Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Reversibility”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35Linked to original sources
PubMed1981

Reversal of glyphosate inhibition of carrot cell culture growth by glycolytic intermediates and organic and amino acids.

Various cytokinins and purines were ineffective in reversing glyphosate (0.25 millimolar)-induced growth inhibition of carrot (Daucus carota L.) cell suspension cultures. Aspartate was particularly effective in reversing glyphosate inhibition, but asparagine and various combinations of lysine, methionine, threonine, and homoserine (eventual products of aspartate metabolism) were not effective. When organic acids of the tricarboxylic acid cycle were added to the medium, particularly good reversal of inhibition could be obtained with alpha-ketoglutarate, succinate, and malate. Citrate provided only moderate reversal but the reversal given by glutamate was comparable to that of aspartate and the more effective tricarboxylic acid cycle intermediates. Pyruvate was somewhat toxic to cells when added early in the cell cycle but was most effective at reversing glyphosate inhibition when added at this time. If pyruvate addition was delayed, it was less toxic but was also a less effective reversing agent for glyphosate inhibition.All of the effective reversing agents for glyphosate inhibition found in this study can serve either directly or indirectly as carbon skeletons for respiration and ammonia assimilation and have previously been shown to be effective detoxifying agents for ammonia in cell culture systems. The results of this study suggest that glyphosate inhibition of growth in this system may be due to depletion of respiratory substrate which may eventually result in ammonia accumulation.

Journal Article↗
PubMed1990

Reversal of methotrexate cytotoxicity to human bone marrow cells and leukemic K562 cells by leucovorin: methotrexate polyglutamates formation as a possible important factor.

Methotrexate (MTX) is metabolized intracellularly to MTX-polyglutamates (MTX-PGs), which markedly inhibit several folate-dependent enzymes. Polyglutamation defect, therefore, is one of the important factors in drug resistance. In this study, reversal of MTX cytotoxicity by l-leucovorin (l-LV) was investigated using normal human bone marrow granulocyte progenitor cells (G-CFCs), and MTX-sensitive and -resistant leukemic K562 cell lines; the latter showed diminished polyglutamation. Cytotoxicity of 10(-7)M MTX to G-CFCs was completely reversed by an equimolar concentration of l-LV, but with higher MTX concentrations, relatively more l-LV was required. The reversal of MTX cytotoxicity by l-LV was more effective against bone marrow cells than MTX-sensitive K562 cells; this reversal seemed to be correlated to the total intracellular MTX levels as well as MTX-PG formation (low in bone marrow cells and high in K562 cells). When MTX-sensitive and -resistant K562 cells were incubated with MTX under conditions in which the total intracellular MTX levels of both cells were similar, successful reversal of MTX toxicity by l-LV was demonstrated in MTX-resistant cells, but not in MTX-sensitive cells, suggesting that an increase of MTX-PG formation in MTX-sensitive cells may explain the failure of l-LV to overcome MTX cytotoxicity. In addition to competitive reversal of MTX cytotoxicity by LV, noncompetitive reversal relating to variable formation of MTX-PGs is suggested to be another important factor in the mechanism of the reversal of MTX cytotoxicity by LV.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed2003

Reversal of the resistance to STI571 in human chronic myelogenous leukemia K562 cells.

STI571, an Abl-specific tyrosine kinase inhibitor, selectively kills Bcr-Abl-containing cells in vitro and in vivo. However, some chronic myelogenous leukemia (CML) cell lines are resistant to STI571. We evaluated whether STI571 interacts with P-glycoprotein (P-gp) and multidrug resistance protein 1 (MRP1), and examined the effect of agents that reverse multidrug resistance (MDR) on the resistance to SI571 in MDR cells. STI571 inhibited the [(125)I]azidoagosterol A-photolabeling of P-gp, but not that of MRP1. K562/MDR cells that overexpress P-gp were 3.67 times more resistant to STI571 than the parental Philadelphia-chromosome-positive (Ph +) CML K562 cells, and this resistance was most effectively reversed by cepharanthine among the tested reversing agents. The concentration of STI571 required to completely inhibit tyrosine phosphorylation in K562/MDR cells was about 3 times higher than that in K562 cells, and cepharanthine abolished the difference. In KB-G2 cells that overexpress P-gp, but not Bcr-Abl, 2.5 micro M STI571 partly reversed the resistance to vincristine (VCR), paclitaxel, etoposide (VP-16) and actinomycin D (ACD) but not to Adriamycin (ADM) or colchicine. STI571 increased the accumulation of VCR, but not that of ADM in KB-G2 cells. STI571 did not reverse resistance to any agent in KB/MRP cells that overexpress MRP1. These findings suggest that STI571 is a substrate for P-gp, but is less efficiently transported by P-gp than VCR, and STI571 is not a substrate for MRP1. Among the tested reversing agents that interact with P-gp, cepharanthine was the most effective agent for the reversal of the resistance to STI571 in K562/MDR cells. Furthermore, STI571 itself was a potent reversing agent for MDR in P-gp-expressing KB-G2 cells.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1990

Enzyme kinetics in reversed micelles. 2. Behaviour of enoate reductase.

Enoate reductase (EC 1.3.1.31) can stereospecifically reduce a variety of alpha,beta-unsaturated carboxylates. Its use was extended to apolar media by incorporating the enzyme into a reversed micellar medium. The kinetics of the enzyme in such a medium have been investigated using 2-methylbutenoic acid as substrate and NADH as a cofactor and compared with the reaction rates in aqueous solution. In aqueous solution the enzyme obeys a ping pong mechanism [Bühler et al. (1982) Hoppe-Seyler's Z. Physiol. Chem 363, 609-625]. In 50 mM Hepes pH = 7.0 with ionic strength of 0.05 M the Michaelis constants for NADH and 2-methylbutenoic acid are 20 microM and 6.0 mM respectively. In reversed micelles the kinetics of the reaction (Michaelis constant, maximum velocity as well as inhibitory effects) were markedly different. The rate of the enzymatic reaction of enoate reductase was studied using various concentrations of 2-methylbutenoic acid and various NADH concentrations. In reversed micelles composed of the anionic detergent sodium di(ethylhexyl)sulphosuccinate, the enzymatic reaction deviates substantially from the values in aqueous solution. Using our model (see preceding paper in this issue of the journal), all kinetics could be explained as evolving from enclosure in reversed micelles without any change in the intrinsic rate parameters of the enzyme. So the enzyme itself is unaffected by incorporation in reversed micelles, but the rate of intermicellar exchange as well as the microheterogeneity of the medium, resulting in very high local concentrations of the substrate, are the most important factors altering the reaction pattern. The effect of the composition of the reversed micellar medium was also investigated using either a nonionic or a cationic surfactant. In these solutions too, exchange and microheterogeneity of the medium proved to be the most important parameters influencing the enzymatic reaction. In all reversed micellar solutions inhibition by the enoate was observed at an overall concentration of 0.5-5 mM, implying that a concentration of substrate equal to the Km value in aqueous solution may already cause inhibition in reversed micelles. At this level no inhibition by NADH was observed. The microheterogeneity of the medium also explains this inhibition of the enzyme at relatively low 2-methylbutenoic acid concentrations.

Colloids↗
PubMed1990

The behavior of proteases in lecithin reverse micelles.

Reverse micelles, formed in isooctane/alcohol by phosphatidylcholines of variable chain length (i.e. 6, 7 or 8 C atoms in the fatty acid moiety) have been studied, mostly in relation to their capability of solubilizing trypsin and alpha-chymotrypsin. It has been found that the capability of the lecithin reverse micellar systems to solubilize water is strongly affected by the chain length of the alkyl group and by the alcohol used as co-surfactant. The C8-lecithin system, i.e. 1,2-dioctanoyl-sn-glycero-3-phosphocholine, in isooctane/hexanol is the system which affords the maximal solubilization of water (up to wo 60, where wo = [H2O]/[lecithin]) and of the enzymes. The water of the water pool of lecithin reverse micelles has been investigated by 1H-NMR; the proton chemical shift as a function of wo was found to be similar to the case of reverse micelles formed by the well known negatively charged surfactant sodium bis(2-ethylhexyl sulfosuccinate). 31P-NMR studies show that the ionization behavior of phosphate groups is similar to that in bulk water, suggesting no anomaly in the pH behavior of this water pool. The stability of trypsin and alpha-chymotrypsin in the various lecithin reverse micellar system is similar and occasionally better than that in aqueous solution. The same holds for the kinetic behavior (kcat and Km have been determined for a few systems). The bell-shaped curve of the pH/activity profile in lecithin reverse micelles is, for both enzymes, shifted towards more alkaline values with respect to water. Bell-shaped curves are also obtained when studying the influence of wo on the enzyme activity, with an optimal wo which is in the range 7-10, a surprisingly small value considering that we are dealing with hydrolases. Circular dichroic studies have been carried out in order to correlate the activity with the protein conformation: for both enzymes, generally no marked perturbations appear as a consequence of the solubilization in the lecithin reverse micelles, but conditions can be found under which significant alterations are present. Certain properties of the two enzymes, which in water solution are very similar, become sharply different in reverse micelles, showing that occasionally the micellization is able to enhance the relatively small structural differences between the two proteins.

Chymotrypsin↗
PubMed1992

Reversed micelles of polymeric surfactants in nonpolar organic solvents. A new microheterogeneous medium for enzymatic reactions.

A new microheterogeneous non-aqueous medium for enzymatic reactions, based on reversed micelles of a polymeric surfactant, was suggested. The surfactant termed CEPEI, was synthesized by successive alkylation of poly(ethyleneimine) with cetyl bromide and ethyl bromide and was found to be able to solubilize considerable amounts of water in benzene/n-butanol mixtures. The hydrodynamic radius of polymeric-reversed micelles was estimated to be in the range 22-51 nm, depending on the water content of the system, as determined by means of the quasi-elastic laser-light scattering. Polymeric reversed micelles were capable of solubilizing enzymes (alpha-chymotrypsin and laccase) in nonpolar solvents with retention of catalytic activity. Due to the strong buffering properties of CEPEI over a wide pH range, it could maintain any adjusted pH inside hydrated reversed micelles. It was found that catalytic behavior of enzymes entrapped in polymeric reversed micelles was rather insensitive to the pH of the buffer solution introduced into the system as an aqueous component, but determined mostly by acid-base properties of the polymeric surfactant itself. Both catalytic activity and stability of entrapped alpha-chymotrypsin and laccase were found to increase with increasing water content of the system. Under certain conditions, the entrapment of alpha-chymotrypsin into CEPEI reversed micelles resulted in a considerable increase in catalytic activity and stability as compared to aqueous solution. CEPEI reversed micelles were demonstrated to be promising enzyme carriers for use in membrane reactors. Owing to the large dimensions of CEPEI reversed micelles, they are effectively kept back by a semipermeable membrane, thus allowing an easy separation of the reaction product and convenient recovery of the enzyme.

Buffers↗
PubMed1996

The effect of linoleic acid on pH inside sodium bis(2-ethylhexyl)sulfosuccinate reverse micelles in isooctane and on the enzymic activity of soybean lipoxygenase.

The effective pH of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles (pHrm), containing buffers of different pH (pHst) and various amounts of linoleic acid, was studied within the range of compositions used to study the activity of soybean lipoxygenase in reverse micelles. Significant shifts of pHrm versus pHst were observed for the solutions of relatively higher pHst, dependent on linoleic acid and buffer concentrations. The effect diminished as pHst became closer to 7. When low-ionic-strength buffers were added to AOT solutions in isooctane, a significant buffering effect of linoleic acid in reverse micelles was observed. Solubilization of > 3 mM linoleic acid in micellar solutions containing 25 mM buffers gave the observed pHrm values almost independent of pHst. This effect diminished with the ionic strength of the buffering solution, but did not vanish even at 200 mM buffer. The observed effects result from the balance between ionization of linoleic acid and its partition between the water pool and the micellar interface. The enzymic activity of soybean lipoxygenase in the AOT reverse micellar solutions of the determined pHrm values was also studied. A significant reduction of the kinetics of the enzymic activity was observed, for all studied reverse micellar solutions. Changes of pHrm, caused by the presence of acidic substrate (linoleic acid) do not explain the observed reduction of activity directly through the effect on the enzyme. Due to unfavourable partition of the substrate between the microphases present in the systems, enhanced by reduction of pH at higher total concentrations of linoleic acid, the saturation of the enzyme with the substrate was not observed in the system and is difficult to attain experimentally in reverse micelles. A shift of the lipoxygenase activity/pHrm profile but negligible shift of the activity/pHst profile, with respect to aqueous buffer solutions, were observed. This indicates that either the information given by pH indicators used does not reveal the true pH of the enzyme in these reverse micelles, or that the sample of the enzyme is pH-insensitive over a broader pH range than results from the relationship observed for linoleic acid in aqueous solutions. The latter conclusion is consistent with the data on lipoxygenase activity towards linoleyl sulfate in aqueous solutions [Bild, G. S., Ramadoss, C. S. & Axelrod, B. (1977) Lipids 12, 732-735]. Conditions for measuring pH-independent lipoxygenase activity in reverse micelles are discussed.

Dioctyl Sulfosuccinic Acid↗
PubMed1991

Axonal transport reversal of acetylcholinesterase molecular forms in transected nerve.

Reversal of anterograde rapid axonal transport of four molecular forms of acetylcholinesterase (AChE) was studied in chick sciatic nerve during the 24-h period following a nerve transection. Reversal of AChE activity started approximately 1 h after nerve transection, and all the forms of the enzyme, except the monomeric ones, showed reversal of transport. The quantity of enzyme activity reversed 24 h after transection was twofold greater than that normally conveyed by retrograde transport. We observed no leakage of the enzyme at the site of the nerve transection and no reversal of AChE activity transport in the distal segment of the severed nerve, a result indicating that the material carried by retrograde axonal transport cannot be reversed by axotomy. Thus, a nerve transection induces both quantitative and qualitative changes in the retrograde axonal transport, which could serve as a signal of distal injury to the cell body. The velocity of reverse transport, measured within 6 h after transection, was found to be 213 mm/day, a value close to that of retrograde transport (200 mm/day). This suggests that the reversal taking place in severed sciatic nerve is similar to the anterograde-to-retrograde conversion process normally occurring at the nerve endings.

Acetylcholinesterase↗
PubMed1994

Reversal of established responses to endothelin-1 in vivo and in vitro by the endothelin receptor antagonists, BQ-123 and PD 145065.

1. Endothelin-1 binds almost irreversibly to its receptors and causes prolonged vasoconstrictions. Here we have studied the reversal of established responses to ET-1 in vivo and in vitro by BQ-123, an ETA receptor-selective antagonist, and/or PD 145065, an ETA/ETB receptor non-selective antagonist. 2. In anaesthetized rats pretreated with hexamethonium, infusion of ET-1 (10(-11) mol kg-1 min-1) increased the mean arterial pressure (MAP) from 93 +/- 1.5 mmHg to 137 +/- 2.4 mmHg after 70 min (n = 29). While the ET-1 infusion was continued an additional infusion of BQ-123 caused a gradual dose-dependent reduction in the pressor effect of ET-1. For instance, after a 60 min infusion of BQ-123 (10(-8) mol kg-1 min-1) the MAP was decreased by 29.3 +/- 4.3 mmHg (n = 4). 3. PD 145065 was a much weaker antagonist of the established pressor effects of ET-1. At 10(-8) mol kg-1 min-1 it had no significant effect and even at 10(-7) mol kg-1 min-1 the elevated blood pressure was only reduced by 11.8 +/- 8.0 mmHg (n = 5) after 60 min. Co-infusion of BQ-123 and PD 145065 caused smaller reductions in the established response to ET-1 than infusion of BQ-123 alone. 4. Sustained contractions of rat aortic rings induced by ET-1 (3 x 10(-9) M) and mediated by ETA receptors were slowly reversed by addition of BQ-123 (10(-5) M) or PD 145065 (10(-5) M). For instance,after 40 min the elevated tone was reduced 85.8 +/- 5.6% (n = 6) by PD 145065, and 77.1 +/- 6.7% (n = 6)by BQ-123. Thus, on the rat aortic rings in vitro both antagonists were equally effective against established responses to ET-1.5. ET-1 increased the perfusion pressure of the rat isolated perfused kidney by 138.1 +/- 7.6 mmHg(n = 14). Subsequent co-infusion of BQ-123 or PD 145065 reversed this increase with PD 145065 being more active. For instance, PD 145065 (10-6 M) reversed the increase in perfusion pressure by 56.9 +/- 8.8% (n = 5) and BQ-123 (10-6 M) reversed it by 22.8 +/- 8.0% (n = 5). This fits well with the vasoconstriction induced by ET-1 in the rat kidney being mediated by ETA and ETB receptors.6. Thus, sustained vasoconstrictions to ET-1 in vitro, mediated by either ETA or ETB receptors, may be reversed slowly by the subsequent application of receptor antagonists. Similarly, endothelin antagonists reverse the pressor effects of ET-1 in vivo although co-antagonism of ETA and ETB receptors or the co-administration of an ETA receptor antagonist, BQ-123, and a mixed antagonist, PD 145065 produces less reversal than the application of an ETA receptor-selective antagonist. This may be because PD 145065 also reduces vasodilatations induced by ET-1 in vivo, or could suggest that because of its peptide structure PD 145065 affects the elimination of ET-1.

Amino Acid Sequence↗
PubMed1995

A method for adjusting caries increments for reversals due to examiner misclassification.

Two types of reversals occur in longitudinal caries studies, remineralization (true reversals) and reversals due to examiner misclassification (D3 to S; F to S). Since 1966, the standard practice has been to subtract examiner reversals from the crude increment (CCI), resulting in a net caries increment (NCI). The use of the NCI has been based on the assumption that examiners make an equal number of false positive and false negative errors both at baseline and follow-up examination. Consequently, the difference between the two caries prevalence scores would provide an unbiased estimate of caries incidence between examinations. The NCI considers all reversals to be true reversals which is an extreme strategy, particulary when the level of diagnosis is set at lower thresholds. In this study we compromised between the NCI and CCI by creating a simple formula to calculate the caries increment using a prevalence-based adjustment for reversals. The formula is ADJCI = y2(1-(y3/(y3 + y4))), where y2 = S to D or F; y3 = D or F to S; y4 = D to D/F or F to F. The impact of this adjustment is illustrated using data from a random sample of 452 older black and white adults followed over a 3-yr period. The ADJCI was more likely to result in higher caries increments and more observed intergroup differences than the NCI, a finding that has implications for clinical trials. For example, the crude 3-yr coronal DS increment for whites was 0.62, the NCI was 0.26 and the ADJCI was 0.56. There were significant black-white differences for the CCI and ADJCI, but not the NCI. We conclude that the NCI was too severe of an adjustment for reversals for this study population.

Adult↗
PubMed1976

Extreme vestibulo-ocular adaptation induced by prolonged optical reversal of vision.

1. These experiments investigated plastic changes in the vestibulo-ocular reflex (VOR) of human subjects consequent to long-term optical reversal of vision during free head movement. Horizontal vision-reversal was produced by head-mounted dove prisms. Four normal adults were continuously exposed to these conditions during 2, 6, 7 and 27 days respectively.2. A sinusoidal rotational stimulus, previously shown to be nonhabituating (1/6 Hz; 60 degrees /sec amplitude), was used to test the VOR in the dark at frequent intervals both during the period of vision-reversal and an equal period after return to normal vision. D.c. electro-oculography (EOG) was used to record eye movement, taking care to avoid changes of EOG gain due to light/dark adaptation of the retina.3. All subjects showed substantial reduction of VOR gain (eye velocity/head velocity) during the first 2 days of vision-reversal. The 6-, 7- and 27-day subjects showed further reduction of gain which reached a low plateau at about 25% the normal value by the end of one week. At this time the attenuation of some EOG records was so marked as to defy extraction of a meaningful sinusoidal signal.4. After removal of the prisms VOR gain recovered along a time course which approximated that of the original adaptive attenuation.5. In the 27-day experiment large changes of phase developed in the VOR during the second week of vision-reversal. These changes generally progressed in a lagging sense, to reach 130 degrees phase lag relative to normal by the beginning of the third week. Accompanying this was a considerable restoration of gain from 25 to 50% the normal value. These adapted conditions, which approximate functional reversal of the reflex, were then maintained steady, even overnight, until return to normal vision on the 28th day.6. Thereafter, whereas VOR phase returned to near-normal in 2 hr, restoration of gain occupied a further 2-3 weeks.7. There was a highly systematic relation between instantaneous gain and phase, even during periods of widely fluctuating change associated with transition from one steady state to another. During such transition there was a tendency for directional preponderance to occur in the VOR.8. All the observed changes were highly specific to the plane of vision-reversal, no VOR changes being observed in the sagittal plane.9. VOR changes were adaptive, in the sense that they were always goal-directed towards the requirements of retinal image stabilization during head movement. They were plastic to the extent that there was extensive and retained remodelling of the reflex towards this goal.10. It is inferred that all the observed changes in gain and phase are compatible with a simple neural network employing known vestibulo-ocular projections via brainstem and cerebellar pathways, providing that the reversed visual tracking task can produce plastic modulation of efficacy in the cerebellar pathway and that this pathway exhibits a dynamic characteristic producing moderate phase lead in a sinusoidal signal at 1/6 Hz.

Journal Article↗
PubMed1978

The physiological effects of monocular deprivation and their reversal in the monkey's visual cortex.

1. 1127 single units were recorded during oblique penetrations in area 17 of one normal, three monocularly deprived and four reverse sutured monkeys. 2. In all animals most cells outside layer IV c were orientation-selective, and preferred orientation usually shifted from cell to cell in a regular progressive sequence. 3. The presence in layer IV c of non-oriented, monocularly driven units, organized in alternating right-eye and left-eye 'stripes' (LeVay, Hubel & Wiesel, 1975) was confirmed. 4. Early monocular deprivation (2--5 1/2 weeks) caused a strong shift of ocular dominance towards the non-deprived eye. However, even outside layer IV c, neural background and some isolated cells could still be driven from the deprived eye in regularly spaced, narrow columnar regions. In layer IV c the non-deprived eye's stripes were almost three times wider, on average, than the deprived. 5. Later monocular deprivation (11--16 months) had no detectable influence on layer IV c but seemed to cause a small shift in ocular dominance outside IV c. Deprivation for 6 1/4 months in an adult had no such effect. 6. After early reverse suturing (at 5 1/2 weeks) the originally deprived eye gained dominance over cells outside layer IV c just as complete as that originally exercised by the eye that was first non-deprived. 7. The later reverse suturing was delayed, the less effective was recapture by the originally deprived eye. Reversal at 8 weeks led to roughly equal numbers of cells being dominated by each eye; fewer cells became dominated by the newly open eye after reverse suturing at 9 weeks and most of them were non-oriented; reversal at 38 1/2 weeks had no effect. 8. Binocular cells, though rare in reverse sutured animals, always had very similar preferred orientations in the two eyes. The columnar sequences of preferred orientation were not interrupted at the borders of ocular dominance columns. 9. Even within layer IV c there was evidence for re-expansion of physiologically determined ocular dominance stripes. After early reverse suture, stripes for the two eyes became roughly equal in width. Possible mechanisms for these changes are discussed.

Action Potentials↗
PubMed2003

Transmission mode time-reversal super-resolution imaging.

The theory of time-reversal super-resolution imaging of point targets embedded in a reciprocal background medium [A. J. Devaney, "Super-resolution imaging using time-reversal and MUSIC," J. Acoust. Soc. Am. (to be published)] is generalized to the case where the transmitter and receiver sensor arrays need not be coincident and for cases where the background medium can be nonreciprocal. The new theory developed herein is based on the singular value decomposition of the generalized multistatic data matrix of the sensor system rather than the standard eigenvector/eigenvalue decomposition of the time-reversal matrix as was employed in the above-mentioned work and other treatments of time-reversal imaging [Prada, Thomas, and Fink, "The iterative time reversal process: Analysis of the convergence," J. Acoust. Soc. Am. 97, 62 (1995); Prada et al., "Decomposition of the time reversal operator: Detection and selective focusing on two scatterers," J. Acoust. Soc. Am. 99, 2067 (1996)]. A generalized multiple signal classification (MUSIC) algorithm is derived that allows super-resolution imaging of both well-resolved and non-well-resolved point targets from arbitrary sensor array geometries. MUSIC exploits the orthogonal nature of the scatterer and noise subspaces defined by the singular vectors of the multistatic data matrix to form scatterer images. The time-reversal/MUSIC algorithm is tested and validated in two computer simulations of offset vertical seismic profiling where the sensor sources are aligned along the earth's surface and the receiver array is aligned along a subsurface borehole. All results demonstrate the high contrast, high resolution imaging capabilities of this new algorithm combination when compared with "classical" backpropagation or field focusing. Above and beyond the application of seismo-acoustic imaging, the time-reversal super-resolution theory has applications in ocean acoustics for target location, and ultrasonic nondestructive evaluation of parts.

Acoustics↗
PubMed2005

Time reversal of noise sources in a reverberation room.

Usually, time reversal is studied with pulsed emissions. Here, the properties of time reversal of the acoustic field emitted by noise sources in a reverberation room are studied numerically, theoretically, and experimentally. A time domain numerical simulation of a two-dimensional enclosure shows that the intensity of a time-reversed noise is strongly enhanced right on the initial source position. A theory based on the link that exists between time reversal of noise and the "well-known" time reversal of short pulse is developed. One infers that the focal spot size equals half a wavelength and the signal to noise ratio only depends on the number of transceivers in the time reversal mirror. This last property is characteristic of the time reversal of noise. Experimental results are obtained in a 5 X 3 X 3 m3 reverberation room. The working frequency range varies from 300 Hz to 2 kHz. The ability of the time reversal process to physically reconstruct the image of two noise sources is studied. To this end, care is given to the technique to separate two close random sources, and also to the influence of temperature fluctuations on the focusing quality.

Acoustics↗
PubMed1982

Roles of cysteine sulfinate and transaminase on in vitro dark reversion of urocanase in Pseudomonas putida.

Urocanase is inactivated in intact cells of Pseudomonas putida and photoactivated by brief exposure of the cells to the UV radiation in sunlight. The dark reversion (inactivation) in vitro is explained by the formation of a sulfite-NAD adduct. Our objective was to investigate the dark reversion in vivo. Various compounds were added to P. putida cells, and the reversion was measured, after sonication, by comparison of the activity before and after UV irradiation. Sulfite, cysteine sulfinate, and hypotaurine enhanced the reversion of urocanase in resting cells. The reversion was time and concentration dependent. Sulfite modified the purified enzyme, but cysteine sulfinate and hypotaurine could not, indicating that those two substances had to be metabolized to support the reversion. Both of those compounds yielded sulfite when they were incubated with cells. Transaminases form sulfite from cysteine sulfinate. P. putida extract contained a transaminase whose activity involved as alpha-keto acid and either cysteine sulfinate or hypotaurine for (i) production of sulfite, (ii) disappearance of substrates, (iii) formation of corresponding amino acids, and (iv) urocanase reversion. Porcine crystalline transaminase caused reversion of highly purified P. putida urocanase with cysteine sulfinate and alpha-ketoglutarate. We conclude that in P. putida cysteine sulfinate or hypotaurine is catabolized in vivo by a transaminase reaction to sulfite, which modifies urocanase to a form that can be photoactivated. We suggest that this photoregulatory process is natural because it occurs in cells with the aid of sunlight and cellular metabolism.

Alanine↗
PubMed1997

Characterization of the reverse gyrase from the hyperthermophilic archaeon Pyrococcus furiosus.

The reverse gyrase gene rgy from the hyperthermophilic archaeon Pyrococcus furiosus was cloned and sequenced. The gene is 3,642 bp (1,214 amino acids) in length. The deduced amino acid sequence has relatively high similarity to the sequences of the Methanococcus jannaschii reverse gyrase (48% overall identity), the Sulfolobus acidocaldarius reverse gyrase (41% identity), and the Methanopynrus kandleri reverse gyrase (37% identity). The P. furiosus reverse gyrase is a monomeric protein, containing a helicase-like module and a type I topoisomerase module, which resembles the enzyme from S. acidocaldarius more than that from M. kandleri, a heterodimeric protein encoded by two separate genes. The control region of the P. furiosus rgy gene contains a typical archaeal putative box A promoter element which is located at position -26 from the transcription start identified by primer extension experiments. The initiating ATG codon is preceded by a possible prokaryote-type ribosome-binding site. Purified P. furiosus reverse gyrase has a sedimentation coefficient of 6S, suggesting a monomeric structure for the native protein. The enzyme is a single polypeptide with an apparent molecular mass of 120 kDa, in agreement with the gene structure. The sequence of the N terminus of the protein corresponded to the deduced amino acid sequence. Phylogenetic analysis indicates that all known reverse gyrase topoisomerase modules form a subgroup inside subfamily IA of type I DNA topoisomerases (sensu Wang [J. C. Wang, Annu. Rev. Biochem. 65:635-692, 1996]). Our results suggest that the fusion between the topoisomerase and helicase modules of reverse gyrase occurred before the divergence of the two archaeal phyla, Crenoarchaeota and Euryarchaeota.

Amino Acid Sequence↗
PubMed1999

Gliding mutants of Myxococcus xanthus with high reversal frequencies and small displacements.

Myxococcus xanthus cells move on a solid surface by gliding motility. Several genes required for gliding motility have been identified, including those of the A- and S-motility systems as well as the mgl and frz genes. However, the cellular defects in gliding movement in many of these mutants were unknown. We conducted quantitative, high-resolution single-cell motility assays and found that mutants defective in mglAB or in cglB, an A-motility gene, reversed the direction of gliding at frequencies which were more than 1 order of magnitude higher than that of wild type cells (2.9 min-1 for DeltamglAB mutants and 2.7 min-1 for cglB mutants, compared to 0.17 min-1 for wild-type cells). The average gliding speed of DeltamglAB mutant cells was 40% of that of wild-type cells (on average 1.9 micrometers/min for DeltamglAB mutants, compared to 4.4 micrometers/min for wild-type cells). The mglA-dependent reversals and gliding speeds were dependent on the level of intracellular MglA protein: mglB mutant cells, which contain only 15 to 20% of the wild-type level of MglA protein, glided with an average reversal frequency of about 1.8 min-1 and an average speed of 2.6 micrometers/min. These values range between those exhibited by wild-type cells and by DeltamglAB mutant cells. Epistasis analysis of frz mutants, which are defective in aggregation and in single-cell reversals, showed that a frzD mutation, but not a frzE mutation, partially suppressed the mglA phenotype. In contrast to mgl mutants, cglB mutant cells were able to move with wild-type speeds only when in close proximity to each other. However, under those conditions, these mutant cells were found to glide less often with those speeds. By analyzing double mutants, the high reversing movements and gliding speeds of cglB cells were found to be strictly dependent on type IV pili, encoded by S-motility genes, whereas the high-reversal pattern of mglAB cells was only partially reduced by a pilR mutation. These results suggest that the MglA protein is required for both control of reversal frequency and gliding speed and that in the absence of A motility, type IV pilus-dependent cell movement includes reversals at high frequency. Furthermore, mglAB mutants behave as if they were severely defective in A motility but only partially defective in S motility.

Bacterial Proteins↗
PubMed2000

The human immunodeficiency virus type 1 TAR RNA upper stem-loop plays distinct roles in reverse transcription and RNA packaging.

The human immunodeficiency virus type 1 (HIV-1) RNA genome is flanked by a repeated sequence (R) that is required for HIV-1 replication. The first 57 nucleotides of R form a stable stem-loop structure called the transactivation response element (TAR) that can interact with the virally encoded transcription activator protein, Tat, to promote high levels of gene expression. Recently, we demonstrated that TAR is also important for efficient HIV-1 reverse transcription, since HIV-1 mutated in the upper stem-loop of TAR showed a reduced ability both to initiate and to complete reverse transcription. We have analyzed a series of HIV-1 mutant viruses to better defined the structural or sequence elements required for natural endogenous reverse transcription and packaging of virion RNA. Our results indicate that the requirement for TAR in reverse transcription is conformation dependent, since mutants with mutations that alter the upper stem-loop orientation are defective for reverse transcription initiation and have minor defects in RNA packaging. In contrast, TAR mutations that allowed the formation of alternative upper stem-loop structure greatly reduced RNA packaging but did not affect reverse transcription efficiency. These results are consistent with direct involvement of the upper stem-loop structure in packaging of genomic RNA and suggest that the TAR RNA stem-loop from nucleotide +18 to +42 interacts with other components of the reverse transcription initiation complex to promote efficient reverse transcription.

Base Pairing↗