The interaction of the potential-sensitive molecular probe merocyanine 540 with phosphorylating beef heart submitochondrial particles under equilibrium and time-resolved conditions

Effect of pressure on the solubilization of a fluorescent merocyanine dye by a nonionic surfactant

The target dye, which is a derivative of Merocyanine 540 bearing a naphthoxazole headgroup, persists as a monomer in ethanol solution but dimerizes in water under ambient conditions. Analysis of the absorption spectrum indicates that the dimer has an oblique geometry with the two molecules being held at an angle of ca. 55°. Applying high pressure to the system forces the two molecules into closer contact, resulting in a decreased partial molar volume of 3.1 cm(3). One molecule of the monomeric dye enters a neutral micelle formed from Triton X-100, where it is highly fluorescent and free of exciton coupling. The result of applied pressure on these latter systems depends on the concentration of surfactant. Above the critical micelle concentration (CMC), applied pressure has little effect other than to increase the viscosity inside the micelle. At very low surfactant concentration, applied pressure forces monomeric dye into the dimeric form, as observed in the absence of Triton X-100. It is notable, however, that the pressure effect on the dimerization constant is exaggerated in the presence of surfactant. At intermediate surfactant concentrations, applied pressure leads to a marked change in the CMC. In particular, applied pressure reduces the partial molar volume of the micelle by ca. 7.9 cm(3) and induces micelle formation at relatively low concentration of surfactant. For example, the CMC falls from ca. 250 μM at atmospheric pressure to only 50 μM at 460 MPa.

pH, serum proteins and ionic strength influence the uptake of merocyanine 540 by WiDr cells and its interaction with membrane structures

It has been suggested that selective uptake of photosensitizers is due to significantly lower pH of the interstitial fluid in tumors compared to normal tissue. Therefore, the cellular uptake of merocyanine 540 (MC 540) was examined at two pH values: 6.8+/-0.1 and 7.4+/-0.1. There was no difference in spectral properties (absorption and fluorescence maxima positions, fluorescence intensity) of the drug in the presence of increasing amounts of either human blood plasma or FCS (0-2%) at the two pH values investigated. Nevertheless, significantly higher amounts of the drug were taken up by WiDr cells at pH 6.8+/-0.1, both in the presence of 10% FCS and in the absence of FCS. The absorption spectra of MC 540 in the presence of egg phosphatidylcholine (PC) liposomes turned out to be NaCl concentration-dependent (0.00-0.30 mol l(-1)). Membrane fluidity, as measured by fluorescence anisotropy of diphenylhexatriene (DPH), was unchanged within the experimental error in the NaCl concentration range 0.01-0.30 mol l(-1). The spectral changes indicated an enhancement of the incorporation of MC 540 into lipid membranes with increasing ionic strength. Such a salt concentration dependence suggests a possible involvement of the surface potential in the interaction of MC 540 with lipid membranes. The results might provide an explanation of the pH dependency of the cellular uptake of MC 540 observed in this study.

The capacitating agent bicarbonate induces protein kinase A-dependent changes in phospholipid transbilayer behavior in the sperm plasma membrane

A flow cytometric procedure was used to follow the effect of bicarbonate, a key inducer of sperm capacitation in vitro, on the transbilayer behavior of C6NBD-phospholipids in the plasma membrane of living acrosome-intact boar spermatozoa under physiological conditions. In the absence of bicarbonate, 97% of C6NBD-phosphatidylserine and 78% of C6NBD-phosphatidylethanolamine was rapidly translocated from the outer leaflet to the inner, whereas relatively little C6NBD-phosphatidylcholine and C6NBD-sphingomyelin was translocated (15% and 5%, respectively). Inclusion of 15 mM bicarbonate/5%CO(2) markedly slowed down the rates of translocation of the aminophospholipids without altering their final distribution, whereas it increased the proportions of C6NBD-phosphatidylcholine and C6NBD-sphingomyelin translocated (30% and 20%, respectively). Bicarbonate activated very markedly the outward translocation of all four phospholipid classes. The changes in C6NBD-phospholipid behavior were accompanied by increased membrane lipid disorder as detected by merocyanine 540, and also by increased potential for phospholipase catabolism of the C6NBD-phospholipid probes. All three changes were mediated via a cAMP-dependent protein phosphorylation pathway. We suspect that the changes result from an activation of the non- specific bidirectional translocase ('scramblase'). They have important implications with respect to sperm fertilizing function.

Interaction of merocyanine 540 with nicotinic acetylcholine receptor membranes from Discopyge tschudii electric organ

Interactions between merocyanine 540 (MC540) and nicotinic acetylcholine receptor (AChR) have been studied by visible absorption spectroscopy using native receptor-rich membranes from Discopyge tschudii electric tissue and liposomes obtained by aqueous dispersion of endogenous lipids extracted from the same tissue. The fact that merocyanine partitions into the membrane when this is in the liquid-crystalline state, exhibiting a characteristic peak at 567 nm, was exploited to obtain quantitative information about the physical state of the AChR-rich membrane. Spectra of MC540 revealed that this molecule was preferentially incorporated into AChR-rich membranes, with an affinity (Kdapp 30 microM) 10-fold higher than that in liposomes (Kdapp 290 microM). Changes were observed in the equilibrium dissociation constant of MC540 at different temperatures: the two-fold higher affinity at 8 degrees C than at 23 degrees C can be rationalized in terms of a higher value of the overall dimerization constant (Kdim) at the lower temperature. The local anaesthetic benzocaine competed for MC540 binding sites with higher potency in AChR-rich native membranes than in liposomes made with endogenous lipids. This competition was found to be AChR concentration-dependent, whereas in liposomes the displacement was constant at different lipid/MC540 molar ratios. Titration experiments yielded an apparent dissociation constant for benzocaine of 0.6 mM and 0.7 mM for liposomes and AChR-rich membranes, respectively. The possible location of the benzocaine binding site is deduced from the competition experiments to be at the lipid annulus surrounding the nicotinic AChR protein.

The components of merocyanine-540 absorption spectra in aqueous, micellar and bilayer environments

Spectral-data-processing and curve-fitting techniques have been applied to the decomposition of merocyanine-540 absorption spectra in aqueous, micellar and bilayer environments. The various resolved component bands have been assigned to dye monomers, dimers, or larger aggregates, either in polar or non-polar environments. The analysis of spectral parameters (lambda max and integrated intensity) of the overall spectra and of each component has revealed that merocyanine 540 is a useful probe in studies of membrane structure and dynamics using visible-absorption spectroscopy. In particular, the monomer lambda max and the integrated intensity, i.e. area, of the dimer population are very useful in this respect. The monomer lambda max is especially sensitive to polarity changes and is thus useful, e.g. in the precise determination of critical micellar concentrations. The fractional area of the dimer increases with the packing density of the phospholipid-hydrocarbon region near the interface and is thus very sensitive to changes in vesicle curvature and to the presence of sterols or intrinsic polypeptides in the bilayer.

The behavior of a fast-responding barbituric acid potential-sensitive molecular probe in bovine heart submitochondrial particles

The barbituric acid probe diBa-C2-(5) responds to the formation of a membrane potential (delta psi) in bovine heart submitochondrial particles (SMP) by a CCCP-reversible, 5-7 nm red shift of the probe absorption spectrum. This shift can be enhanced by the addition of nigericin, an observation that indicates that the probe is specifically sensitive to delta psi. Probe-SMP binding analyses indicate that, relative to the resting state, the ratio of the dye dissociation constant to the maximum number of binding sites decreases by a factor of 30 when delta psi is generated. This observation suggests that the origin of the potential-dependent shift of the probe absorption spectrum is increased occupancy of the SMP membrane by diBa-C2-(5). The time course of the ATP-induced diBa-C2-(5) spectral shift in SMP was complete in nominally 0.2 s and could be described by a single-exponential rate equation. There was no evidence for a slower-phase signal when the data collection time period was increased to 250 s. The apparent first-order rate constants obtained from the single exponential analyses of the barbituric acid ATP-generated signal, however, were a linear function of probe concentration at fixed SMP membrane concentration. The resulting second-order rate constant obtained by linear regression was nominally 1 x 10(7) M (dye)-1 s-1; this value is two to three orders of magnitude higher than that of a number of other well-established probes of delta psi in mitochondrial preparations. Based on the invariance of the kinetics of the oxidation of cytochromes c and c1 by ATP-driven reversed electron transport in the presence and absence of the probe, diBa-C2-(5) does not appear to permeate the SMP membrane on a time scale of milliseconds to several minutes.

Response of the electrochromic dye, merocyanine 540, to membrane potential in rat liver mitochondria

Merocyanine binds extensively to rat liver mitochondria in spite of the presence of a sulfonic acid group which would suggest only limited penetration through the membrane. Passive binding shows both tight and weak binding components and is dependent on salt concentration and ionic strength in accord with the Gouy-Chapman theory. The binding of merocyanine to mitochondria is accompanied by both a fluorescence enhancement and a spectral shift. Induction of an electrical field by either respiration or K+ diffusion potential results in a partial reversal of the spectral shift seen on dye binding. At low temperature, the merocyanine spectral response to an electrical field is biphasic, consisting of a fast phase with a t1/2 of less than 1 sec at 15 degrees C and a slower phase which may vary considerably in rate and extent with conditions. The spectral shift during the two phases appears similar, but differ in sensitivity to ionic strength and temperature. The spectral shift during the fast phase at 15 degrees C indicates that the major component is a decrease in bound monomer and an increase in the aqueous dimer, indicating an "on-off" mechanism. It is suggested that the fast and slow phases of the merocyanine response may be due to two different populations of dye, possibly located at the outer and inner surfaces, respectively, of the mitochondrial membrane. The electrophoretic movement of the dye located in the membrane interior would result in the temperature-sensitive slow phase response. Demonstration of the proportionality of the fast phase response to the magnitude of the membrane potential suggests the usefulness of merocyanine in studies with mitochondrial systems.

A stopped-flow kinetic study of the interaction of potential-sensitive oxonol dyes with lipid vesicles

The interaction of the dyes oxonol V and oxonol VI with unilamellar dioleoylphosphatidylcholine vesicles was investigated using a fluorescence stopped-flow technique. On mixing with the vesicles, both dyes exhibit an increase in their fluorescence, which occurs in two phases. According to the dependence of the reciprocal relaxation time on vesicle concentration, the rapid phase appears to be due to a second-order binding of the dye to the lipid membrane, which is very close to being diffusion-controlled. The slow phase is almost independent of vesicle concentration, and it is suggested that this may be due to a change in dye conformation or position within the membrane, possibly diffusion across the membrane to the internal monolayer. The response times of the dyes to a rapid jump in the membrane potential has also been investigated. Oxonol VI was found to respond to the potential change in less than 1 s, whereas oxonol required several minutes. This has been attributed to lower mobility of oxonol V within the lipid membrane.

Light-induced permeabilization and merocyanine 540 staining of mouse spleen cells

Merocyanine 540 (M540) is a potential-sensitive, hydrophobic dye that preferentially incorporates into the 'fluid' domains of cellular membranes, distinguishing between hemopoietic cells according to their differentiation state. A bright staining with M540 is usually achieved by UV illumination of the cells during staining. We show by flow cytometric analysis that: (1) staining is greatly enhanced by UV illumination of mouse spleen cells before addition of the dye; (2) UV treatment causes an increased permeability toward propidium iodide and intracellular fluorescein as well; (3) the increment in M540 fluorescence precedes permeabilization to propidium iodide, while the latter precedes leakage of fluorescein. We also describe an overshoot and accelerated recovery of M540 fluorescence after photobleaching by a 514 nm laser beam. It is suggested that penetration of M540 to the more fluid inner membrane structures explains the fluorescence increment in both experiments.

Impermeant potential-sensitive oxonol dyes: I. Evidence for an "on-off" mechanism

This series of papers addresses the mechanism by which certain impermeant oxonol dyes respond to membrane-potential changes, denoted delta Em. Hemispherical oxidized cholesterol bilayer membranes provided a controlled model membrane system for determining the dependence of the light absorption signal from the dye on parameters such as the wavelength and polarization of the light illuminating the membrane, the structure of the dye, and delta Em. This paper is concerned with the determination and analysis of absorption spectral changes of the dye RGA461 during trains of step changes of Em. The wavelength dependence of the absorption signal is consistent with an "on-off" mechanism in which dye molecules are driven by potential changes between an aqueous region just off the membrane and a relatively nonpolar binding site on the membrane. Polarization data indicate that dye molecules in the membrane site tend to orient with the long axis of the chromophore perpendicular to the surface of the membrane. Experiments with hyperpolarized human red blood cells confirmed that the impermeant oxonols undergo a potential-dependent partition between the membrane and the bathing medium.

Interaction of the extrinsic potential-sensitive molecular probe diS-C3-(5) with pigeon heart mitochondria under equilibrium and time-resolved conditions

Some aspects of the interaction of the extrinsic, potential-sensitive, molecular probe diS-C3-(5) with pigeon heart mitochondria are reported in this paper. Binding studies based on fluorimetry indicate that the ratio of the dissociation constant to the maximum number of binding sites, KD/n, is larger for succinate-containing mitochondria than that for cyanide-inhibited preparations. These observations suggest that the basis of the energy-dependent diS-C3-(5) optical signals is the ejection of the probe from the mitochondrial membrane. A more detailed analysis indicated that the major change in the binding parameters is a reduction in the maximum number of binding sites, n, when a charge gradient is formed at the expense of substrate. Using rapid mixing techniques, the time course of the passive association of diS-C3-(5) with mitochondria, that of the glutamate- and ATP-dependent optical signals, and the effect of this probe on the rate at which the energy-dependent cytochrome c oxidase Soret band shift signal develops have been monitored. Retardation the ATP-dependent cytochrome c oxidase Soret band shift signal suggests that the probe readily permeates the mitochondrial membrane. The first-order rate law that the glutamate-dependent signal obeys suggests that the rate-limiting step in the development of this signal is the dissociation of the dye from the mitochondrial membrane or the permeation of this membrane by the probe. The faster phase of the ATP-induced signal likely reflects the initial transfer of dye from the bulk aqueous phase followed by a slower probe permeation process that obeys a first-order rate law. This probe appears to distribute across the mitochondrial membrane in accordance with the transmembrane potential as judged by its effect on the ATP-dependent cytochrome c oxidase Soret band shift signal. DiS-C3-(5) also appears to inhibit the NADH dehydrogenase.