Determination of surface potential of biological membranes

Ionizable Nitroxides for Studying Local Electrostatic Properties of Lipid Bilayers and Protein Systems by EPR

Electrostatic interactions are known to play a major role in the myriad of biochemical and biophysical processes. Here, we describe biophysical methods to probe local electrostatic potentials of proteins and lipid bilayer systems that are based on an observation of reversible protonation of nitroxides by electron paramagnetic resonance (EPR). Two types of probes are described: (1) methanethiosulfonate derivatives of protonatable nitroxides for highly specific covalent modification of the cysteine's sulfhydryl groups and (2) spin-labeled phospholipids with a protonatable nitroxide tethered to the polar head group. The probes of both types report on their ionization state through changes in magnetic parameters and degree of rotational averaging, thus, allowing the electrostatic contribution to the interfacial pKa of the nitroxide, and, therefore, the local electrostatic potential to be determined. Due to their small molecular volume, these probes cause a minimal perturbation to the protein or lipid system. Covalent attachment secures the position of the reporter nitroxides. Experimental procedures to characterize and calibrate these probes by EPR, and also the methods to analyze the EPR spectra by simulations are outlined. The ionizable nitroxide labels and the nitroxide-labeled phospholipids described so far cover an exceptionally wide range of ca. 2.5-7.0 pH units, making them suitable to study a broad range of biophysical phenomena, especially at the negatively charged lipid bilayer surfaces. The rationale for selecting proper electrostatically neutral interface for probe calibration, and examples of lipid bilayer surface potential studies, are also described.

Surface electrostatics of lipid bilayers by EPR of a pH-sensitive spin-labeled lipid

Many biophysical processes such as insertion of proteins into membranes and membrane fusion are governed by bilayer electrostatic potential. At the time of this writing, the arsenal of biophysical methods for such measurements is limited to a few techniques. Here we describe a, to our knowledge, new spin-probe electron paramagnetic resonance (EPR) approach for assessing the electrostatic surface potential of lipid bilayers that is based on a recently synthesized EPR probe (IMTSL-PTE) containing a reversibly ionizable nitroxide tag attached to the lipids' polar headgroup. EPR spectra of the probe directly report on its ionization state and, therefore, on electrostatic potential through changes in nitroxide magnetic parameters and the degree of rotational averaging. Further, the lipid nature of the probe provides its full integration into lipid bilayers. Tethering the nitroxide moiety directly to the lipid polar headgroup defines the location of the measured potential with respect to the lipid bilayer interface. Electrostatic surface potentials measured by EPR of IMTSL-PTE show a remarkable (within ±2%) agreement with the Gouy-Chapman theory for anionic DMPG bilayers in fluid (48°C) phase at low electrolyte concentration (50 mM) and in gel (17°C) phase at 150-mM electrolyte concentration. This agreement begins to diminish for DMPG vesicles in gel phase (17°C) upon varying electrolyte concentration and fluid phase bilayers formed from DMPG/DMPC and POPG/POPC mixtures. Possible reasons for such deviations, as well as the proper choice of an electrostatically neutral reference interface, have been discussed. Described EPR method is expected to be fully applicable to more-complex models of cellular membranes.

Spin-labeled pH-sensitive phospholipids for interfacial pKa determination: synthesis and characterization in aqueous and micellar solutions

The synthesis and characterization of spin-labeled phospholipids (SLP)--derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE)--with pH-reporting nitroxides that are covalently attached to the lipid's polar headgroup are being reported. Two lipids were synthesized by reactions of PTE with thiol-specific, pH-sensitive methanethiosulfonate spin labels methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester (IMTSL) and S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl)benzyl methanethiosulfonate (IKMTSL). The pKa values of the IMTSL-PTE lipid measured by EPR titration in aqueous buffer/isopropyl alcohol solutions of various compositions were found to be essentially the same (pKa approximately 2.35), indicating that in mixed aqueous/organic solvents, the amphiphilic lipid molecules could be shielded from changing bulk conditions by a local shell of solvent molecules. To overcome this problem, the spin-labeled lipids were modeled by synthesizing IMTSL- and IKMTSL-2-mercaptoethanol adducts. These model compounds yielded the intrinsic pKa0's for IMTSL-PTE and IKMTSL-PTE in aqueous buffers as 3.33 +/- 0.03 and 5.98 +/- 0.03, respectively. A series of EPR titrations of IMTSL-PTE in mixed water/isopropyl alcohol solution allowed for calibrating the polarity-induced pKa shifts, deltapKapol, vs bulk solvent dielectric permittivity. These calibration data allowed for estimating the local dielectric constant, epsilon(eff), experienced by the reporter nitroxide of the IMTSL-PTE lipid incorporated into the nonionic Triton X-100 micelles as 60 +/- 5 and 57 +/- 5 at 23 and 48 degrees C, respectively. For micelles formed from an anionic surfactant sodium dodecyl sulfate (SDS) the electrostatic-induced pKa shift, deltapKael = 2.06 +/- 0.04 units of pH, was obtained by subtracting the polarity-induced contribution. This shift yields psi = -121 mV electric potential of the SDS micelle surface.

Hydroperoxyl, superoxide and pH gradients in the mitochondrial matrix: a theoretical assessment

The negative surface charge of many cellular membranes concentrates protons and rarefies superoxide in their vicinity. It was speculated that the low pH near membranes should facilitate superoxide protonation, thereby concentrating hydroperoxyl radical in this region. This process would exacerbate both lipid peroxidation and the transfer of oxidative damage between cellular compartments, as hydroperoxyl is a good initiator of lipid peroxidation and permeates lipid bilayers. Surface-charge-enhancement of hydroperoxyl production in mitochondria--which are main intracellular sources of superoxide--should be particularly relevant. Using a simple model of superoxide metabolism in the mitochondrial matrix, we calculated the gradients of pH, superoxide, and hydroperoxyl, and assessed the previous hypothesis in the light of available experimental data. The following predictions ensued: (i) Near the mitochondrial inner membrane, gradients of superoxide concentration with amplitude up to 36% of the maximal concentration, and pH gradients of up to 0.19 units between membrane and bulk. (ii) These electrostatically induced gradients die out within approximately 4 nm of the membrane. (iii) At high (hundreds of nanometres) inter-cristae separations, owing to enzyme-catalyzed dismutation of superoxide, both superoxide and hydroperoxyl become rarefied towards the midpoint between cristae. (iv) Surface charge should neither enhance superoxide protonation nor concentrate hydroperoxyl near biological membranes.

Cercospora beticola toxins. VII. Fluorometric study of their interactions with biological membranes

The interactions of two beticolins, Cercospora beticola toxins, and of their magnesium complexes with liposomes or plasma membrane were studied. The fluorometric pH titration curves of beticolins in liposomes and in plasma membranes reveal the presence of the dissociated form of beticolins. The concentration of the magnesium complex in these membranes increases at high pH. The partition coefficient of beticolin-1 on liposomes is 3-fold higher than that of beticolin-2 and the fluorescence of both compounds on liposomes is similar. The addition of magnesium to liposomes causes a 40-fold and 20-fold increase in the partition coefficient of beticolin-1 and -2, respectively, as a result of the interactions between membrane, magnesium and beticolins. Beticolins react to a delta pH across the liposome membrane but the formation of the magnesium complex completely abolishes this effect.

The application of pH-sensitive spin labels to studies of surface potential and polarity of phospholipid membranes and proteins

The effects of pH titration on the EPR spectra of imidazolidine nitroxides located at the surface of mixed bilayers composed of dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidylcholine (DMPC), and at the surface of the protein, human serum albumin (HSA), have been investigated. It is found that the shift in pKa of the amino group of the imidazolidine radical from its value of 4.6 in water depends both on the interfacial polarity (delta pKapol) and on the electrostatic surface potential (delta pKael) when it is positioned at the bilayer/water interface by an anchoring hydrocarbon tail. The polarity shift is determined to be: delta pKapol = -1.3 units at the surface of DMPC bilayers at 17 degrees C, corresponding to an effective interfacial dielectric constant of epsilon approximately 37, and depends on the temperature with a coefficient of d delta pKapol/dT approximately -0.01 per degree. The electrostatic shift at the surface of DMPG bilayers is delta pKael = +1.6 units in 0.1 M KCl, which corresponds to an electrostatic surface potential of -95 mV. This electrostatic shift depends strongly both on ionic strength and on the fraction of charged lipid in the DMPC/DMPG mixtures, in a manner that agrees with the predictions of electrostatic double-layer theory. It is found that the shift in pKa of an imidazolidine radical covalently bound at the surface of HSA is determined mainly by the surface electrostatics (delta pKapol approximately 0) and corresponds to an electrostatic potential of +33 mV in 0.01 M KCl at a pH below the isoelectric point of the protein.

Regulation of Ca2+ transport in brain mitochondria. II. The mechanism of the adenine nucleotides enhancement of Ca2+ uptake and retention

ADP greatly enhances the rate of Ca2+ uptake and retention in Ca2+ loaded mitochondria. Atractyloside, a specific inhibitor of the ADP/ATP translocator, completely inhibits the ADP effect, while bongkrekate, another specific inhibitor of the translocator enhances the effect of ADP. These results indicate that locking the ADP/ATP translocator in the M-state is sufficient to produce the ADP effect. Cyclosporin A, a specific inhibitor of the Ca2(+)-induced membrane permeabilization does not substitute for ADP, indicating that ADP directly affect the rate of electrogenic Ca2+ uptake. The effect of the translocator conformation on the rate of electrogenic Ca2+ uptake is independent of the concentration of Pi and is not caused by changes in membrane potential. However, locking the carrier in the M-state appears to increase the negative surface charge on the matrix face of the inner membrane. This may lead to an enhanced rate of Ca2+ dissociation from the electrogenic carrier at the matrix surface. The rate of Na(+)-independent Ca2+ efflux is only slightly inhibited by locking the carrier in the M-state, presumably due to the same mechanism. In the presence of ADP, Pi inhibits the Na(+)-independent efflux. In the presence of physiological concentrations of spermine, Pi and Mg2+, the rate of Ca2+ uptake, Ca2+ retention and Ca2+ set points depend sharply on ADP concentration at the physiological range of ADP. Thus, changes of cytosolic ADP concentration may lead to change in the rate of Ca2+ uptake by mitochondria and thus modulate the excitation-relaxation cycles of cytoplasmic free calcium.

Regulation of Ca2+ transport in brain mitochondria. I. The mechanism of spermine enhancement of Ca2+ uptake and retention

Spermine enhances electrogenic Ca2+ uptake and inhibits Na(+)-independent Ca2+ efflux in rat brain mitochondria. As a result, Ca2+ retention by brain mitochondria increases greatly and the external free Ca2+ level at steady-state can be lowered to physiologically relevant concentrations. The stimulation of Ca2+ uptake by spermine is more pronounced at low concentrations of Ca2+, effectively lowering the apparent Km for Ca2+ uptake from 3 microM to 1.5 microM. However, the apparent Vmax is also increased. At low Ca2+ concentrations, Ca2+ uptake is diffusion-limited. Spermine strongly inhibits Ca2+ binding to anionic phospholipids and it is suggested that this increases the rate of surface diffusion which reduces the apparent Km for uptake. The same effect could inhibit the Na(+)-independent efflux if the rate of efflux is limited by Ca2+ dissociation from the efflux carrier. In brain mitochondria (but not in liver) the spermine effect depends on the presence of ADP. In a medium that contains physiological concentrations of Pi, Mg+, K+, ADP and spermine, brain mitochondria sequester Ca2+ down to 0.1 microM and below, depending on the matrix Ca2+ load. Moreover, brain mitochondria under the same conditions buffer the external medium at 0.4 microM, a concentration at which the set point becomes independent of the matrix Ca2+ content. Thus, mitochondria appear to be capable of modulating calcium oscillations in brain cells.