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At least 19 recordsLinked to original sources

PubMed2004

Mechanism of chromate reduction by the Escherichia coli protein, NfsA, and the role of different chromate reductases in minimizing oxidative stress during chromate reduction.

Chromate [Cr(VI)] is a serious environmental pollutant, which is amenable to bacterial bioremediation. NfsA, the major oxygen-insensitive nitroreductase of Escherichia coli, is a flavoprotein that is able to reduce chromate to less soluble and less toxic Cr(III). We show that this process involves single-electron transfer, giving rise to a flavin semiquinone form of NfsA and Cr(V) as intermediates, which redox cycle, generating more reactive oxygen species (ROS) than a divalent chromate reducer, YieF. However, NfsA generates less ROS than a known one-electron chromate reducer, lipoyl dehydrogenase (LpDH), suggesting that NfsA employs a mixture of uni- and di-valent electron transfer steps. The presence of YieF, ChrR (another chromate reductase we previously characterized), or NfsA in an LpDH-catalysed chromate reduction reaction decreased ROS generation by c. 65, 40, or 20%, respectively, suggesting that these enzymes can pre-empt ROS generation by LpDH. We previously showed that ChrR protects Pseudomonas putida against chromate toxicity; here we show that NfsA or YieF overproduction can also increase the tolerance of E. coli to this compound.

Amino Acid Sequence↗

PubMed1999

Chromatic induction with remote chromatic contrast varied in magnitude, spatial frequency, and chromaticity.

Chromatic induction from a surround is attenuated by chromatic contrast within a remote region outside of the surround (Shevell & Wei, 1998, Vision Research, 38, 1561-1566). The present study reports hue-cancellation measurements that show the attenuation depends on the magnitude, spatial frequency and chromaticity of remote chromatic contrast. Spatial-frequency tuning is shown by maximal attenuation of induction with remote contrast elements of the same size as the test. Experiments with various chromaticities of remote contrast show that S-cone stimulation within the remote region has a much weaker effect than L-/M-cone chromatic contrast, and does not depend on whether the S-cone stimulation is uniform or uneven across the region. Overall, the results show that remote L/M contrast affects classical chromatic induction, with its effect depending on the spatial frequency and magnitude of contrast. The influence of remote S-cone stimulation, on the other hand, is relatively weak and depends on only the S-cone spatial average, at least when S-cone stimulation by the test and its immediate surround is minimal (as in all experiments here).

Color Perception↗

PubMed1988

Transformation of C3H/10T1/2 mouse embryo cells to focus formation and anchorage independence by insoluble lead chromate but not soluble calcium chromate: relationship to mutagenesis and internalization of lead chromate particles.

The genotoxicity of soluble and insoluble hexavalent chromium compounds was studied in mammalian cell assays which detect base substitution, deletion, addition, and frameshift mutations [6-thioguanine resistance in Chinese hamster ovary cells], primarily base substitution mutations [ouabain resistance in Chinese hamster ovary and C3H/10T1/2 Cl 8 mouse embryo fibroblasts (10T1/2)] and morphological transformation [focus formation] in 10T1/2 cells. Soluble hexavalent CaCrO4, administered in either acute (5-h) or subacute (24-h) dosing regimens, induced dose-dependent cytotoxicity and mutation to 6-thioguanine resistance in Chinese hamster ovary cells but no mutation to ouabain resistance or focus formation in transformation assays, although the acute treatment induced a high frequency of conversion of 10T1/2 cells to adipocytes. Cell lines established from cloned adipocytic cells were not morphologically transformed and did not grow in soft agarose. PbCrO4 did not induce mutation to either 6-thioguanine or ouabain resistance but did induce a reproducible dose-dependent, low frequency of focus formation in 10T1/2 cells. Cell lines established from PbCrO4-induced foci stably formed foci when coseeded with 10T1/2 cells, had 3-5-fold increased saturation densities relative to nontransformed 10T1/2 cells, and formed colonies in soft agarose, indicating their likelihood to be neoplastic. Long term exposure of 10T1/2 cells to either CaCrO4 or PbCl2, even at 85% cytotoxic concentrations, or pretreatment of cells with either CaCrO4 or PbCl2 followed by treatment with the alternate compound, did not induce morphological transformation. Treatment of cells with insoluble hexavalent PbCrO4 resulted in progressive and extensive vacuolization of cells in contact with the particles. Progressive cytoplasmic engulfment of PbCrO4 particles was observed using scanning electron microscopy, although PbCrO4 particles were not observed inside vacuoles. These results indicate that the soluble clastogens K2Cr2O7 and CaCrO4 were probably mutagenic by a non-base substitution mechanism but could not transform 10T1/2 cells. In contrast, PbCrO4 was not detectably mutagenic but induced transformation, which could not be explained solely by acute or chronic exposure to dissolution products of either lead or chromate alone. Since PbCrO4 particles were found to be intracytoplasmic in extensively vacuolated cells, we suggest that the unique physiochemical properties of PbCrO4 particles, leading to their internalization and the resultant associated cellular stress response, may be related to the transformation induced by this compound.

Adipose Tissue↗

PubMed1992

Plasmid chromate resistance and chromate reduction.

Compounds of hexavalent chromium (chromates and dichromates) are highly toxic. Plasmid genetic determinants for chromate resistance have been described in several bacterial genera, most notably in Pseudomonas. Resistance to chromate is associated with decreased chromate transport by the resistant cells. The genes for a hydrophobic polypeptide, ChrA, were identified in chromate resistance plasmids of Pseudomonas aeruginosa and Alcaligenes eutrophus. ChrA is postulated to be responsible for the outward membrane translocation of chromate anions. Widespread bacterial reduction of hexavalent chromate to the less toxic trivalent chromic ions is also known. Chromate reduction determinants have not, however, been found on bacterial plasmids or transposons. In different bacteria, chromate reduction is either an aerobic or an anaerobic process (but not both) and is carried out either by soluble proteins or by cell membranes. Chromate reduction may also be a mechanism of resistance to chromate, but this has not been unequivocally shown.

Amino Acid Sequence↗

PubMed2000

Infant color vision: sharp chromatic edges are not required for chromatic discrimination in 4-month-olds.

In our previous demonstrations of chromatic discrimination in infants, we have used test and surround fields of different chromaticities that abutted each other at sharp chromatic edges. In order to see whether sharp chromatic edges are necessary for infants to make chromatic discriminations, 16-week-old infants were tested with three stimulus configurations in which sharp chromatic edges were eliminated. The three edge manipulations involved black borders, a dark surround, or blurred edges around the chromatic test field. In each case red, green, and violet test fields were used. Although performance decreased when sharp chromatic edges were eliminated, observers' percent correct scores remained clearly above chance for eight of the nine discriminations (three colors x three edge manipulations). We argue that all three edge manipulations reduce the likelihood of mediation of chromatic discrimination by M (magnocellular) cells. These data thus provide evidence that young infants have functional P (parvocellular) pathways, and use them for making chromatic discriminations.

Color Perception↗

PubMedDate not supplied

Spatial and temporal chromatic contrast: Effects on chromatic discrimination for stimuli varying in L- and M-cone excitation.

Discrimination for equiluminant chromatic stimuli that vary in L- and M-cone excitation depends on the chromaticity difference between the test field and the surrounding area. The current study investigated the effect of the proximity in space and time of a surround to the test field on chromatic contrast discrimination. The experimental paradigm isolated spatial, temporal, and spatial-and-temporal chromatic contrast effects on discrimination. Chromatic contrast discrimination thresholds were assessed by a four-alternative spatial forced-choice procedure. Stimuli were either metameric to the equal energy spectrum, or varied in L-cone activation along a line of constant S-cone activation. A model based on primate parvocellular pathway physiology described the data. Spatial and temporal contrast produced equivalent reductions in chromatic discriminability as the chromatic difference between the test and surround increased. For all test chromaticities, discrimination was best in the absence of chromatic contrast. Chromatic contrast discrimination is determined by either the spatial or temporal contrast component of the signal.

Color Perception↗

PubMed1977

The significance of chromate ingestion in patients allergic to chromate.

A double-blind study was carried out to demonstrate the significance of chromate ingestion in the persistence of chromic chromate dermatitis. Each of 31 chromate-allergic patients was given one tablet containing 7.1 mg potassium dichromate, plus a placebo tablet. The dermatitis of 11 of the 31 patients flared after the ingestion of chromate, but not after placebo. 3 patients had equivocal reactions to both tablets; the dermatitis worsened in 2 patients following ingestion of the placebo but not after the chromate tablet. No statistically significant difference could be found between reactivity to chromate and eczema of dyshidrotic morphology. Nor was it possible to correlate the degree of patch test reactivity to the reaction following chromate ingestion. It is concluded that a low-chromate diet is of value in the management of patients with chronic chromate dermatitis.

Administration, Oral↗

PubMed1989

[Chromatic study of composite resins. Report 1. The influence of the color measuring instruments and the background colors on the chromatic values].

The purpose of this study was to investigate the influence of the structural difference of the color measuring instruments and the background colors on the chromatical values of the composite resins. Composite resin disks which were 10 mm in diameter and 1 mm and 2 mm in thickness, and made by the light-cured composite resin Silux Plus (3M Co.) were chromatically measured. The colors of the composite resin used in this study were universal, yellow, dark-yellow and light. The color measuring instruments used in this study were the Murakami Color Research Laboratory Fast Spectrophotometer CMS-500 used with the Flexible Sensor FS-1, FS-2 and FS-3, and the Murakami Color Research Laboratory Color Difference Meter CD-270 used with the Improved Light Guide CD-270. The diameters of the head which receives the light from the object were: FS-1 (2 mm), FS-2 (3 mm), FS-3 (5 mm) and Improved Light Guide CD-270 (2 mm). The chromatical values of the resin specimens were measured without back color, backed by a white plate and backed by a black plate. The chromatical values of the composite resin disks were measured with the different color measuring instruments under the different background colors. The XYZ, L*a*b* and xy values were compared and the following results were obtained. 1) The chromatical XYZ, L*a*b* and xy values of the composite resins showed different value patterns among the color measuring instruments. 2) Different influences of the background colors on the chromatical values of the composite resins were obtained among the different colors and different thicknesses of the resins in the cases of some color measuring instruments were used. 3) The most precise color measuring instrument for chromatical measuring of the composite resins among the instruments used in this study was the Fast Spectrophotometer CMS-500 used with the FS-3. 4) Prior to the comparison of the chromatical values measured by the different color measuring instruments, the chromatical values obtained by measurement with the same specimens should be compared and the structural differences, the precision and the sensitivities of the instruments should be understood.

Color↗

PubMed1999

Chromate efflux by means of the ChrA chromate resistance protein from Pseudomonas aeruginosa.

Everted membrane vesicles of Pseudomonas aeruginosa PAO1 harboring plasmid pCRO616, expressing the ChrA chromate resistance protein, accumulated four times more (51)CrO(4)(2-) than vesicles from plasmidless cells, indicating that a chromate efflux system functions in the resistant strain. Chromate uptake showed saturation kinetics with an apparent K(m) of 0.12 mM chromate and a V(max) of 0. 5 nmol of chromate/min per mg of protein. Uptake of chromate by vesicles was dependent on NADH oxidation and was abolished by energy inhibitors and by the chromate analog sulfate. The mechanism of resistance to chromate determined by ChrA appears to be based on the active efflux of chromate driven by the membrane potential.

Bacterial Proteins↗

PubMedDate not supplied

Chromate allergy: total chromium and hexavalent chromate in the air.

This is a study on atmospheric concentration of total chromium and hexavalent chromate and its role in chromate sensitivity. Air concentration of total chromium and hexavalent chromate in a construction factory, a busy city area, a suburban area, a residential area, and a heavy industrial area were measured by air sampling pumps. Hexavalent chromate was not detected in any sampled areas. Two (concreting areas) of 8 locations in the construction factory had total chromium of 0.2 and 2.3 micrograms/m3 in the atmosphere. It appeared that the atmospheric concentration of total chromium and hexavalent chromate was negligible. These findings indicate that unexplained chromate sensitivity, as so often seen in patients attending a contact dermatitis clinic, is not attributable to exposure to hexavalent chromate in the air.

Chromates↗

PubMed1989

The interactions between chromatic aberration, defocus and stimulus chromaticity: implications for visual physiology and colorimetry.

It has long been recognised that chromatic aberration can introduce luminance artifacts into nominally isoluminant colour stimuli. In this study the effects of chromatic aberration (along with those of defocus and stimulus spatial frequency) on the chromaticity of the retinal image are considered. Such optical effects have important methodological and functional implications for visual physiology. The "Silent Substitution" principle is a fundamental feature of modern colorimetry, being employed in both psychophysical and electrophysiological approaches to the visual system. The theoretical colour spaces introduced by MacLeod and Boynton (1979) and Derrington et al. (1984) are also ultimately based on this principle. All such applications of the silent substitution principle are sensitive to the optical effects of chromatic aberration, defocus, spatial frequency and stimulus chromaticity. The spatial acuity of the mechanisms of colour vision are appreciably lower than those of the luminance system (Mullen, 1985). In addition chromatic aberration has been shown to be a cue to ocular accommodation (Fincham, 1951). The analysis presented in this study suggests a possible explanation for these findings in terms of the ecological and computational constraints placed on the visual system by chromatic aberration.

Color Perception↗

PubMedDate not supplied

Chromatic input to cells of the magnocellular pathway: mean chromaticity and the relative phase of modulated lights.

If the relative phase of red and green modulated lights is changed, at low temporal frequencies the response of cells of the magnocellular (MC) pathway has been found to be minimal not to counterphase, chromatic modulation (as expected of a luminance mechanism) but shifted to some phase intermediate between luminance and chromatic modulation. The results could only be modeled by assuming interaction between achromatic and chromatic inputs to MC cells. The 'phase shift' resembled that seen with psychophysical threshold measurements using the same stimuli. Psychophysical results also showed that the phase shift is dependent on the chromaticity of a background. The results reported here show that the direction of the phase shift in MC cells is reversed by changing the background from long to short wavelengths and is consistent with psychophysical observations. Cell behavior was again modeled by assuming vector summation of achromatic and chromatic inputs. The reversal of phase-shift direction requires a reversal in polarity of the chromatic input. The underlying physiological mechanism may involve summation of chromatic signals of opposite polarity; if the relative size of these signals depends on the background, this may determine the direction of phase shift.

Animals↗

PubMedDate not supplied

Interactions of chromaticity and luminance in edge identification depend on chromaticity.

The goal of this work was to study interactions of chromaticity and luminance in edge identification. Two horizontal spatial sawtooth patterns, one with positive and the other with negative harmonics, were compared in a two-alternative forced-choice (2-AFC) procedure. The observer identified which pattern had sharp upper or lower edges. The fundamental frequency was 2 cycles/deg (cpd), with 5 cycles presented in a 2.5-deg square field. The pattern was presented as a 1-s raised temporal cosine, replacing part of an 8-deg background. Stimuli were specified in a cone troland (l, s, Y) chromaticity space, with correction for individual equiluminance at a nominal 115 td, and individual tritan direction. A preliminary set of interleaved staircases established edge identification for the six directions of the (l, s, Y) space. Three compound stimuli combining two orthogonal directions were chosen and included with the end-points in five randomly interleaved staircases. For combinations of Y with l-chromaticity, or l- with s-chromaticity, probability summation was observed. Combinations of Y with s-chromaticity revealed opponency. Data for +s, +Y and -s, -Y were subadditive; data for +s, -Y and -s, +Y were additive. Control studies using detection rather than edge identification revealed probability summation for all combinations. Luminance edges did not enhance stimuli with l-chromaticities. There was an interaction of luminance edges with s-chromaticities. Dim "blues" and bright "yellows" showed linear summation. Bright "blues" and dim "yellows" showed opponency.

Choice Behavior↗

PubMed2001

Chromate reduction by a pseudomonad isolated from a site contaminated with chromated copper arsenate.

A pseudomonad (CRB5) isolated from a decommissioned wood preservation site reduced toxic chromate [Cr(VI)] to an insoluble Cr(III) precipitate under aerobic and anaerobic conditions. CRB5 tolerated up to 520 mg of Cr(VI) liter(-1) and reduced chromate in the presence of copper and arsenate. Under anaerobic conditions it also reduced Co(III) and U(VI), partially internalizing each metal. Metal precipitates were also found on the surface of the outer membrane and (sometimes) on a capsule. The results showed that chromate reduction by CRB5 was mediated by a soluble enzyme that was largely contained in the cytoplasm but also found outside of the cells. The crude reductase activity in the soluble fraction showed a K(m) of 23 mg liter(-1) (437 microM) and a V(max) of 0.98 mg of Cr h(-1) mg of protein(-1) (317 nmol min(-1) mg of protein(-1)). Minor membrane-associated Cr(VI) reduction under anaerobiosis may account for anaerobic reduction of chromate under nongrowth conditions with an organic electron donor present. Chromate reduction under both aerobic and anaerobic conditions may be a detoxification strategy for the bacterium which could be exploited to bioremediate chromate-contaminated or other toxic heavy metal-contaminated environments.

Aerobiosis↗

PubMed1984

Lung cancer mortality among workers making lead chromate and zinc chromate pigments at three English factories.

Lung cancer mortality among 1152 men working at three English chromate pigment factories was studied from the 1930s or 1940s until 1981. Workers at factory C were exposed only to lead chromate and experienced normal mortality (Obs/Exp deaths 7/6.45). Workers at factories A and B were exposed to both lead and zinc chromate; mortality was normal among those who had only low exposure (O/E 7/6.95). For workers with high or medium exposure lung cancer mortality was significantly raised among men remaining at least a year after entering service at factory A during 1932-54 (O/E 21/9.45) and at factory B during 1948-67 (O/E 11/2.50). At factory A, 1933-46 entrants staying only 3-11 months were not affected (O/E 6/5.04) and 1955-63 entrants also appeared unaffected (O/E 2/2.00); working conditions there improved in 1955. The hazard at factories A and B affected workers who left after one year as well as those with longer service, and latent intervals were unusually short. The results indicate that moderate or heavy exposure to zinc chromate may give rise to a severe risk of developing lung cancer, but that exposure which is relatively mild or lasts less than a year may not constitute an effective risk. The results provide no indication that lead chromate induces lung cancer in man, even under conditions conducive to lead poisoning.

Adult↗

PubMed1989

Dependence of the chromatic valence function on chromatic standards.

The red, green, yellow, and blue curve portions of chromatic valence functions (CVFs) were measured by an iso-cancellation technique using three or four different chromatic standards (cancellation stimuli of fixed intensity and chromaticity) for each of the four curve portions. Curve shapes of the red, green, and yellow CVF portions varied with different chromatic standards, while that of blue did not. These results of chromatic standard dependency in CVFs are discussed with respect to opponent equilibria in a color mixture space.

Adult↗

PubMed1993

Chromatic discrimination with variation in chromaticity and luminance: data and theory.

Boynton and Kambe developed a model of chromatic discrimination in which thresholds are mediated by two independent mechanisms: the short-wavelength sensitive (S-) cones (S-cone axis), and the middle-wavelength sensitive (M-) and long-wavelength sensitive (L-) cones (M/L-cone axis). In this study, we used a Maxwellian view optical system to investigate fundamental properties of the model as a function of chromaticity and luminance. We confirmed that discriminations along the S-cone axis were dependent on S-cone excitation level. However, changes in chromaticity and changes in mean luminance were not described by a single threshold-vs-radiance (TVR) template. We developed a model to account for the different effects of changing S-cone excitation by varying mean chromaticity and by varying mean luminance. M/L-cone discriminations showed a minimum at the L-cone excitation to white, indicating strong opponency. The thresholds increased with luminance approaching a Weber region and showing parallel functions for differing chromaticities. These data are fit by a model allowing retinal gain controls and spectral opponency.

Adult↗

PubMed1989

The gap effect revisited: slow changes in chromatic sensitivity as affected by luminance and chromatic borders.

Chromatic discrimination was studied with two half-fields that were either precisely juxtaposed or were separated by a narrow gap. When present, the gap was in some conditions filled with light isoluminant to the test fields. When the fields were juxtaposed, chromatic sensitivity declined with viewing duration. For a discrimination based solely on S cone activity, separating steadily-viewed fields by either a luminance or a purely chromatic gap caused similar enhancements of sensitivity. Neither type of gap had much effect when the fields were flashed. The results may be interpreted as showing that either a luminance or chromatic contour can spatially delimit the two half-fields, thus preventing a slow spatial integration from reducing the discriminability of the two sides of the field.

Adult↗