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PubMed2005

Nei deficient Escherichia coli are sensitive to chromate and accumulate the oxidized guanine lesion spiroiminodihydantoin.

Growth inhibition and oxidized guanine lesion formation were studied in a number of base excision repair (BER) deficient Escherichia coli (E. coli) following chromate exposure. The only BER deficient bacterial strain that demonstrated significant growth inhibition by chromate, in comparison to its matched wild-type cell line, was the Nei deficient (TK3D11). HPLC coupled with electrospray ionization mass spectrometry showed that the Nei deficient E. coli accumulated the further oxidized guanine lesion, spiroiminodihydantoin (Sp), in genomic DNA at levels that were approximately 20-fold greater than its wild-type counterpart. However, no accumulation of the putative intermediate of Sp, 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG), was observed in the Nei deficient strain. A MutM-/MutY- double deletion mutant that was deficient in BER enzymes for the recognition and repair of 8-oxodG demonstrated no sensitivity toward chromate nor was there an associated increase in Sp accumulation over that of its wild type. However, the MutM-/MutY- double deletion mutant did show approximately 20-fold accumulation of 8-oxodG upon chromate exposure over that of the wild type and the Nei deficient E. coli. These data demonstrate that the Nei BER enzyme is critical for the recognition and repair of the Sp lesion in bacterial cell lines and demonstrates the protective effect of a specific BER enzyme on DNA lesions formed by chromate. To our knowledge, these are the first studies to show the formation and biological significance of the Sp lesion in a cellular system. This study has significant mechanistic and toxicological implications for how chromate may serve as an initiator of carcinogenesis and suggests a role for specific repair enzymes that may ameliorate the carcinogenic potential of chromate.

Cell Proliferation↗
PubMed1998

Aerobic chromate reduction by Bacillus subtilis.

We have studied the reduction of hexavalent chromium (chromate) to the less toxic trivalent form by using cell suspensions and cell-free extracts from the common soil bacterium, Bacillus subtilis. B. subtilis was able to grow and reduce chromate at concentrations ranging from 0.1 to 1 mM K2CrO4. Chromate reduction was not affected by a 20-fold excess of nitrate-compound that serves as alternate electron acceptor and antagonizes chromate reduction by anaerobic bacteria. Metabolic poisons including sodium azide and sodium cyanide inhibited chromate reduction. Reduction was effected by a constitutive system associated with the soluble protein fraction and not with the membrane fraction. The reducing activity was heat labile and showed a Km of 188 microns CrO4(2)-. The reductase can mediate the transfer of electrons from NAD(P)H to chromate. The results suggest that chromate is reduced via a detoxification system rather than dissimilatory electron transport.

Aerobiosis↗
PubMed2000

Repeated exposures to cobalt or chromate on the hands of patients with hand eczema and contact allergy to that metal.

The present study aimed at evaluating the effects of daily repeated exposures to low cobalt or chromate concentrations on the hands of patients with hand eczema and cobalt or chromate allergy. For 2 weeks, the patients immersed a finger for 10 min daily into the appropriate metal salt solution in water. During the 1st week, this was a 10 or 50 mg/l cobalt concentration or a 10 mg/l chromate concentration, and, during the 2nd week, a 100 or 200 mg/l cobalt concentration or a 100 mg/l chromate concentration. This regimen elicited a flare of hand eczema only in the chromate-exposed chromate-sensitive patients. During the exposure period, accumulation of cobalt or chromate in the nail was demonstrated. Standardization of chemical methods of quantification of skin exposure to allergens, combined with experimental exposure studies in patients with specific contact allergy, will increase the possibility of providing evidence-based medicine in the area of allergic contact dermatitis in the future.

Aged↗
PubMed1992

Prognosis of occupational chromate dermatitis.

To elucidate further the natural history and prognosis of occupational chromate dermatitis, 120 affected patients, diagnosed between 1980 and 1989, were reviewed. The incidence of chromate dermatitis in Western Australia appeared to remain unchanged over the decade. 65% of patients were construction workers with cement-induced chromate dermatitis. Workers at greatest risk of sensitization were those mixing bagged cement at the work site. The median age at onset of symptoms was 34 years, with 48% having been exposed to chromate for 5 years or less. Only 37% presented to the dermatologist within 12 months of developing symptoms. 76% of patients had ongoing dermatitis at the time of review. Although 48% of the study population had completely changed their occupation to avoid chromate exposure, symptoms persisted in 69%. A delayed diagnosis of chromate sensitivity was noted to be a predictor of chronicity. In view of the potential chronicity of chromate dermatitis and its associated social and occupational impairment, we recommend the addition of ferrous sulphate while mixing bagged cement at the work site. This simple technique targets the workers at greatest risk of becoming sensitized.

Adolescent↗
PubMed1990

Cloning, nucleotide sequence, and expression of the chromate resistance determinant of Pseudomonas aeruginosa plasmid pUM505.

The chromate resistance determinant of Pseudomonas aeruginosa plasmid pUM505 was cloned into broad-host-range vector pSUP104. The hybrid plasmid containing an 11.1-kilobase insert conferred chromate resistance and reduced uptake of chromate in P. aeruginosa PAO1. Resistance to chromate was not expressed in Escherichia coli. Contiguous 1.6- and 6.3-kilobase HindIII fragments from this plasmid hybridized to pUM505 but not to P. aeruginosa chromosomal DNA and only weakly to chromate resistance plasmids pLHB1 and pMG6. Further subcloning produced a plasmid with an insert of 2,145 base pairs, which was sequenced. Analysis of deletions revealed that a single open reading frame was sufficient to determine chromate resistance. This open reading frame encodes a highly hydrophobic polypeptide, ChrA, of 416 amino acid residues that appeared to be expressed in E. coli under control of the T7 promoter. No significant homology was found between ChrA and proteins in the amino acid sequence libraries, but 29% amino acid identity was found with the ChrA amino acid sequence for another chromate resistance determinant sequenced in this laboratory from an Alcaligenes eutrophus plasmid (A. Nies, D. Nies, and S. Silver, submitted for publication).

Amino Acid Sequence↗
PubMedDate not supplied

Modification of the erythrocyte anion carrier by chromate.

It was confirmed that chromate is taken up by human erythrocytes via the general anion carrier. The chromate flux is unidirectional and chromium is accumulated within red cells presumably due to intracellular reduction of Cr(VI) to Cr(III). The analysis of the initial rates of uptake of chromate revealed two distinct uptake mechanisms at low (0.001-0.01 mM) and at high (0.05-1.0 mM) chromate concentrations. After prolonged incubation with 1 mM chromate, the subsequent rate of uptake of chromate was decreased. It is suggested that the decreased uptake is due to a modification of the anion-transport protein by chromate.

Anion Transport Proteins↗
PubMed1986

Mechanism of DNA cleavage induced by sodium chromate(VI) in the presence of hydrogen peroxide.

Reactivities of chromium compounds with DNA were investigated by the DNA sequencing technique using 32P 5'-end-labeled DNA fragments, and the reaction mechanism was investigated by ESR spectroscopy. Incubation of double-stranded DNA with sodium chromate(VI) plus hydrogen peroxide or potassium tetraperoxochromate(V) led to the cleavage at the position of every base, particularly of guanine. Even without piperidine, the formation of oligonucleotides was observed, suggesting the breakage of the deoxyribose-phosphate backbone. ESR studies using hydroxyl radical traps demonstrated that hydroxyl radical is generated both during the reaction of sodium chromate(VI) with hydrogen peroxide and the decomposition of potassium tetraperoxochromate(V), and that hydroxyl radical reacts significantly not only with mononucleotides but also with deoxyribose 5-phosphate. ESR studies using a singlet oxygen trap demonstrated that singlet oxygen is also generated both by the same reaction and decomposition, and reacts significantly with deoxyguanylate, but scarcely reacts with other mononucleotides. Furthermore, ESR studies suggested that tetraperoxochromate(V) is formed by the reaction of sodium chromate(VI) with hydrogen peroxide. These results indicate that sodium chromate(VI) reacts with hydrogen peroxide to form tetraperoxochromate(V), leading to the production of the hydroxyl radical, which causes every base alteration and deoxyribose-phosphate backbone breakage. In addition, sodium chromate(VI) plus hydrogen peroxide generates singlet oxygen, which subsequently oxidizes the guanine residue. The mechanism by which both hydroxyl radical and singlet oxygen are generated during the reaction of sodium chromate(VI) with hydrogen peroxide was presented. Finally, the possibility that this reaction may be one of the primary reactions of carcinogenesis induced by chromate(VI) is discussed.

Chromates↗
PubMed1982

Chemical aspects on chromate in cement.

A certain amount of chromate can be added to calcium aluminate and calcium aluminate ferrite, 2 compounds present in clinker, without being extractable with water. Solutions of sulfate and the chemically related selenate and molybdate can extract about 2/3 of the added chromate. Also a certain amount of molybdate can be added to calcium aluminate without being extractable with water. About 2/3 of the added molybdate can be extracted with solutions of sulfate, selenate and chromate. Chromate present in clinker can partly be extracted with solutions of sulfate, selenate and molybdate. The present results indicate that chromate is partly present in crystals of calcium aluminate and calcium aluminate ferrite and that the correlation between chromate and sulfate in cement is due to an equilibrium between these two compounds inside and outside the crystals. The results also indicate that chromate, sulfate, selenate and molybdate can substitute each other. As cement always contains gypsum, which is partly water soluble, the possibility to make chromatefree cement is limited.

Aluminum↗
PubMed1994

Ontogenic changes in the nephrotoxicity of chromate correlate with the glutathione oxidoreduction system.

The role of GSH concentration and GSSG reductase activity in age differences in chromate nephrotoxicity was investigated. Young and adult rats were injected with 2 and 1 mg sodium chromate/100 g body weight (BW), respectively, which led to equal Cr concentrations in renal tissue. Cr nephrotoxicity was lower in young than in adult rats. It was shown that from 30 minutes after the chromate injection GSSG reductase activity in renal tissue was increased in adult but decreased in young rats by the chromate. GSSG reductase activity was increased in young rats by pretreatment with phenobarbital. The consequence was an enhancement of chromate nephrotoxicity as shown by proteinuria. Renal GSH concentration is lower in young rats and limiting for chromate reduction in vitro in these animals. Therefore, GSH concentration was increased by pretreatment with N-acetylcysteine, which enhanced chromate nephrotoxicity significantly. These results reflect the important role of the GSH oxidoreduction system in chromate nephrotoxicity and its relationship to age differences.

Acetylcysteine↗
PubMed1996

Visual hallucinosis: the major clinical determinant of distorted chromatic contour perception in Parkinson's disease.

Recently distorted chromatic contour perception has been demonstrated in Parkinson's disease (PD). The aim of our study is to determine the clinical factors which influence chromatic contour perception in PD. Chromatic and achromatic contour perception, colour discrimination and clinical data were evaluated in 73 patients with PD. We used a computer-aided method to determine the chromatic fusion time (CFT) which indicates the acuity of monochromatic contour perception. Chromatic CFT was generally shortened in patients as compared to controls (p < 0.01), whereas achromatic CFT was not significantly different. Variance analysis revealed the ability of colour discrimination and the risk of visual hallucinations as statistically significant (p < 0.05) variables influencing contour perception of certain stimuli. In contrast, disease stage, disease duration and disease severity have no relevant effect on chromatic contour perception in Parkinson's disease. On the basis of those properties one may suggest that distorted chromatic contour perception is due to an impairment at a central stage of visual processing in PD and an imbalance of the serotonergic system. Whether CFT is a reliable method to predict the individual risk of hallucinosis in PD has to be evaluated.

Aged↗
PubMed1994

Visual evoked potentials in three-dimensional color space: correlates of spatio-chromatic processing.

Visual evoked potentials (VEPs) were measured for sinusoidal gratings with spatio-chromatic modulation defined in a three-dimensional color space. The spatio-chromatic modulation of the gratings can be decomposed into contributions from an achromatic luminance varying component, an isoluminant component which modulates only the activities of L cones and M cones, and an isoluminant component corresponding to modulation of only S-cone activity. The emphasis of this report is the nature of VEPs resulting from isoluminant spatio-chromatic modulation. The VEP response was characterized along a number of spatial, temporal, and chromatic stimulus dimensions: contrast, spatial frequency, chromaticity in the isoluminant plane, chrominance/luminance ratio, orientation, and temporal frequency. Isoluminant VEPs resulting from stimuli modulating L and M cones are compared with those from S-cone modulation. When appropriate spatiotemporal conditions are employed, both types produce robust VEPs; however, the S-pathway VEPs show considerably longer latencies than do those from LM-pathway activation. The VEP results are compared to psychophysical and single unit electrophysiological observations. VEP latencies exhibit the lowpass character of psychophysical chromatic contrast sensitivity functions but VEP amplitudes show bandpass tuning along both the S and LM axes. An oblique effect, i.e. shorter latencies for horizontal and vertical gratings than for diagonal, is observed in the isoluminant VEP. S-pathway VEPs are used to demonstrate an electophysiological correlate of transient tritanopia. Normative amplitude and latency data for VEPs from selectivity stimulated chromatic mechanisms provide a baseline for clinical electrodiagnostic applications.

Adaptation, Ocular↗
PubMed1996

Color discrimination along the cardinal chromatic axes with VECPs as an index of function of the parvocellular pathway. Correspondence of intersubject and axis variations to psychophysics.

Chromatic information is carried only by the parvocellular pathway, giving the neurophysiologist the opportunity for eliciting specific responses. Further subdivision of the parvo chromatic system in two opponent chromatic mechanisms is potentially of great interest, given that the anatomical correlate seems to reside in subclasses of parvo ganglion cells that show differences both in size and in susceptibility to disease. We separately recorded responses arising from each chromatic opponent mechanism using visual stimuli chosen to belong to one of the "cardinal" chromatic axes. A calibrated color monitor, driven by a high resolution (14 bits/gun) computer board, was used for visualization of 1 c/deg isoluminant color gratings, sinusoidally modulated in time at 4 Hz. VECPs were recorded at several color contrasts along both cardinal axes, allowing extrapolation of contrast thresholds. Psychophysical thresholds were derived in the same stimulus conditions for comparison and found to correlate very well with the electrophysiologically derived values, both as intersubject and axis differences. The S-(L+M) opponent mechanism consistently yielded higher thresholds, smaller amplitude, and higher phase lag than the L-M mechanism. This finding was largely explained by the perceptual non-uniformity of the CIE chromaticity diagram. Correcting the VECP data for the perceptual differences yielded comparable responses, supporting the view that the two mechanisms are similarly represented in the cortex. In conclusion, recording of cortical responses to color contrast stimuli belonging to the cardinal chromatic axes seems a reliable procedure and may prove to be useful in performing clinical evaluations that refine the assessment of the physiology of the visual system.

Adult↗
PubMed2004

Chromatic induction from S-cone patterns.

Chromatic induction from patterned backgrounds depends on the spatial as well as the chromatic aspects of the background light. Color appearance with patterned and uniform backgrounds was compared using chromaticities distinguished by only the S cones; all backgrounds were equivalent to equal-energy white in terms of L-cone and M-cone stimulation. The measurements showed larger shifts in color appearance with a patterned chromatic background than with a uniform background at any chromaticity within the pattern. The measurements also showed that inducing light within different spatial regions could cause opposite shifts in color appearance: inducing light near a test field shifted appearance toward the inducing chromaticity (assimilation), while the same light some distance from the test shifted appearance away from the inducing chromaticity (simultaneous contrast). The shifts in color appearance were accounted for by a neural receptive field with S-cone spatial antagonism.

Color Perception↗
PubMed2000

Interactions between chromatic adaptation and contrast adaptation in color appearance.

Color appearance depends on adaptation processes that adjust sensitivity both to the average color in the stimulus (through light or chromatic adaptation) and to the variations in color (through contrast adaptation). We explored how these different forms of adaptation interact, by examining how the state of chromatic adaptation depends on the time-varying color contrasts in the stimulus, and conversely, how adaptation to the mean determines the stimulus contrasts underlying contrast adaptation. Light adaptation levels remain very similar whether observers adapt to a static chromaticity or to large temporal modulations in cone excitation that vary at rates of 0.5 Hz or higher. This suggests that up to the sites of light adaptation, the response to moderate contrasts is effectively linear and that the adaptation effectively averages over several seconds of the stimulus. For slower flicker rates color is differentially biased by the last half-cycle of the flicker, and perceived contrast may be altered by response polarization. This polarization selectively saturates responses to moderate (but not low) contrasts along the color direction complementary to the mean color bias, implying that the response changes occur within multiple mechanisms tuned to different chromatic axes. Chromatic adaptation often adjusts only partially to the mean color of the stimulus, and thus leaves a residual bias in the color appearance of the field. Contrast adaptation reduces perceived contrast relative to this residual color, and not relative to the stimulus that appears achromatic. Similarly, contrast discrimination thresholds appear lower around the residual color than around the achromatic point. Thus under biased states of chromatic adaptation alternative measures of 'zero contrast' can be dissociated, suggesting that they do not depend on a common null point within the channels encoding chromatic contrast.

Adaptation, Ocular↗
PubMed2001

Nonlocal interactions in color perception: nonlinear processing of chromatic signals from remote inducers.

The perceived color of an object depends on the chromaticity of its immediate background. But color appearance is also influenced by remote chromaticities. To quantify these influences, the effects of remote color fields on the appearance of a fixated 2 degrees test field were measured using a forced-choice method. Changes in the appearance of the test field were induced by chromaticity changes of the background and of 2 degrees color fields not adjacent to the test field. The appearance changes induced by the color of the background corresponded to a fraction of between 0.5 and 0.95 of the cone contrast of the background change, depending on the observer. The magnitude of induction by the background color was modulated on average by 7.6% by chromaticity changes in the remote color fields. Chromaticity changes in the remote fields had virtually no inducing effect when they occurred without a change in background color. The spatial range of these chromatic interactions extended over at least 10 degrees from the fovea. They were established within the first few hundred milliseconds after the change of background color and depended only weakly on the number of inducing fields. These results may be interpreted as reflecting rapid chromatic interactions that support robustness of color vision under changing viewing conditions.

Color Perception↗
PubMed2002

Chromatic input to motion processing in the absence of attention.

While several previous psychophysical and neurophysiological studies have demonstrated chromatic (red/green) input to motion processing, the nature of this input is still a matter of debate. In particular, recent controversy has developed regarding whether chromatic motion processing relies on lower-level processes [J. Neurosci. 14 (1994) 4854; 19 (1999) 6571] versus higher-level, attention- or salience-based mechanisms [Science 257 (1992) 1563; Proc. Natl. Acad. Sci. 96 (1999a) 8289; 96 (1999b) 15374]. In this study, we investigated the degree to which chromatic motion is influenced by attentional mechanisms by employing a dual-task paradigm, which allowed us to compare the strength of chromatic motion under conditions of poor versus full attention. Here, we found that for equiluminant red/green gratings, chromatic motion processing is as robust in poor, as in full, attention conditions. This lack of an attentional effect suggests that chromatic motion processing must rely, at least in part, on lower-level (i.e., pre-attentive) motion mechanisms. For non-equiluminant (e.g., red brighter than green) gratings, however, attention significantly modulates chromatic motion strength. Possible explanations for this latter result are discussed in the context of inherent salience differences between the bright-red and dim-green stripes of the heterochromatic grating.

Attention↗
PubMed2002

Interactions between chromatic- and luminance-contrast-sensitive stereopsis mechanisms.

It is well known that chromatic information can assist in solving the stereo correspondence problem. It has also been suggested that there are two independent first-order stereopsis mechanisms, one sensitive to chromatic contrast and the other sensitive to luminance contrast (Vision Research 37 (1997) 1271). Could the effect of chromatic information on stereo correspondence be subserved by interactions between these mechanisms? To address this question, disparity thresholds (1/stereoacuity) were measured using 0.5 cpd Gabor patches. The stimuli possessed different relative amounts of chromatic and luminance contrast which could be correlated or anti-correlated between the eyes. Stereoscopic performance with these compound stimuli was compared to that with purely isoluminant and isochromatic stimuli at different contrasts. It was found that anti-correlated chromatic contrast severely disrupted stereopsis with achromatic stimuli and that anti-correlated luminance contrast severely disrupted stereopsis with chromatic stimuli. Less dramatic, but still significant, was the improvement in stereoacuity obtained using correlated colour and luminance contrast. These data are consistent with there being positive and negative interactions between chromatic and achromatic stereopsis mechanisms that take place after the initial encoding of disparity information, but before the extraction of stereoscopic depth. These interactions can be modelled satisfactorily assuming probability summation of depth sign information between independent mechanisms.

Color Perception↗
PubMed2002

The perceived speed of drifting chromatic gratings is mechanism-dependent.

The perceived speed of chromatic motion was investigated for gratings that stimulate each chromatic mechanism [L-M and S-(L+M)] in isolation and for gratings that stimulate both chromatic systems. The observers' task consisted of adjusting the speed of a drifting achromatic grating to match the perceived speed of an isoluminant chromatic grating, drifting at 8 deg/s (temporal frequency of 4 Hz). Every observer reported a substantial decrease in perceived speed for chromatic gratings modulated along the S-(L+M) (blue-yellow) cardinal axis compared to other directions in color space. One observer even reported motion standstill for gratings modulated along this axis. Further testing demonstrates that the perceived speed of an isoluminant chromatic grating depends solely on the extent to which it stimulates the L-M (red-green) mechanism. Thus, under the conditions that were tested, the S-(L+M) postreceptoral mechanism does not appear to contribute significantly to determining the perceived speed of chromatic motion.

Color Perception↗