Biomedical subjects

Z Tokaji

Publications and source records attributed to Z Tokaji.

13 recordsLinked to original sources

PubMed2000

Blue light regeneration of bacteriorhodopsin bleached by continuous light.

Photobleaching of bacteriorhodopsin (BR) by continuous light has recently been demonstrated. This bleaching consists of at least two subsequent product states. One of them is absorbing maximally in the blue spectral region. Our present study shows that upon illumination of the bleached sample with blue light a back photoprocess appears, resulting in regeneration of the original BR state. From a technical point of view, the observed phenomenon is similar to the reverting effect of blue light on the photocycle. An important difference is that the photobleached state of BR is much more stable than any of the photocycle intermediates, and may provide an advantage for several technical applications.

Bacteriorhodopsins↗

PubMed1999

Bleaching of bacteriorhodopsin by continuous light.

A new two step photobleaching process is observed under continuous illumination of bacteriorhodopsin. This photobleaching is considerable even at physiological temperatures and becomes large at 50-60 degrees C. The photobleaching also increases with increasing pH from 7 to 10. We suggest that the bleaching at its final stage could be due to the dissociation of the retinal and a local thermal denaturation-like process. These facts may question the generally held belief that BR is a stable protein in vivo for a long period of time. Our results may have relevance also to practical applications of bacteriorhodopsin where the stability of bacteriorhodopsin is a key issue. In certain instances, the use of bacteriorhodopsin may require cooled conditions. Here, we defined the conditions under which bacteriorhodopsin is stable. The permanent photobleaching offers a new way of picture imaging and information input for bacteriorhodopsin-based optical devices.

Bacteriorhodopsins↗

PubMed1998

Quantitative model for the cooperative interaction of the bacteriorhodopsin molecules in purple membranes.

The trimeric, asymmetric and sequential model for the cooperative interaction of the bacteriorhodopsin molecules in purple membranes [Zs. Tokaji, Biophys. J. 65 (1993) 1130-11341 is being extended in the paper. Analyses of data from absorption kinetic measurements with preexcitation and green background illumination, and photoselection measurements on oriented samples confirm the main features of this cooperative interaction and support the validity of the extended model. This model includes the observed heterogeneity of the non-excited state of the bacteriorhodopsin molecules in purple membranes, and the agreement with the data suggests that molecules in any other state than the bacteriorhodopsin ground state can alter the photocycle of their neighbors. The presented results seem to contradict other models for the cooperation of the bacteriorhodopsin molecules.

Bacteriorhodopsins↗

PubMed1997

Cooperativity-induced optical anisotropy changes during the photocycle of bacteriorhodopsin.

Previous photoselection measurements showed that if the excitation is weak no optical anisotropy changes appear in immobilized purple membranes during the photocycle. The present study demonstrates that surprisingly at stronger excitations the anisotropy changes versus time. At 412 nm the dichroic ratio decreases after a few milliseconds, while at 570 nm the similar decrease is followed by an increase. The phenomenon cannot be described by tiltings of the retinal chromophore. It is the consequence of the cooperative interaction among the photocycling bacteriorhodopsin molecules that regulates the yields of more than one (expectedly two main) parallel pathways existing in the millisecond time domain of the photocycle.

Anisotropy↗

PubMed1997

N-like intermediate in the photocycle of the acid purple form of bacteriorhodopsin.

The photocycle of the acid-purple form of bacteriorhodopsin (BR) was studied by multiwavelength absorption kinetic measurements. The data were evaluated using a new method based on the differences in the kinetics of the absorption changes at different wavelengths [Tokaji, Zs. (1995) FEBS Lett. 357, 156-160]. The evaluation indicates that, in contrast to the previous suggestion, the photocycle of the acid purple BR contains an intermediate that, in its spectral and kinetic properties, resembles the N intermediate of the alkaline BR photocycle.

Bacteriorhodopsins↗

PubMed1996

Actinic light density dependence of the O intermediate of the photocycle of bacteriorhodopsin.

The O intermediate of the photocycle of bacteriorhodopsin (BR) was studied by absorption kinetic measurements at different actinic light densities. With increasing exciting flash intensity, the relative yield of O slightly increases, while that of Mf strongly decreases at the expense of Ms. Kinetic calculations and the optical anisotropy of O show that O can be formed only from Mf although Mf and O have different light intensity dependences. In order to resolve the apparent contradiction, a phenomenologically new cooperative regulatory mechanism seems to be necessary.

Absorption↗

PubMed1995

Cooperativity-regulated parallel pathways of the bacteriorhodopsin photocycle.

The paper demonstrates that the actinic light density dependence of the millisecond part of the bacteriorhodopsin (BR) photocycle at high pH predicts a model, which is the same in the sequence of the intermediates as concluded previously on the basis of double flash experiments [1992, FEBS Lett. 311, 267-270]. This model consists of the Mf-->N-->BR and M(s)-->BR parallel pathways, the relative yields of which are regulated by cooperative interaction of the BR molecules. The decay of M(s) is always slower than the decay of Mf and described as a direct reprotonation of the Schiff-base from the bulk, and the recovery of the ground-state nearly at the same time. M(s) is decomposed into M'f and M's. The first does not reprotonate, and similarly to Mf, it is suggested to be before the conformational change (switch), which latter process would be just before the decay of Mf. A simple way for the determination of the kinetics is also used. This confirms that the amount of N decreases with increasing fraction cycling and shows that the decay rate of N is independent of the fraction cycling. The differences in the kinetics are compared to each other, and they seem to allow a new way of kinetic evaluation at least under special conditions. The aim of this paper was briefly explained in my poster presented on the VIth International Conference on Retinal Protein (see [14]).

Bacteriorhodopsins↗

PubMed1994

Influence of excitation energy on the bacteriorhodopsin photocycle.

Kinetic curves for the bacteriorhodopsin (BR) photocycle were obtained both at 570 and at 412 nm at a series of increasing levels of intensity of the exciting laser. Singular value decomposition (SVD) of these curves showed two transitions in the kinetic profiles that occurred at specific levels of actinic light. This means that the photocycle was influenced by photon density in two ways. In a separate application of SVD, time-resolved optical spectra were analyzed at each of many levels of exciting laser intensities. The studies showed that the transition at the low level of laser intensity was due principally to an increase in the amount of BR that was turning over. The transition at the higher level of laser intensity showed a fundamental change in kinetics of the photocycle. At low intensity levels, the fast form of M (Mf) predominated, whereas at high levels the slow form of M (Ms) predominated. A distinction was found between Mf and Ms, in that the former decayed directly to the O intermediate whereas the latter decayed directly to BR.

Bacteriorhodopsins↗

PubMed1993

Actinic light density dependence of the bacteriorhodopsin protocycle.

The photocycle of bacteriorhodopsin (BR) was studied in the 0.3 microsecond to 10 s time interval after excitation, using a wide range of actinic light intensities (10 ns half-duration, 0.06-60 mJ/cm2), at neutral and alkaline pH values. The relative weights of the rapidly and the slowly decaying components of the M intermediate (Mf and M(s), respectively) and the yield of the third millisecond component, N(R,P), are the function of the exciting light intensity (density), while their lifetimes are not. The relative weight of M(s) is found to be a linear function of the portion of the BR molecules undergoing the photocycle. This suggests the existence of a cooperative interaction of the BR molecules arranged in the crystalline purple membrane sheets. Another source of M(s) is also found, which results a nonvanishing relative weight of M(s) even at very weak actinic light density values. The explanation for this may be a branching, or the heterogeneity of BR itself or with its environment. It is shown that the relative weights of the rising and decaying components of the M form(s) do not correlate directly with each other.

Bacteriorhodopsins↗

PubMed1993

Dimeric-like kinetic cooperativity of the bacteriorhodopsin molecules in purple membranes.

The kinetics of the absorption changes accompanying the photocycle of bacteriorhodopsin (BR) strongly depend on the intensity of the exciting short laser pulse. The decrease in the flash intensity dependence of the M kinetics after different extents of bleaching of the purple membranes by hydroxylamine proves the existence of a cooperative interaction between the photocycling BR molecules. The yield of the slow component of the M decay (M(s)) is a quadratic function of the extent of the fraction cycling. The slope of the relative weight of M(s) versus the fraction cycling is 0.5. This slope indicates a dimeric-like cooperative interaction, although the structural units of the purple membranes are the trimers of the BR molecules. For the most probable cooperative mechanism an asymmetric trimeric interaction is suggested, which accounts for the apparently dimeric features. A photocycling molecule may influence only one of its two neighbors in the trimer. From this asymmetric feature a deformative interaction is expected to be the cooperative mechanism, which would be an allosteric regulating mechanism in the purple membrane.

Allosteric Regulation↗

PubMed1992

Kinetics of the N intermediate and the two pathways of recovery of the ground-state of bacteriorhodopsin.

Absorption kinetic measurements at alkaline pH, in which bacteriorhodopsin (BR) is pre-excited by another flash, indicate that a part of the recovery of the BR ground-state is faster than the decay of the N intermediate of the photocycle. This fact proves the existence of a parallel pathway in the late part of the BR photocycle (the decay of Ms into BR), which does not include the N intermediate. We demonstrate that the decay of the Mf intermediate does not lead to any direct recovery of the BR ground-state, and that excitation of N does not form an M-like intermediate. Mf decays directly into the N intermediate, and photoexcitation of N leads to the formation of a red-shifted form, O*. The kinetics of this red-shifted intermediate are also presented.

Bacteriorhodopsins↗

PubMed1991

Alternative translocation of protons and halide ions by bacteriorhodopsin.

Bacteriorhodopsin (bR568) in purple membrane near pH 2 shifts its absorption maximum from 568 to 605 nm forming the blue protein bRacid605, which no longer transports protons and which shows no transient deprotonation of the Schiff base upon illumination. Continued acid titration with HCl or HBr but not H2SO4 restores the purple chromophore to yield bRHCl564 or bRHBr568. These acid purple forms also regain transmembrane charge transport, but no transient Schiff base deprotonation is observed. In contrast to bR568, no rate decrease of the bRacidpurple transport kinetics is detected in 2H2O; however, the transport rate decreases by a factor of approximately 2 in bRHBr568 compared with bRHCl564. The data indicate that in the acid purple form bR transports the halide anions instead of protons. We present a testable model for the transport mechanism, which should also be applicable to halorhodopsin.

Anions↗

PubMed1991

Light-induced, long-lived perturbation of the photocycle of bacteriorhodopsin.

The relative weight of the slowly decaying M intermediate of the photocycle of bacteriorhodopsin increases upon increasing the energy density of the short (10 ns) actinic laser pulse. Moreover, when a pre-exciting flash is applied to the BR sample, the absolute amplitude of the Ms is higher in the signal induced by a second flash, applied with a delay from 100 microseconds to 100 ms. These facts together prove that either the leftover BR ground-state population becomes different due to the pre-excitation, or there is a cooperative interaction between the BR molecules.

Bacteriorhodopsins↗