Bioavailability of iron from coal fly ash: mechanisms of mobilization and of biological effects

Particulate air pollution contains iron that may be involved in the pathological effects after inhalation. This article reviews work demonstrating that ambient particulate samples (Standard Reference Material [SRM] 1648 and SRM 1649, from the National Institute of Science and Technology) contain iron that can be mobilized from the particle in vitro and inside human lung epithelial (A549) cells. The mobilized iron can then catalyze the formation of reactive oxygen species (ROS). Work is also reviewed on the generation and size fractionation of coal fly ash (CFA) from three commercially important coal types, as well as size fractionation of three types of noncombustion particles. The availability of iron from these particles to A549 cells was measured by citrate mobilization in vitro and induction of the iron storage protein ferritin in particle-treated cells. The amount of bioavailable iron decreased with increasing particle size. The ability of particles to induce synthesis of the proinflammatory cytokine interleukin-8 (IL-8) was also determined. As with the bioavailability of iron, there was an inverse correlation with size. Further work showed that iron in CFA is responsible for IL-8 induction. Mössbauer spectroscopy of a CFA sample before and after desferrioxamine B treatment to remove bioavailable iron showed that the bioavailable iron was associated with the glassy aluminosilicate fraction of the particle. In conclusion, this work shows that bioavailable iron is responsible for ROS production by SRMs and IL-8 induction by CFA in A549 cells. The source of this bioavailable iron in CFA is glassy aluminosilicates, which are found at higher levels in smaller sizes of CFA.

The formation of reactive oxygen species catalyzed by neutral, aqueous extracts of NIST ambient particulate matter and diesel engine particles

It is important to characterize the chemical properties of particulate matter in order to understand how low doses, inhaled by a susceptible population, might cause human health effects. The formation of reactive oxygen species catalyzed by neutral, aqueous extracts of two ambient particulate samples, National Institute of Standards & Technology (NIST) Standard Reference Materials (SRM) 1648 and 1649, and two diesel particulate samples, NIST SRM 1650 and SRM 2975, were measured. The formation of reactive oxygen species was estimated by measuring the formation of malondialdehyde from 2-deoxyribose in the presence of ascorbic acid; H2O2 was not added to this assay. SRM 1649, ambient particulate matter collected from Washington, DC, generated the most malondialdehyde, while SRM 2975, diesel particulate matter collected from a forklift, yielded the least amount. Desferrioxamine inhibited the formation of malondialdehyde from the particulate samples providing additional data to support the observation that transition metals were involved in the generation of reactive oxygen species. Six transition metal sulfates (iron sulfate, copper sulfate, vanadyl sulfate, cobalt sulfate, nickel sulfate, and zinc sulfate) were assayed for their ability to generate reactive oxygen species under the same conditions used for the particulate samples in order to facilitate comparisons between particles and these transition metals. The concentration of transition metals was measured in aqueous extracts of these particulate samples using ion-coupled plasma mass spectrometry (ICP-MS) analysis. There was qualitative agreement between the concentrations of Fe, Cu, and V and the amount of malondialdehyde produced from extracts of these particulate samples. These data suggest that transition metals can be dissolved from particles in neutral, aqueous solutions and that these metals are capable of catalyzing the formation of reactive oxygen species.

Using in vitro iron deposition on asbestos to model asbestos bodies formed in human lung

Recent studies have shown that iron is an important factor in the chemical activity of asbestos and may play a key role in its biological effects. The most carcinogenic forms of asbestos, crocidolite and amosite, contain up to 27% iron by weight as part of their crystal structure. These minerals can acquire more iron after being inhaled, thereby forming asbestos bodies. Reported here is a method for depositing iron on asbestos fibers in vitro which produced iron deposits of the same form as observed on asbestos bodies removed from human lungs. Crocidolite and amosite were incubated in either FeCl(2) or FeCl(3) solutions for 2 h. To assess the effect of longer-term binding, crocidolite was incubated in FeCl(2) or FeCl(3) and amosite in FeCl(3) for 14 days. The amount of iron bound by the fibers was determined by measuring the amount remaining in the incubation solution using an iron assay with the chelator ferrozine. After iron loading had been carried out, the fibers were also examined for the presence of an increased amount of surface iron using X-ray photoelectron spectroscopy (XPS). XPS analysis showed an increased amount of surface iron on both Fe(II)- and Fe(III)-loaded crocidolite and only on Fe(III)-loaded amosite. In addition, atomic force microscopy revealed that the topography of amosite, incubated in 1 mM FeCl(3) solutions for 2 h, was very rough compared with that of the untreated fibers, further evidence of Fe(III) accumulation on the fiber surfaces. Analysis of long-term Fe(III)-loaded crocidolite and amosite using X-ray diffraction (XRD) suggested that ferrihydrite, a poorly crystallized hydrous ferric iron oxide, had formed. XRD also showed that ferrihydrite was present in amosite-core asbestos bodies taken from human lung. Auger electron spectroscopy (AES) confirmed that Fe and O were the only constituent elements present on the surface of the asbestos bodies, although H cannot be detected by AES and is presumably also present. Taken together for all samples, the data reported here suggest that Fe(II) binding may result from ion exchange, possibly with Na, on the fiber surfaces, whereas Fe(III) binding forms ferrihydrite on the fibers under the conditions used in this study. Therefore, fibers carefully loaded with Fe(III) in vitro may be a particularly appropriate and useful model for the study of chemical characteristics associated with asbestos bodies and their potential for interactions in a biosystem.

Mobilization of iron from coal fly ash was dependent upon the particle size and source of coal: analysis of rates and mechanisms

The observed iron mobilization rate from size-fractionated coal fly ash is consistent with the model predictions for a limiting case of mass transfer where the dominant resistance is diffusion through a layer of depleted solid between the surface of spherical particles and a shrinking core of unreacted material. The rate of mobilization of iron from coal fly ash under physiologically relevant conditions in vitro was previously shown to depend on the size of the ash particles and on the source of the coal, and these in vitro measurements have been shown to correlate with indirect measurements of excess iron in cultured cells. Existing iron mobilization data were compared to mathematical models for mass transfer and chemical reaction in solid-liquid heterogeneous systems. Liquid-phase diffusion resistance can be ruled out as the rate-limiting mechanism for iron mobilization as the model predictions for this case are clearly inconsistent with the measurements. Other plausible hypotheses, such as a rate limited by a heterogeneous surface reaction, cannot be conclusively ruled out by the available data. These mathematical analysis methods are applicable to the design of future experiments to determine the rate-limiting mechanism for the mobilization of iron and of other transition metals from both ambient air samples and surrogates for major sources of particulate air pollution.

Mössbauer spectroscopy indicates that iron in an aluminosilicate glass phase is the source of the bioavailable iron from coal fly ash

Iron speciation by Mössbauer spectroscopy indicates that ferric iron in an aluminosilicate glass phase is the source of the bioavailable iron in coal fly ash and that this iron species is associated with combustion particles, but not with crustal dust derived from soil minerals. Urban particulate has been shown to be a source of bioavailable iron and has been shown to be able to induce the formation of reactive species in cell culture experiments. Crustal dust and laboratory-generated coal fly ash have been studied as surrogates for two sources of metal-bearing particles in ambient air. As much as a 60-fold difference in the amount of iron mobilized by the chelator citrate was observed between fly ash and crustal dust samples with similar total iron contents. The extent of iron mobilization by citrate in vitro has been shown to correlate with indirect measures of excess iron in cultured cells and with assays for reactive oxygen species generation in vitro. Mössbauer spectroscopy of coal fly ash, before and after treatment with the chelator desferrioxamine B, showed that the iron in an aluminosilicate glass phase was preferentially removed. The removal of the glass-phase iron greatly reduced the amount of iron that could be mobilized by citrate and prevented the particles from inducing interleukin-8 in cultured human lung epithelial (A549) cells. Ferric iron in aluminosilicate glass is associated with particles formed at high temperatures followed by rapid cooling. The observation that ferric iron in aluminosilicate glass is the source of bioavailable iron in coal fly ash suggests that particles from ambient sources and other specific combustion sources should be examined for the presence of this potential source of bioavailable iron.

Interleukin-8 levels in human lung epithelial cells are increased in response to coal fly ash and vary with the bioavailability of iron, as a function of particle size and source of coal

Particulate air pollution contains iron, and some of the pathological effects after inhalation may be due to radical species produced by iron-catalyzed reactions. We tested the hypothesis that iron present in coal fly ash (CFA) could induce the expression and synthesis of the inflammatory cytokine interleukin-8 (IL-8). CFA, containing as much as 14% iron, was used as a model combustion source particle. Three coal types were used to generate three size fractions enriched in particles [submicron (<1 micrometer), fine (<2.5 micrometer), or coarse (2.5-10 micrometer]), as well as the fraction of >10 micrometer. Treatment of human lung epithelial (A549) cells for 4 h with CFA from Utah enriched in <1 micrometer particles (20 microgram/cm(2)) resulted in a 2.6-fold increase in mRNA levels for IL-8. IL-8 levels were increased in the medium by as much as 8-fold when cells were treated with the fraction enriched in the smallest size Utah CFA for 24 h. IL-8 production was completely inhibited when the CFA was pretreated with the metal chelator desferrioxamine B, suggesting that a transition metal was responsible for the induction, probably iron. Treatment with a soluble form of iron, ferric ammonium citrate (FAC), mimicked the IL-8 level increase observed with CFA. There was a direct relationship, above a threshold level of bioavailable iron, between the levels of IL-8 and bioavailable iron in A549 cells treated with CFA or FAC. Further, the relationship between IL-8 and bioavailable iron for CFA was indistinguishable from that for FAC. These results strongly suggest that iron can induce IL-8 in A549 cells and that iron was the likely component of CFA that induced IL-8. CFA-induced IL-8 production was inhibited by tetramethylthiourea or dimethyl sulfoxide, suggesting that radical species were involved in the induction. These results demonstrate that iron present in CFA may be responsible for production and release of inflammatory mediators by the lung epithelium through generation of radical species and suggest that iron may contribute to the exacerbation of respiratory problems by particulate air pollution.

Mechanisms of DNA oxidation

Oxidative damage of DNA caused by a variety of chemical and physical agents appears to be linked to cancer. However, it is becoming increasingly clear that endogenous generation of oxidants, such as hydroxyl radical and peroxynitrite, lead to oxidation of DNA, and this may cause cancer in individuals where no obvious exposure to chemical or physical agents known to be carcinogenic has occurred. The mechanisms for generation of these two oxidants in living organisms will be discussed and their reactivities with DNA to produce oxidized products (e.g., 8-oxo-dG) will be presented with special emphasis on the individual characteristics of the generation and reactivity of each oxidant.

In vitro effects of coal fly ashes: hydroxyl radical generation, iron release, and DNA damage and toxicity in rat lung epithelial cells

Oxygen radical generation due to surface radicals, inflammation, and iron release has been suggested as the mechanism of adverse effects of quartz, such as emphysema, fibrosis, and carcinogenic effects. Therefore, we measured iron release, acellular generation of hydroxyl radicals, and oxidative DNA damage and cytotoxicity in rat lung epithelial (RLE) cells by different coal fly ashes (CFA) that contain both quartz and iron. Seven samples of CFA with different particle size and quartz content (up to 14.1%) were tested along with silica (alpha-quartz), ground coal, and coal mine dust (respirable) as positive control particles, and fine TiO(2) (anatase) as a negative control. Five test samples were pulverized fuel ashes (PFA), two samples were coal gasification (SCG) ashes (quartz content <0.1%), and one sample was a ground coal. No marked differences between SCG and PFA fly ashes were observed, and toxicity did not correlate with physicochemical characteristics or effect parameters. Stable surface radicals were only detected in the reference particles silica and coal mine dust, but not in CFA. On the other hand, hydroxyl radical generation by all fly ashes was observed in the presence of hydrogen peroxide, which was positively correlated with iron mobilization and inhibited by deferoxamine, but not correlated with iron or quartz content. Also a relationship between acellular hydroxyl radical generation and oxidative DNA damage in RLE cells by CFA was observed. Differences in hydroxyl radical generation and oxidative damage by the CFA were not related to iron and quartz content, but the respirable ashes (MAT023, 38, and 41) showed a very extensive level of hydroxyl radical generation in comparison to nonrespirable fly ashes and respirable references. This radical generation was clearly related to the iron mobilization from these particles. In conclusion, the mechanisms by which CFA and the positive references (silica, coal mine dust) affect rat lung epithelial cells seem to be different, and the data suggest that quartz in CFA does not act the same as quartz in silica or coal mine dust. On the other hand, the results indicate an important role for size and iron release in generation and subsequent effects of reactive oxygen species caused by CFA.

Regulation of nitric oxide synthase induction by iron and glutathione in asbestos-treated human lung epithelial cells

Treatment of human lung epithelial (A549) cells with crocidolite asbestos resulted in the induction of the inducible form of nitric oxide synthase (iNOS), production of NO, and a dramatic decrease in intracellular reduced glutathione (GSH). Iron, mobilized from the crocidolite fibers (27% iron by weight), and the formation of NO were required for the formation of 2'-deoxy-7-hydro-8-oxoguanosine in the DNA of the A549 cells, but not for the decrease in GSH. Therefore, we investigated the role of GSH and iron in the induction of iNOS in A549 cells by crocidolite. Iron was required for the induction of iNOS by crocidolite. A fivefold higher amount of chrysotile asbestos (3% iron by weight) was required to cause a similar decrease in intracellular GSH and induction of iNOS. In the absence of asbestos, treatment with either buthionine sulfoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase, or ferric ammonium citrate (FAC), a soluble form of iron, did not result in induction of iNOS. However, iNOS was induced when A549 cells were treated simultaneously with BSO and FAC. The presence of 5 mM N-acetylcysteine prevented induction of iNOS in crocidolite-treated A549 cells. These observations suggest that the induction of iNOS resulted from a decrease in intracellular GSH and the presence of iron from the asbestos fibers.

Mobilization of iron from coal fly ash was dependent upon the particle size and the source of coal

Particulate air pollution, including coal fly ash, contains iron, and some of the pathological effects after inhalation may be due to reactive oxygen species produced by iron-catalyzed reactions. The objective of this study was to determine whether iron, present in coal fly ash, was mobilized, leading to ferritin induction in human airway epithelial cells, and whether the size of the particles affected the amount of iron mobilized. Three types of coal were used to generate the three size fractions of fly ash collected. The Utah coal fly ash was generated from a bituminous b coal, the Illinois coal fly ash from a bituminous c coal, and the North Dakota coal fly ash from a lignite a coal. Three size fractions were studied to compare the amount of iron mobilized in human airway epithelial (A549) cells and by citrate in cell-free suspensions. The size fractions selected were fine (<2.5 microm) and coarse (2.5-10 microm) components of PM10, airborne particulate matter <10 microm in diameter, and the fraction greater than 10 microm. Coal fly ash samples were incubated with 1 mM citrate to determine if iron associated with coal fly ash could be mobilized. Iron was mobilized by citrate from all three size fractions of all three coal types to levels as high as 56.7 nmol of Fe/mg of coal fly ash after 24 h. With all three coal types, more iron was mobilized by citrate from the <2.5 microm fraction than from the >2.5 microm fractions. Further, the mobilized iron was in the Fe(III) form. To determine if iron associated with the coal fly ash could be mobilized by A549 cells, cells were treated with coal fly ash, and the amount of the iron storage protein ferritin was determined after 24 h. Ferritin levels were increased by as much as 11.9-fold in cells treated with coal fly ash. With two of the three types of coal studied, more ferritin was induced in cells treated with the <2.5 microm fraction than with the >2.5 microm fractions. Further, inhibition of the endocytosis of the coal fly ash by the cells resulted in ferritin levels that were near that of the untreated cells, suggesting that iron was mobilized intracellularly, not in the culture medium. The results of this study suggest that differences in particle size and speciation of iron may affect the release of iron in human airway epithelial cells.

Participation of iron and nitric oxide in the mutagenicity of asbestos in hgprt-, gpt+ Chinese hamster V79 cells

Crocidolite asbestos is known to cause cellular damage, leading to asbestosis, bronchogenic carcinoma, and mesothelioma in humans. The mechanism responsible for the carcinogenicity of asbestos is not known. Iron associated with asbestos is thought to play a role by catalyzing the formation of reactive oxygen species, which may cause DNA damage, leading to mutations and cancer. Here, we examined whether asbestos can induce mutations in Chinese hamster hgprt+ V79 cells or transgenic hgprt-, gpt+ V79 cells (G12). Treatment with 6 microg/cm2 crocidolite for 24 h caused a 2-fold increase in the mutation frequency at the gpt locus of G12 cells, but no increase at the hgprt locus of V79 cells. The mutation frequency at the gpt locus of G12 cells increased with increasing treatment dose of crocidolite. The mutations induced by crocidolite appeared to be due to the generation of reactive oxygen species catalyzed by iron associated with the fibers, because treatment of G12 cells in iron-free medium with fibers from which redox active iron had been removed with desferrioxamine B prevented all of the gpt- mutations above untreated control levels. In addition, treatment of cells with a soluble form of iron, 1.5 mM ferric ammonium citrate, resulted in an increase in mutation frequency at the gpt locus of approximately 1.5 fold above that of untreated G12 cells with no increase in mutations at the hgprt locus of V79 cells with ferric ammonium citrate. We also investigated the effect of nitric oxide on the mutagenicity of crocidolite in G12 cells. When G12 cells were treated with 3 microg/cm2 of crocidolite in the presence of nitric oxide-generating compound, 200 microM diethyltriamine/NO, the mutation frequency increased to a level that was more than additive for crocidolite or diethyltriamine/NO treatment alone. These results strongly suggest that the presence of iron and nitric oxide may either lead to the generation of another reactive, mutagenic species, such as peroxynitrite, or that nitric oxide inhibits a DNA repair enzyme(s), leading to an increase in mutations.

Efflux of reduced glutathione after exposure of human lung epithelial cells to crocidolite asbestos

This study investigated glutathione (GSH) homeostasis in human lung epithelial cells (A549) exposed to crocidolite. Exposure of A549 cells to 3 micrograms/cm2 crocidolite resulted in a decrease in intracellular reduced glutathione by 36% without a corresponding increase in GSH disulfide. After a 24-hr exposure to crocidolite, 75% of the intracellular GSH lost was recovered in the extracellular medium, of which 50% was in reduced form. Since the half-life of reduced GSH in culture medium was less than 1 hr, this suggests that reduced GSH was released continuously from the cells after treatment. The release of GSH did not appear to result from nonspecific membrane damage, as there was no concomitant release of lactate dehydrogenase or 14C-adenine from loaded cells after crocidolite treatment for 24 hr. Crocidolite exposure resulted in the formation of S-nitrosothiols but no increase in the level of GSH-protein mixed disulfides or GSH conjugates. Exposure of A549 cells to crocidolite for 24 hr decreased gamma glutamylcysteine synthetase (gamma-GCS) activity by 47% without changes in the activities of GSH reductase, GSH peroxidase, GSH S-transferase, or glucose-6-phosphate dehydrogenase. Treatment of cells with crocidolite pretreated with the iron chelator desferrioxamine B resulted in the same level of intracellular GSH depletion and efflux and the same decrease in gamma-GCS activity as treatment with unmodified crocidolite, which suggests that iron-catalyzed reactions were not responsible for the GSH depletion.

Mobilization of iron from urban particulates leads to generation of reactive oxygen species in vitro and induction of ferritin synthesis in human lung epithelial cells

Many of the biochemical effects of asbestos in cultured cells have been shown to be due to iron, which can be as high as 27% by weight. Urban air particulates also contain iron, and some of the pathological effects after inhalation may be due to reactive oxygen species produced by iron-catalyzed reactions. Two standard reference material (SRM) urban air particulate samples were used for the studies described here. SRM 1648 (3.9% iron by weight) was collected in the St. Louis, MO, area, and SRM 1649 (3% iron by weight) was collected in the Washington, DC, area. To determine if iron associated with urban particulates could be mobilized, as it is from asbestos, SRMs 1648 and 1649 were incubated with 1 mM citrate or EDTA, in the presence or absence of ascorbate. Iron was mobilized from both particulates by either chelator, especially in the presence of ascorbate. Citrate, in the presence of ascorbate, mobilized 30.9 nmol of Fe/mg of SRM 1648 and 65.1 nmol of Fe/mg of SRM 1649 in 24 h. EDTA, in the presence of ascorbate, mobilized 53.8 nmol of Fe/mg of SRM 1648 and 98.8 nmol of Fe/mg of SRM 1649 in 24 h. To determine whether reactive oxygen species were being produced by the particulate iron, each particulate was incubated with phi X174 RFI DNA in the presence or absence of ascorbate. Single-strand breaks (SSBs) were produced by either particulate, but only in the presence of ascorbate. Incubation of SRM 1648 or 1649 (0.5 mg/mL) with DNA in the presence of ascorbate and citrate resulted in 20% or 34% DNA with SSBs, respectively. Incubation of SRM 1648 or 1649 (0.1 mg/mL) with DNA in the presence of ascorbate and EDTA resulted in 26% or 45% DNA with SSBs, respectively. To determine if iron associated with urban particulates could be mobilized by human lung epithelial cells (A549), cells were treated with particulates and the amount of the iron storage protein ferritin was determined at the end of treatment. The 6.4- or 8.4-fold increase in ferritin observed in cells treated with SRM 1648 or 1649, respectively, over that of control (untreated) cells strongly suggested that iron was mobilized in the cultured cells. If similar mobilization and reactivity of the iron occurs in the lung, this may explain some of the pathological effects of urban particulates.

Induction of ferritin synthesis in human lung epithelial cells treated with crocidolite asbestos

Crocidolite asbestos is a known human carcinogen containing 27% iron by weight. It has previously been shown that iron was mobilized intracellularly from crocidolite after treatment of human lung epithelial cells (A549) and that the toxicity of the fibers was directly related to how much mobilized iron was in the < 10,000 MW (low-molecular-weight, LMW) fraction [C. C. Chao, L.G. Lund, K. R. Zinn, and A. E. Aust (1994) Arch. Biochem. Biophys. 314, 384-391]. The data here show that iron mobilization from crocidolite began immediately after treatment of the A549 cells and increased linearly with time. However, the synthesis of ferritin, an iron storage protein, did not begin until after 4 h of treatment, reaching a sustained maximum after 12 h. Mobilized iron was preferentially incorporated into the nonferritin-protein fraction up to 7 h after treatment, when the amount of iron mobilized was low and before significant accumulation of newly synthesized ferritin had occurred. This suggested that these cultured cells needed additional iron for synthesis of iron-requiring proteins and that iron mobilized from crocidolite could be utilized directly for this purpose. Subsequent to this, additional mobilized iron was incorporated into newly synthesized ferritin. Even though iron from crocidolite was incorporated into newly synthesized ferritin or into other proteins, the amount of iron from crocidolite in the LMW fraction remained constant during the 24 h. Thus, it appeared that synthesis of ferritin may not have fully protected the cells from the toxic effects of iron mobilized from crocidolite.

Participation of nitric oxide and iron in the oxidation of DNA in asbestos-treated human lung epithelial cells

Treatment of human lung epithelial (A549) cells with crocidolite resulted in the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the DNA, synthesis of mRNA for the inducible form of nitric oxide synthase (NOS), and increased intracellular nitrite (NO2-), a stable oxidation product of NO. Iron, associated with crocidolite, was involved in both NO2- and 8-OHdG formation. Addition of the NOS inhibitor, aminoguanidine (AG), reduced intracellular NO2- and prevented formation of 8-OHdG in crocidolite-treated cells, suggesting that NO was required in 8-OHdG formation. Addition of an NO-generating compound, diethyltriamine/NO, with AG and crocidolite resulted in recovery of 8-OHdG, further supporting a role for NO in oxidation of deoxyguanosine.

The effect of iron binding on the ability of crocidolite asbestos to catalyze DNA single-strand breaks

Crocidolite or crocidolite pretreated with desferrioxamine-B (DF crocidolite) was exposed to ferrous chloride solutions to determine whether iron could be bound from solution. Native crocidolite was capable of binding up to 57 nmol Fe2+/mg fiber in 60 min, while the DF crocidolite was capable of binding only 5.5 nmol Fe2+/mg fiber. The rate of iron binding for the first 5 min of exposure was independent of the concentration of iron in the solution, suggesting that there was a group of rapidly saturable sites, approximately 1.5 x 10(18) binding sites/m2 crocidolite surface, which were responsible for the immediate binding. This process was followed by a slower binding phase, likely occurring at other sites. Crocidolite and DF crocidolite, with various amounts of iron bound, were assayed for their abilities to catalyze the formation of DNA single-strand breaks (SSBs) in phi X174 RFI DNA. Native crocidolite with additional iron bound did not significantly change in its ability to cause DNA SSBs in 15 or 30 min incubations, even though more iron could be mobilized from the iron-treated crocidolite at 4 or 24 h. DF crocidolite, after the addition of iron, had a significantly increased ability to form DNA SSBs. DF crocidolite with 0, 3.0 or 5.5 mmol Fe2+/mg catalyzed the formation of DNA SSBs in 21, 42 or 51% of the DNA respectively in the presence of EDTA and ascorbate. Fibers were also incubated in tissue culture medium with or without iron salts. The fibers incubated in the iron-containing medium had an increased ability to form DNA SSBs. These results suggest that fibers such as crocidolite may be capable of binding iron from intracellular sources. This additional iron may be as reactive as the intrinsic iron and may increase the reactive lifetime of the fiber.

Mediated, thin-layer cell, coulometric determination of redox-active iron on the surface of asbestos fibers

Redox-active iron on the surface of asbestos fibers was detected and quantified using a thin-layer cell, coulometric method with soluble mediators to shuttle electrons between the mineral fibers and the solid electrode. The working and counter electrodes consisted of gold films on a glass slide with reference electrodes of silver. Asbestos fibers were entrapped in a thin-layer cell of 25 microns thickness. Hexaammineruthenium(II) or o-dianisidine (dication) was used as the reducing or oxidizing mediator, respectively. Hexaammineruthenium(III) undergoes a one-electron reduction, and protonated o-dianisidine undergoes a sequential two-electron oxidation. The measurement involved determination of the total charge for the oxidation or reduction of surface-immobilized Fe(II) or Fe(III) on the asbestos fibers. Analysis of the results showed that crocidolite and amosite have 4.3 +/- 0.7 and 3.3 +/- 0.7 nmol/mg of total redox-active iron that is accessible to the mediators, respectively. This corresponded to a surface coverage of accessible redox-active iron of approximately 4.3 x 10(-11) mol/cm2 for crocidolite and 9.5 x 10(-11) mol/cm2 for amosite. Furthermore, Fe(II) constituted 76% or 25% of the accessible redox-active iron on the surface of crocidolite or amosite, respectively. The method may be applied to other types of solid materials with redox-active species on their surfaces.

Effect of iron acquisition on induction of DNA single-strand breaks by erionite, a carcinogenic mineral fiber

It has been proposed that iron from inhaled fibers, such as asbestos, is responsible for their toxicity and carcinogenicity. The natural fibrous zeolite erionite is far more carcinogenic than asbestos, but contains little or no iron. Fibers that persist in the lung accumulate iron on their surfaces. The ability of erionite to induce formation of single-strand breaks in phi X174 RFI DNA was investigated before and after binding of iron. Unmodified erionite did not induce DNA single-strand breaks with or without ascorbate and/or the iron chelator EDTA. When erionite (1 mg/ml) was suspended in 25, 50, 100, or 500 microM FeCl2, it readily bound iron (25, 50, 93, or 176 nmol Fe/mg, respectively, in 1 h), apparently through ion exchange. When erionite was suspended in FeCl3 at identical concentrations for 1 h, it also bound iron (24, 46, 89, or 239 nmol Fe/mg). The ferric iron did not appear to bind primarily through ion exchange, but appeared to be deposited on the surface, probably in the form of hydroxides/oxyhydroxides. When ferrous iron was bound to erionite, single-strand breaks in DNA were induced in the absence of ascorbate, but when ferric iron was bound, DNA single-strand breaks were observed only in the presence of ascorbate. When EDTA or citrate was added, this activity increased with the amount of iron mobilized from the fiber. If iron acquired by erionite in vivo is similarly mobilized, it may damage biomolecules throughout the cell, including DNA. This may explain the toxicity and carcinogenicity of erionite.

Iron mobilization from crocidolite asbestos by human lung carcinoma cells

Neutron-activated crocidolite, containing 55Fe and 59Fe, was used to determine whether iron was mobilized from crocidolite phagocytized by cultured human lung carcinoma cells (A549 cells). Cells were treated with neutron-activated crocidolite in medium at pH 6.8 or 7.4 for 24 h. The mobilization of iron into two subcellular fractions, 10,000g supernatant (total iron) or < 10,000 MW [low-molecular-weight (LMW)] was monitored using scintillation counting. Iron was mobilized from crocidolite at a rate similar to that observed in vitro when citrate was incubated with crocidolite for 24 h at pH 7.4, but the amount mobilized was greater when cells were cultured at pH 6.8 than at 7.4. Iron mobilization was not due to the medium nor did it appear to be due to differences in the amount of crocidolite phagocytized. At the highest concentration of crocidolite used for treatment at pH 7.4 (4.5 micrograms/cm2), a total of 3600 pmol iron/10(6) cells was mobilized of which 54 pmol/10(6) cells was in a LMW fraction. After estimation of the volume of the cells, this was calculated to be equivalent to an intracellular concentration of 1.4 mM iron of which 22 microM was in the LMW fraction. Cell survival decreased linearly as the iron mobilized into the LMW fraction increased, independent of the pH of the culture medium being used. These results suggest that iron mobilization from crocidolite into a LMW fraction may represent "iron overload" in cells which have phagocytized the fibers and may be responsible for crocidolite-dependent cytotoxicity and possibly other crocidolite-dependent biological effects.

Iron associated with asbestos bodies is responsible for the formation of single strand breaks in phi X174 RFI DNA

The ability of amosite cored asbestos bodies isolated from human lungs to catalyse damage to phi X174 RFI DNA in vitro was measured and compared with that of uncoated amosite fibres with a similar distribution of length. Asbestos bodies (5000 bodies) suspended for 30 minutes in 50 mM NaCl containing 0.5 micrograms phi X174 RFI DNA, pH 7.5, did not catalyse detectable amounts of DNA single strand breaks. Addition of the reducing agent ascorbate (1 mM), however, resulted in single strand breaks in 10% of the DNA. Asbestos bodies in the presence of a low molecular weight chelator (1 mM) and ascorbate catalysed the formation of single strand breaks in 21% of the DNA with citrate or 77% with ethylenediamine tetra-acetic acid (EDTA), suggesting that mobilisation of iron may increase damage to DNA. Preincubation for 24 hours with desferrioxamine B, which binds iron (Fe (III)) and renders it redox inactive, completely inhibited the reactivity of asbestos bodies with DNA, strongly suggesting that iron was responsible. Amosite fibres (5000 fibres/reaction), with a similar length distribution to that of the asbestos bodies, did not catalyse detectable amounts of single strand breaks in DNA under identical reaction conditions. The results of the present study strongly suggest that iron deposits on the amosite core asbestos bodies were responsible for the formation of DNA single strand breaks in vitro. Mobilisation of iron by chelators seemed to enhance the reactivity of asbestos bodies with DNA. It has been postulated that the in vivo deposition of the coat material on to fibres may be an attempt by the lung defenses to isolate the fibre from the lung surface and thus offer a protective mechanism from physical irritation. These results suggest, however, that the iron that is deposited on asbestos fibres in vivo may be reactive, potentially increasing the damage to biomolecules, such as DNA, above that of the uncoated fibres.

Effect of long-term removal of iron from asbestos by desferrioxamine B on subsequent mobilization by other chelators and induction of DNA single-strand breaks

The long-term removal of iron from crocidolite or amosite by desferrioxamine B (DF) at pH 7.5 or 5.0 was studied. Crocidolite or amosite (1 mg/ml) was suspended in 50 mM NaCl at pH 7.5 or 5.0 with the addition of 1 mM DF for up to 90 days. Although the rate of iron mobilization decreased with time, iron was continuously mobilized from both forms of asbestos at pH 5.0 or 7.5. The amount of iron mobilized from crocidolite was at least twice that mobilized from amosite at either pH. Iron was mobilized more rapidly from crocidolite at pH 5.0 than at 7.5 for the first 15 days, but at later times the amount being mobilized at pH 7.5 became equal to or slightly greater than that at 5.0. For amosite, the mobilization at pH 5.0 was always greater than that at pH 7.5. Next, the effect of iron removal from asbestos by DF on subsequent iron mobilization by a second chelator (EDTA or citrate) and on induction of DNA single-strand breaks (SSBs) was studied. Asbestos, treated for up to 15 days with DF at pH 7.5, was washed to remove ferrioxamine and excess DF, then incubated with EDTA or citrate (1 mM). The rates of iron mobilization from both forms of asbestos by a second chelator decreased as more and more iron was removed by DF. Induction of DNA SSBs also decreased, reflecting the unavailability of iron to catalyze the damage. The results suggest three things. First, if long-term mobilization of iron from asbestos occurs in vivo as has been observed in vitro, it may play a role in the long-term biological effects of asbestos. Second, more rapid mobilization of iron from asbestos fibers may occur when the fibers are phagocytized by cells and maintained in phagosomes where the pH is 4.0-5.0. Third, treatment of asbestos by iron chelators, such as DF, prior to exposure to cultured cells or whole animals, may reduce the biological effects of asbestos resulting from iron, but may not completely eliminate them.

Photochemical reduction of ferric iron by chelators results in DNA strand breaks

The induction of single-strand breaks (SSB) in phi X174 RFI DNA by Fe(III) chelates under cool-white fluorescent (CWF) light or in darkness was investigated. Under CWF light, ferric iron (25 microM) without the addition of a low-molecular-weight (LMW) chelator or with the addition of a LMW chelator, such as citrate, nitrilotriacetate (NTA), or EDTA, generated SSB in 11, 30, 13, or 0% of the treated, closed-circular DNA, respectively. Addition of H2O2 increased the formation of DNA SSB to 93, 75, 62, or 25, respectively. In the dark, DNA SSB were not detected in the absence of H2O2. The addition of H2O2 resulted in 85% (none), 8% (citrate), 18% (EDTA), or 42% (NTA) DNA with SSB. The formation of DNA SSB was completely inhibited by a strong Fe(II)-specific chelator, ferrozine. Various .OH scavengers (e.g., mannitol, 5,5-dimethyl-1-pyrroline-N-oxide, dimethyl sulfoxide) completely inhibited DNA SSB formation in the presence of a LMW chelator, but only partially inhibited in the absence. These results suggest that Fe(II) and .OH or similarly reactive species may be involved in the induction of DNA SSB. The evolution of CO2 from chelators, such as EDTA or citrate, required CWF light and Fe(III), suggesting the participation of chelators in the reaction with Fe(III). Under CWF light, catalase inhibited and superoxide dismutase (SOD) stimulated formation of DNA SSB, indicating H2O2 was generated from O2-. and was required for DNA damage. In the dark, SOD inhibited formation of DNA SSB, suggesting O2-. was required for reaction(s) other than formation of H2O2. These results suggest that in the light the photochemical reduction of Fe(III) by the chelators results in formation of DNA SSB without the addition of H2O2. In the dark, the addition of H2O2 was required for the formation of DNA SSB, suggesting a different mechanism for DNA strand break formation.

Iron mobilization from crocidolite asbestos greatly enhances crocidolite-dependent formation of DNA single-strand breaks in phi X174 RFI DNA

The ability of the iron associated with asbestos to catalyze damage to phi X174 RFI DNA was determined and compared with iron mobilized from asbestos. Asbestos (1 mg/ml) suspended for 30 min in 50 mM NaCl containing 0.5 micrograms phi X174 RFI DNA, pH 7.5, did not catalyze detectable amounts of DNA single-strand breaks (SSB). However, addition of ascorbate (1 mM) resulted in 19, 26, 7 or 8% DNA with SSB for crocidolite, amosite, chrysotile or tremolite respectively. The percentage of DNA with SSB induced by each form of asbestos was directly related to its iron content. Inclusion of desferrioxamine B, which binds Fe(III) rendering it redox inactive, completely inhibited asbestos-dependent formation of DNA SSB, suggesting that iron was responsible for catalyzing the formation of DNA SSB. Mobilization of Fe(II) from crocidolite by citrate, EDTA or nitrilotriacetate (1 mM) in the absence of ascorbate resulted in 15, 33 or 63% DNA with SSB respectively. This activity was completely inhibited by compounds considered to be .OH scavengers, i.e. mannitol, 5,5-dimethyl-1-pyrroline N-oxide or salicylate (100 mM). Preincubation of crocidolite with citrate (1 mM) for 24 h resulted in mobilization of 52 microM iron and increased ascorbate-dependent induction of DNA SSB compared with crocidolite that was preincubated without citrate. Iron mobilized by citrate was entirely responsible for crocidolite-dependent formation of DNA SSB as evidenced by complete inhibition with desferrioxamine B. Therefore, the results of the present study strongly suggest that iron was responsible for asbestos-dependent generation of oxygen radicals, which resulted in the formation of DNA SSB. Mobilization of iron by chelators, followed by redox cycling, greatly enhanced crocidolite-dependent formation of DNA SSB. Thus, mobilization of iron in vivo by low mol. wt chelators may lead to the increased production of reactive oxygen species resulting in damage to biomolecules, such as DNA.

Iron-catalyzed reactions may be responsible for the biochemical and biological effects of asbestos

The most carcinogenic forms of asbestos contain iron to levels as high as 36% by weight and catalyze many of the same biochemical reactions that freshly prepared solutions of iron do, i.e. oxygen consumption, generation of reactive oxygen species, lipid peroxidation and DNA damage. The participation of iron from asbestos in these reactions has been demonstrated using the iron chelator desferrioxamine B which inhibits iron-catalyzed reactions. Iron appears to be redox active on the asbestos fiber, but chelation and subsequent iron mobilization from asbestos by a variety of chelators, e.g. citrate, EDTA or nitrilotriacetate, makes the iron more redox active resulting in greater oxygen consumption and production of oxygen radicals in the presence of reducing agents. Iron also appears to be important for some of the asbestos-dependent biological effects on tissues or cells in culture, such as phagocytosis, cytotoxicity, lipid peroxidation and DNA damage. Therefore, redox cycling of iron to generate oxygen radicals at the surface of the fiber and/or in solution, as mobilized, low molecular weight chelates, may be very important in eliciting some of the biological effects of asbestos in vivo.

Mobilization of iron from crocidolite asbestos by certain chelators results in enhanced crocidolite-dependent oxygen consumption

The reactivity of iron on crocidolite asbestos with dioxygen was determined and compared with iron mobilized from crocidolite. Ferrozine, a strong Fe(II) chelator, was used to demonstrate that iron on crocidolite was redox active. More Fe(II) was mobilized from crocidolite (1 mg/ml) by ferrozine anaerobically (11.2 nmol/mg crocidolite/h) than aerobically (6.6 nmol/mg/h) in 50 mM NaCl, pH 7.5, suggesting that Fe(II) on crocidolite reacts with O2 upon aqueous suspension. However, suspension of crocidolite in 50 mM NaCl, pH 7.5, did not result in a measurable amount of O2 consumption. The addition of reducing agents (1 mM) increased the amount of Fe(II) on crocidolite, and addition of ascorbate resulted in 0.4 nmol O2 consumed/mg crocidolite/min. Therefore, iron on crocidolite had limited redox activity in the presence of ascorbate. However, mobilization of iron from crocidolite increased its redox activity. Citrate, nitrilotriacetate (NTA), or EDTA (1 mM) mobilized 79, 32, or 58 microM iron, respectively, in preincubations up to 76 h, and increased O2 consumption upon addition of ascorbate to 2.8, 7.6, or 22.0 nmol O2 consumed/mg/min, respectively. This activity depended only upon the presence of a component(s) mobilized from crocidolite by the chelators. Pretreatment of crocidolite with the iron chelator desferrioxamine B (10 mM) inhibited O2 consumption. The results of the present study suggest that iron on or in crocidolite is responsible for the redox activity of crocidolite, but that mobilization of iron by chelators such as citrate, NTA, or EDTA greatly enhances its redox activity. Thus, iron mobilization from crocidolite in vivo by low-molecular-weight chelators may lead to the increased production of reactive oxygen species which may damage biomolecules, such as DNA.

Iron mobilization from asbestos by chelators and ascorbic acid

The ability of chelators and ascorbic acid to mobilize iron from crocidolite, amosite, medium- and short-fiber chrysotile, and tremolite was investigated. Ferrozine, a strong Fe(II) chelator, mobilized Fe(II) from crocidolite (6.6 nmol/mg asbestos/h) and amosite (0.4 nmol/mg/h) in 50 mM NaCl, pH 7.5. Inclusion of ascorbate increased these rates to 11.4 and 4.9 nmol/mg/h, respectively. Ferrozine mobilized Fe(II) from medium-fiber chrysotile (0.6 nmol/mg/h) only in the presence of ascorbate. Citrate and ADP mobilized iron (ferrous and/or ferric) from crocidolite at rates of 4.2 and 0.3 nmol/mg/h, respectively, which increased to 4.8 and 1.0 nmol/mg/h in the presence of ascorbate. Since ascorbate alone mobilized iron from crocidolite (0.5 nmol/mg/h), the increase appeared to result from additional chelation by ascorbate. Citrate also mobilized iron from amosite (1.4 nmol/mg/h) and medium-fiber chrysotile (1.6 nmol/mg/h). Mobilization of iron from asbestos appeared to be a function not only of the chelator, but also of the surface area, crystalline structure, and iron content of the asbestos. These results suggest that iron can be mobilized from asbestos in the cell by low-molecular-weight chelators. If this occurs, it may have deleterious effects since this could result in deregulation of normal iron metabolism by proteins within the cell resulting in iron-catalyzed oxidation of biomolecules.

Inhibition of 2-aminofluorene mutagenesis in bacteria by inducers of cytochrome P-450d

Certain chemical inducers of rat liver microsomal cytochrome P-450d are tightly bound to the cytochrome. We investigated the ability of two inducers of cytochrome P-450d, Aroclor 1254 and isosafrole, to inhibit the microsomal activation of 2-aminofluorene to a mutagen as measured in Salmonella typhimurium. Butanol treatment of microsomes from isosafrole-treated rats removed an inhibitory metabolite of isosafrole and increased 2-aminofluorene mutagenesis by approximately 2-fold over controls. Butanol treatment of microsomes from Aroclor 1254-treated rats failed to either remove any of the Aroclor 1254 associated with microsomal cytochrome P-450 or affect 2-aminofluorene-induced mutagenesis. However, addition of Aroclor 1254 to butanol-treated microsomes from isosafrole-treated rats almost completely inhibited 2-aminofluorene mutagenesis. Aroclor 1254 completely inhibited the cytochrome P-450d-dependent estradiol 2-hydroxylase activity of butanol-treated microsomes from isosafrole-treated rats. Thus, we suspect that certain congeners from Aroclor 1254, a widely used mixture for induction of cytochrome P-450 activities, could inhibit cytochrome P-450d and partially mask its ability to metabolize some chemicals to mutagens.

Mutations and homologous recombination induced in mammalian cells by metabolites of benzo[a]pyrene and 1-nitropyrene

Metabolites of two structurally related chemical carcinogens, benzo[a]pyrene and 1-nitropyrene, were compared for their ability to cause cytotoxicity and induce mutations in normally repairing or nucleotide excision repair-deficient diploid human fibroblasts; for their ability to induce mutations in a defined gene sequence, supF, when a plasmid containing adducts formed by these carcinogens replicates in human 293 cells; and for their ability to induce homologous recombination between duplicated genes in mouse L cells. Both of the metabolites tested, i.e., (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha, epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and 1-nitrosopyrene (1-NOP), form adducts on guanine. BPDE binds principally at the N2 position of guanine; 1-NOP binds to guanine at the C8 position. Results of the studies in diploid human cells indicated that when compared on the basis of equal numbers of DNA adducts, BPDE is more effective than 1-NOP in inducing mutations in DNA repair-proficient cells, but when compared in repair-deficient xeroderma pigmentosum human cells that do not remove such adducts from their DNA, the frequency of mutants induced per adduct is equal. These results suggest that during the time available for repair of potentially mutagenic lesions, repair-proficient human cells excise 1-NOP adducts more rapidly than they excise BPDE adducts. Molecular analysis of the specific kinds of mutations induced when a plasmid containing BPDE residues was allowed to replicate in human cells showed that BPDE induces mainly base substitution mutations, predominantly G:C to T:A transversions.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of bacterial mutations by aminopyrazoles, compounds which cause mammary cancer in rats

An aminopyrazole PD 71627 (5-amino-1,3-dimethyl-1H-pyrazol-4-yl) (2-fluorophenyl)methanone, and two amide derivatives, PD 108298, N-[4-(2-fluorobenzoyl)-1,3-dimethyl-1H-pyrazol-5-yl]-2- ([3-(2-methyl-1- piperidinyl)-propyl]amino) acetamide-(Z)-2-butanedioate (1:2), and PD 109394, 2-(diethylamino)-N-[4-(2- fluorobenzoyl)-1,3-dimethyl-1H-pyrazol-5-yl]acetamide hydrochloride, proposed neuroleptic drugs, were found to elicit mammary adenocarcinomas in male rats after 13 weeks of treatment. These compounds were assessed for their ability to induce His+ revertants (rev) in five strains of Salmonella typhimurium (TA98, TA100, TA1535, TA1537 and TA1538) in the presence and absence of S9 activation. All were found to be potent mutagens in TA98 and TA100 after a 20 min pre-incubation with Aroclor 1254-induced rat liver S9. However, the activity of the amino-pyrazole PD 71627 was much greater than the amide derivatives, PD 108298 or PD 109394, with activity of 11,800 rev/mumol, 670 rev/mumol, and 230 rev/mumol respectively in TA100, the strain showing the greatest response. A comparison of liver S9 fractions from rats untreated or pretreated with phenobarbital (PB) or Aroclor 1254 showed that S9 from animals pretreated with PB provided the greatest activation capability for the aminopyrazole PD 71627 (59,300 rev/mumol in TA100). Three structural analogs of the aminopyrazole PD 71627, two without the amine and one with a methyl substituent on the amine, were compared with PD 71627 for induction of revertants in TA100 and TA98. The compounds without the amine had no mutagenic activity while the methyl derivative induced 3100 rev/mumol in TA100 after preincubation with Aroclor 1254-induced S9. This confirmed that the amine on the pyrazole ring was required for mutagenic activity. The results of these studies support the hypothesis that these compounds cause cancer in animals as a result of DNA damage.

Comparison of the frequency of diphtheria toxin and thioguanine resistance induced by a series of carcinogens to analyze their mutational specificities in diploid human fibroblasts

The mutagenic specificities of ethylnitrosourea (ENU), X-rays (+/-)7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7, 8,9,10-tetrahydrobenzo[a]pyrene (BPDE), ICR-191, and N-acetoxy-2-acetylaminofluorene (N-AcO-AAF) were analyzed and compared in diploid human fibroblasts and Salmonella typhimurium. In the human fibroblasts, we compared the frequency of diphtheria toxin (DT)-resistant mutants, presumably induced in the gene coding for elongation factor-2, with the frequency of 6-thioguanine (TG) resistance induced by mutations in the gene coding for hypoxanthine(guanine)phosphoribosyltransferase (HPRT). Recovery of DT-resistant (DTr) cells requires that the mutant EF-2 retain the ability to carry on protein synthesis since the normal EF-2 will be inactivated by DT selection. Therefore, the DTr mutation cannot involve major changes in the gene. In contrast, cells can acquire TG resistance by any mechanism which eliminates HPRT activity, e.g., base substitution, frameshift, deletion, loss of chromosomes. Each agent was assessed by calculating the ratio of the slopes of the dose-response plots (induced variant frequency as a function of dose of the agent used) for the two markers (DTr/TGr variants.). In S. typhimurium we examined the reversion frequency in four histidine-requiring strains bearing forward mutations of the frameshift (TA1538, TA98) or missense (TA1535, TA100) type. ENU, which was predominantly a base substitution mutagen in the bacteria, gave a ratio of DTr to TGr variants of 1.5. As expected of an agent inducing gross chromosomal changes, X-rays induced no revertants in bacteria and in human cells gave a ratio of 0.1. ICR-191 which was predominantly a frameshift mutagen in bacteria gave a ratio of 0.15. In the set of bacterial strains containing the plasmid pKM101, BPDE reverted both frameshift and base substitution mutations. It did not cause reversions in the other set of strains. In human cells BPDE gave a response similar to ENU, i.e., a ratio of DTr/TGr variants of 1.5. As reported by others, N-AcO-AAF was predominantly a frameshift mutagen in bacteria. However, in the human cells it gave a ratio of DTr/TGr variants of 1.5, similar to ENU and BPDE. These results suggest that in human cells, BPDE and N-AcO-AAF, like ENU, yield predominantly base substitutions, while ICR-191 and X-rays largely produce mutations by mechanisms which result in more extensive alterations in the gene.

Identifying human cells capable of metabolizing various classes of carcinogens

Human cells that appear capable of metabolizing various classes of carcinogens have been identified using one of two methods: metabolism of tritiated benzo(a)pyrene to aqueous-acetone soluble forms or inhibition of cellular DNA synthesis. Each of the assay systems was optimized and the results on 15 human epithelial cell lines were compared. One or more cell lines were found to activate each of four classes of carcinogens examined: polycyclic hydrocarbons, aromatic amines, heterocyclic hydrocarbons, and nitrosamines. Cells that appeared capable of metabolizing polycyclic hydrocarbons or aromatic amines by these methods were also found to produce metabolites which were cytotoxic to cocultivated human xeroderma pigmentosum fibroblasts after a 48-hr exposure to the carcinogen.

Human cell-mediated benzo(a)pyrene cytotoxicity and mutagenicity in human diploid fibroblasts

A human epithelial cell-mediated cytotoxicity and mutagenicity assay system for benzo(a)pyrene [B(a)P] has been developed with human fibroblasts as the target cells. Lethally X-irradiated human kidney carcinoma-derived epithelial cells, which had constant levels of B(a)P-metabolizing activity, were cocultivated with target human skin fibroblasts (XP12BE), which lack excision repair capability for B(a)P-DNA adducts. The optimal conditions determined for the cell-mediated cytotoxicity assay were a 48-hr exposure to B(a)P concentrations ranging from 0.1 to 1 microM at a metabolizing cell:target cell ratio of at least 1:1. Under these conditions, the frequency of mutations to 6-thioguanine resistance induced in the target XP12BE cells by B(a)P as well as the binding of tritium-labeled B(a)P to DNA was also shown to be concentration dependent. High-pressure liquid chromatography analysis of enzymatically degraded B(a)P-DNA adducts revealed two peaks: a major peak (82%) which cochromatographed with the guanosine adduct-standard synthesized from anti-isomeric-7,8-dihydrodiol-9,10-epoxide of B(a)P; and a minor peak (18%) which cochromatographed with the guanosine adduct-standard synthesized from syn-isomeric-7,8-diol-9,10-epoxide of B(a)P. Human liver carcinoma- and lung carcinoma-derived cell lines, capable of metabolizing B(a)P, proved equal to or better than the kidney carcinoma-derived cell line in producing cytotoxic B(a)P metabolites in the cell-mediated cytotoxicity assay with target XP12BE cells.

Homogeneous pyruvate kinase isolated from yeast by two different methods is indistinguishable from pyruvate kinase in cell-free extract

In this report, we have compared homogeneous yeast (Saccharomyces cerevisiae) pyruvate kinase to enzyme from cell-free extracts in several different ways: 1) isoelectric focusing of cell-free extracts indicates one peak of pyruvate kinase activity whose isoelectric point is the same as that of the pure enzyme; 2) antibody prepared to the pure enzyme produces a single, fused precipitin line against enzyme in the cell-free extract and pure enzyme; 3) immunoelectrophoresis of cell-free extract produces one precipitin arc which has the same mobility as that of the pure enzyme; and 4) immunoprecipitation of the pure enzyme from cell-free extract with subsequent solubilization in 1% sodium dodecyl sulfate and electrophoresis on sodium dodecyl sulfate-polyacrylamide gels produces a single protein band attributable to pyruvate kinase which co-migrates with the purified enzyme. Within the limits of the sensitivity of the methods employed, we conclude that the homogeneous pyruvate kinase prepared from yeast lysed either by Manton-Gaulin homogenization (Aust, A., Yun, S.-L., and Suelter, C. (1975) Methods Enzymol. 42, 176-182) or by toluolysis (Yun, S.-L., Aust, A.E., and Suelter, C.H. (1977) J. Biol. Chem. 251, 124-128) is identical with pyruvate kinase in cell-free extract.

A revised preparation of yeast (Saccharomyces cerevisiae) pyruvate kinase

A revised preparation of pyruvate kinase from saccharomyces cerevisiae is reported. By purifying this cold-labile enzyme at room temperature, an improved recovery and specific activity was obtained. More than 350 mg of pure enzyme with a specific activity of 350 to 400 units/mg at 30 degrees were obtained from a pound of fresh yeast. The last step of the preparation, passage of the enzyme over Sephadex G-100, was required to remove a contaminating protease. The molecular parameters of the new preparation are: molecular weight, 209,000; four subunits of identical size; E 280 nm, 0.51; pI 6.6; and pH optimum, 6.28. Kinetic parameters are: Km for P-enolpyruvate and ADP, 0.09 and 0.18 mM in the presence of saturating Fru-1,6-P2, and 1.8 and 0.34 mM in the absence of Fru-1,6-P2; Ka for Fru-1,6-P2, 0.014 mM. No free NH2-terminal amino acid could be detected. Amino acid composition was determined and compared with other pyruvate kinase preparations.