a radical quencher, so its role in preventing chromium-induced damage is two-fold (Sugden and Stearns 2000).

Both environmental pollution and dietary intake have been proven to be important in the physiological concentrations of transition metals that can lead to a variety of cancers through oxidative mechanisms. All cancers are a result of mutations, strand breakages, or other chromosomal abberations that result either from direct oxidation of DNA by reactive radical species or metal complexes, or from indirect damage resulting from metal induced lipid peroxidation. This research, in conjunction with the PG&E case emphasizes the importance of monitoring environmental contamination and water supplies, both for humans and for animals.

The other major way to control this kind of disease is to maintain a balance diet, avoiding excessive amounts of iron, and including daily intake of antioxidants such as vitamin E and vitamin C, in addition to minerals like selenium. By becoming more aware of how many cancers are induced, in future years we may be able to greatly reduce their prevalence.