Adsorption of ionized and neutral pentachlorophenol to phosphatidylcholine membranes

Effect of pentachlorophenol on calcium accumulation in barnacle muscle cells

1. The effect of extracellularly applied pentachlorophenol (PCP) was studied on the membrane potential (Vm) and Ca2+ uptake in isolated single skeletal muscle cells of Balanus nubilus. 2. When compared with the controls, 0.1 mM PCP induced a significant (P < 0.05) increase in Ca2+ uptake accompanied by membrane depolarization (9 mV at 45 min incubation). This depolarization was reduced by 11% of extracellular Ca2+ (Cao2+) was replaced by Tris+ and by 50% if extracellular Na+ was also replaced by Tris+. 3. The Ca2+ channel blocker, verapamil (0.1 mM), completely inhibited the PCP-induced Ca2+ uptake as well as the membrane depolarization either in the absence or presence of Cao2+. Experiments on voltage-clamped cells show that the PCP-induced Ca2+ uptake was independent of the PCP-induced depolarization. 4. The results indicate that PCP induces activation of a verapamil-sensitive Ca2+ influx pathway (presumably L-type Ca2+ channels) independent of Vm. The permeation of Ca2+, Na+ and Tris+ through this pathway produces membrane depolarization in the following order of effectiveness: Ca2+ > Na+ > Tris+.

Correlation of synergistic cytotoxic effects of environmental chemicals in human fibroblasts with their lipophilicity

The cytotoxic combination effects of 2,4-D with 12 xenobiotics having different lipophilicity were investigated in human fibroblasts at their no effect concentrations (NOEC). Each of the chemicals tested in binary combinations enhanced the toxicity of 2,4-D. These synergistic combination effects were independent of the chemical structure of the test compounds. However, the NOEC's of the xenobiotics used in the combinations varied by a factor of 10,000. For strongly lipophilic compounds the lowest NOEC's were needed to induce synergistic cytotoxicity. A linear regression analysis of the concentrations (NOEC's) of the 12 combined xenobiotics against their lipophilicity revealed a correlation with r = 0.96 for 11 agents. This close correlation may be explained by the membrane damaging properties of lipophilic compounds which enhance the uptake of hydrophilic agents.

Distribution of hydrophobic ionizable xenobiotics between water and lipid membranes: pentachlorophenol and pentachlorophenate. A comparison with octanol-water partition

We have studied distribution of pentachlorophenol (PCP)--a major environmental pollutant--between egg-phosphatidylcholine (egg-PC) membranes and water. The objectives were (1) to compare the membrane-water partition of the un-ionized (HA) and ionized (A) PCP, and (2) to establish similarities and differences between the partition of PCP into lipid membranes and into octanol. The studies were made with egg-PC liposomes. It is shown that the distribution isotherms can be understood in terms of the Langmuir-Stern-Grahame adsorption model. The model is applicable to both the HA and A species; it takes into account the electrostatic interactions at the membrane-water interface charged by the adsorbed pentachlorophenate. Relationships between the membrane surface adsorption and bulk partition characteristics were presented and used to relate the partition of PCP into egg-PC membranes to those for octanol-water systems. Results (egg-PC membranes): bulk distribution coeff. gamma HA = 2.9 x 10(5), gamma A = 1.6 x 10(4), association constant KmHA = 2.9 x 10(5) M-1, KmA = 0.7 x 10(5) M-1, adsorption site area PsHA = 0.6 nm2, PsA = 3.5 nm2, and linear partition coeff. Beta mHA = 550 microns, beta mA = 30 microns. Comparable to gamma HA and gamma A for octanol-water are P(ow)(HA) approximately 1.3 x 10(5) and P(ow)(A) approximately 30. The major difference is in the distribution of ionized PCP which is several hundred times greater for egg-PC membranes compared to octanol. The difference is associated with the properties of the membrane-water interface.

AHA- heterodimer of a class-2 uncoupler: pentachlorophenol

AHA- heterodimers formed by association of neutral molecules of weak acid (HA) with its conjugate anion (A-) have been proposed to be the charged membrane-permeable species of class-2 uncouplers. Past attempts to extract and identify AHA- heterodimers failed. We have measured optical spectra of HA+A- (1:1) solutions of pentachlorophenol (PCP) in various solvents and in the presence of PC liposomes. Optical studies were supplemented by nuclear magnetic resonance measurements of HA+A- (1:1) solutions of PCP in dichloroethane to gain insight into the formation of AHA- species in lipid membranes. From these experiments, we found evidence for AHA- formation in non-hydrogen-bonding solvents, then reported the AHA- formation constant Kf and the molar absorptivity epsilon AHA-(lambda). Kf decreases with increasing dielectric constant, kappa, from 1210 +/- 130 M-1 for dichloroethane (kappa 10.7), to 340 +/- 34 M-1 for acetonitrile (kappa 37.5); Kf also decreases with increasing concentration of water. In hydrogen-bonding solvents, octanol (kappa 10.3) and methanol (kappa 33.5) and in liposomes, AHA- heterodimers are not observed optically. We estimate Kf for PCP in lipid bilayers from a combination of data on membrane electrical conductivity and surface density of adsorbed PCP. Our estimate for lipid bilayer, 0.005 < Kf < 0.5 M-1, is consistent with our inability to detect the AHA- species optically in liposome suspensions. We propose that penetration of water into the membrane inhibits formation of AHA- in lipid bilayers.

Analysis of membrane-bound acceptors. A correction function for non-specific accumulation of poorly water-soluble hydrophobic or amphipathic ligands based on the ligand partition concept

Non-specific ligand accumulation into membrane material, which may contribute considerably to the experimental signal obtained in binding studies with labelled amphipathic and hydrophobic ligands, may be accounted for by linear partition of the ligands into the membrane phase. For application to binding data obtained at a single membrane-lipid concentration, a fitting procedure is proposed which allows one to correct for non-specific ligand partition. If the assumption is met that the amount of acceptor-bound ligand is small compared to the total amount present in the system, one can validly interpret the data in terms of total ligand concentrations. The apparent dissociation constants Kd(app) thus obtained should be corrected for the often large effects of the size of the partition compartment(s), by performing assays at several membrane-lipid concentrations. The importance of the latter correction is stressed and an approach for obtaining the characteristic effective dissociation constants Kd' is indicated. The procedure also yields estimates of the ligand/membrane partition coefficients.

Domains and anomalous adsorption isotherms of dipalmitoylphosphatidylcholine membranes and lipophilic ions: pentachlorophenolate, tetraphenylborate, and dipicrylamine

Dipalmitoylphosphatidylcholine (DPPC) vesicles acquire negative surface charge on adsorption of negatively charged pentachlorophenolate (PCP-), and lipophilic ions tetraphenylborate (TPhB-), and dipicrylamine (DPA-). We have obtained (a) zeta-potential isotherms from the measurements of electrophoretic mobility of DPPC vesicles as a function of concentration of the adsorbing ions at different temperatures (25-42 degrees C), and (b) studied the effect of PCP- on gel-to-fluid phase transition by measuring the temperature dependence of zeta-potential at different PCP- concentrations. The zeta-potential isotherms of PCP- at 25, 32, and 34 degrees C correspond to adsorption to membrane in its gel phase. At 42 degrees C the zeta-potential isotherm corresponds to membrane in its fluid phase. These isotherms are well described by a Langmuir-Stern-Grahame adsorption model proposed by McLaughlin and Harary (1977. Biochemistry. 15:1941-1948). The zeta-potential isotherm at 37 degrees C does not follow the single-phase adsorption model. We have also observed anomalous adsorption isotherms for lipophilic ions TPhB- and DPA- at temperatures as low as 25 degrees C. These isotherms demonstrate a gel-to-fluid phase transition driven by ion adsorption to DPPC membrane during which the membrane changes from weakly to a strongly adsorbing state. The anomalous isotherm of PCP- and the temperature dependence of zeta-potential can be described by a two-phase model based on the combination of (a) Langmuir-Stern-Grahame model for each phase, (b) the coexistence of gel and fluid domains, and (c) depression of gel-to-fluid phase transition temperature by PCP-. Within the anomalous region the magnitude of zeta-potential rapidly increases concentration of adsorbing species, which was characterized in terms of a Esin-Markov coefficient. This effect can be exploited in membrane-based devices. Comments are also made on the possible effect of PCP, as an uncoupler, in energy transducing membranes.

An investigation of the sensitivity of the ouabain-insensitive sodium efflux in single barnacle muscle fibers to pentachlorophenol

The aim of the present work was to explore the possibility that pentachlorophenol (PCP) influences the behavior of the resting Na efflux in single muscle fibers from the barnacle, Balanus nubilus. It is shown here that PCP causes a transitory rise in the Na efflux in both unpoisoned and ouabain-poisoned fibers and that the response is dose-dependent, the minimal effective concentration in ouabain treated fibers being less than 10(-6) M. The efficacy of PCP is significantly greater than that of 2,3,4-trichlorophenol. 2,3-Dichlorophenol is ineffective. This is also the case with phenol. The magnitude of the response to PCP is a function of external pH. Lowering pHe increases the response. The response has an absolute requirement for external Ca2+ and is a sigmoidal function of external Ca2+ concentration. Since treatment of these fibers with PCP in high concentration leads to prompt contraction, experiments were designed to determine whether the observed rise in ouabain-insensitive Na efflux is due to a fall in myoplasmic pCa and whether trigger Ca2+ originates from the bathing medium. The results obtained show that prior injection of ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA) or 1,2-bis(2-aminophenoxyethane-N,N,N',N'-tetraacetic acid (BAPTA) leads to a drastic reduction in the response to PCP. They also show that prior external application of verapamil or devapamil stops the response to PCP from occurring. Both Cd2+ and Co2+ are also effective but only temporarily. Last, the effects of ryanodine and 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) were tested, since the former is known to block the sarcoplasmic reticulum Ca2+ release channel, and the latter to impair the action of agents known to release Ca2+ from internal depots. Both ryanodine and TMB-8 are found to reduce the response to PCP. Taken together, these observations support the hypothesis that PCP stimulates the ouabain-insensitive Na efflux by increasing the internal free Ca2+ and that the increase in internal Ca2+ is due to the entry of trigger Ca2+ from the outside via Ca2+ channels, as well as release of Ca2+ by the sarcoplasmic reticulum via its channel. They also indicate that the efficacy of PCP depends on the 5 Cl atoms present in its aromatic ring and pHe.

Interaction with functional membrane proteins--a common mechanism of toxicity for lipophilic environmental chemicals?

1. The effects of several phenols, anilines and aliphatic alcohols on yeast plasma membrane H(+)-ATPase and purine transport system as well as on Na+, K(+)-ATPase and adenosine uptake by Chinese hamster ovary cells (CHO) were investigated. 2. In all cases an inhibition was observed, which could be correlated with the octanol/water partition coefficients of the substances tested, thus making quantitative structure-activity predictions possible. 3. The observed effects correlated well with the influence of the chemicals on cell growth. 4. The results suggest a common mechanism of toxicity by the action of hydrophobic xenobiotics on biomembranes.

Adsorption of aminopyridines to phosphatidylserine membranes

Aminopyridines belong to the class of compounds which facilitate synaptic transmission at low calcium concentration, an effect associated with the block of K+ channels, enhanced entry of calcium into presynaptic terminals and greater release of transmitter. We have measured the zeta-potential of phosphatidylserine vesicles in the presence of aminopyridines and some related compounds in order to relate the strength of association of the aminopyridines with their biological effectiveness. The dependence of zeta-potential on the concentration of aminopyridines was analyzed in terms of the Langmuir-Stern-Grahame adsorption model. The rank order of the association constants (in M-1) obtained in the study was as follows: 3,4-diaminopyridine (6.5), 4,5-diaminopyrimidine (3.8), 4-aminopyridine (2.6), 3-aminopyridine (1.8), 2-aminopyridine (1.6), 4-dimethylaminopyridine (0.5), 4-aminopyridine methiodide (0.2), and, as control, calcium (12.1). The comparison of association constants with published results of the electric potential maps obtained by the CNDO/2 method suggests that binding to phosphatidylserine membrane increases with the density of excess charge on the protonated aminopyridine ring. We find that the sequence of potencies of aminopyridines in blocking K+ channels, in releasing transmitter, and in the shifts of calcium concentration dependence of synaptic transmission are about the same as the sequence of association constants with the phosphatidylserine membrane. Assuming that the binding domain for aminopyridines in the presynaptic terminal has similar adsorption properties as the phosphatidylserine membrane, we estimate the electric potential difference between the domain and the external solution to be between -300 and -340 mV.

Adsorption to dipalmitoylphosphatidylcholine membranes in gel and fluid state: pentachlorophenolate, dipicrylamine, and tetraphenylborate

We measured the dependence of electrophoretic mobility of dipalmitoylphosphatidylcholine (DPPC) vesicles on the aqueous concentration of negatively charged ions of pentachlorophenol (PCP), dipicrylamine (DPA), and tetraphenylborate (TPhB). The objective was to determine how the physical state of hydrocarbon chains of lipids affects adsorption of lipophilic ions. The studies were done at 25 and 42 degrees C to determine adsorption properties of DPPC membrane in the gel and fluid state, respectively. From the analysis of zeta-potential isotherms in terms of Langmuir-Stern-Grahame model we obtained the association constant, K, the area of the adsorption site, Ps, and the linear partition coefficient, beta.

RESULTS

K, (x 10(4)M-1): K(gel): PCP (0.49 +/- 0.28), DPA (25 +/- 10), TPhB (31 +/- 10); K(fluid): PCP (4.5 +/- 0.9), DPA (74 +/- 21), TPhB (59 +/- 14); Ps, (nm2): Ps(gel): PCP (5.4 +/- 2.3), DPA (5.9 +/- 2), TPhB (5.0 +/- 1.7); Ps(fluid): PCP (4.5 +/- 0.4), DPA (5.2 +/- 0.4), TPhB (4.1 +/- 0.2); beta, (x 10(-5) m): beta(gel): PCP (0.15 +/- 0.09), DPA (7.1 +/- 0.3), TPhB (10 +/- 7); beta(fluid): PCP (1.7 +/- 0.3), DPA (24 +/- 7), TPhB (24 +/- 6). It was interesting to find that the adsorption site area for PCP, DPA, and TPhB were very similar for both the gel and fluid membranes; also, the areas were independent of the size and molecular structure of the adsorbing species. Using a simple discrete charge model the adsorption site areas for all species were consistent with a dielectric constant of 8-10 and with an ion adsorption depth of 0.4-0.6 nm below the water/dielectric interface. The delta delta G0 = delta G0(gel) - delta G0(fluid) was found to be about twice as large for PCP than for DPA and TPhB. This indicates that PCP will be significantly more adsorbed in the fluid and disordered regions of biomembranes, whereas the distribution of DPA and TPhB is expected to be relatively more even.

Pentachlorophenol-induced change of zeta-potential and gel-to-fluid transition temperature in model lecithin membranes

We have determined zeta-potentials for dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) membranes by measuring the electrophoretic mobility of multilayered vesicles and the temperatures of the gel-to-ripple-to-fluid phase transitions of sonicated vesicles by a photometric method. Some conclusions are: (1) The zeta-potentials of DMPC and DPPC vesicles become negative due to adsorption of ionized pentachlorophenol (PCP), (2) their magnitude changes, step-like, on gel-to-fluid transition and (3) the temperature of the step-like change in zeta-potential decreases with an increase in PCP concentration. (4) PCP exhibits a large effect on membrane structure: It induces an isothermal phase change from the ordered to disordered state, which is enhanced by monovalent salt in the aqueous phase. (5) Both ionized and unionized PCP decrease the melting phase transition temperature and abolish the pretransition, (6) the unionized species increases the melting transition width and (7) the ionized species is more potent in abolishing the pretransition. (8) The shorter chain lipid (DMPC) is more sensitive to the presence of PCP; the maximum decrease in delta Tt is 13 K (DMPC) and 7 K (DPPC) in the presence of ionized PCP. We have shown experimentally, by comparing the delta Tt from photometric studies with the density of adsorbed PCP derived from zeta-potential isotherms, that (9) the shift of the melting phase transition temperature increases linearly with the density of adsorbed PCP. (10) In contrast to membranes made of negatively charged lipids, the transition temperature of DMPC and DPPC membranes in the presence of PCP further decreases in the presence of monovalent salt. The salt effect is due to screening of the membrane surface leading to enhanced adsorption of ionized PCP and a depression in transition temperature. (11) It is shown that both the adsorption and the changes of gel-to-fluid phase transition temperature can be described in terms of the Langmuir-Stern-Grahame model and (12) proposed that future studies of membrane toxicity of PCP should be focused on its pH dependence.