Use of electron spin resonance spectroscopy of spin labels for studying drug-induced membrane perturbation

Spin-Label Studies on Synaptic Membrane Lipids: Effect of Cholinergic Agonists, Antagonists, and Cholinesterase Inhibitors

The effects on membrane fluidity of inhibitors of acetylcholinesterase (A ChE) as well as those of certain nicotinic agonists have been examined in synaptosomes prepared from the electroplax of Torpedo californica. Electron paramagnetic resonance (EPR) spectrometry was used to monitor changes in the order parameters of three spin-labeled fatty acid probes, doxyl stearic acid derivatives, intercalated in the lipid bilayer of synaptosomal membranes. Neither inhibition of A ChE nor the application of nicotinic ligands significantly influenced the viscosity of the membrane bilayer as judged by changes in the order parameters of the probes. A small, narrow line component of the EPR spectra disappeared when high concentrations of acetylcholine (ACh) were added to the synaptosomes. This effect was not seen with other cholinergic agonists, e.g., car-bachol. The results of this investigation suggest that if anti-A ChEs produce damaging effects on the lipid components of nerve membranes, the effects are indirect, resulting from accumulation of high ACh levels and not from a direct action of the agent on the membrane lipid.

Influence of 2-chlorodeoxyadenosine (cladribine) on human erythrocytes

2-Chlorodeoxyadenosine (2-CdA, cladribine) is one of the newest chemotherapy drugs which has been around and in use for a few years. Drug in tumour cells causes the inhibition of DNA synthesis and repair processes in replication cells, and the accumulation of DNA strand breaks in nonproliferating cells. The present study was undertaken to characterize the influence of cladribine on the fluidity of the lipid bilayer and protein conformation in human erythrocytes. The effect of cladribine on the erythrocyte membrane structure was examined by electron spin resonance (ESR) spectroscopy and fluorescence measurements. It was observed that under the studied conditions (c: 0.1-5 microg/ml, t = 1 h, 37 degrees C), cladribine localised mainly in the erythrocyte membrane and affected its organization. The alterations in the fluidity were observed mainly in the deeper regions of the cell membrane. The incorporation of drug into human erythrocytes also caused negligible conformational alterations of membrane cytoskeletal proteins and did not change the internal viscosity of the cells. We can conclude from these data that 2-CdA in vitro is significantly much less toxic to erythrocytes than anthracycline drugs, which are used in treatment of leukemias. However, the higher concentrations of 2-CdA (about 5 microg/ml) can be also toxic to erythrocytes.

The combined effect of IDA and glutaraldehyde on the properties of human erythrocytes

The effect of IDA and glutaraldehyde on the properties of human erythrocytes was examined by Electron Spin Resonance spectroscopy and fluorescence measurements. In this study glutaraldehyde was used as the agent linking the drug to the erythrocyte membrane. We have demonstrated that idarubicin (IDA) alone caused only negligible changes of the membrane fluidity. When IDA preincubated erythrocytes were treated with glutaraldehyde, the alterations in the fluidity were observed in the polar parts as well as in the deeper regions of the cell membrane. The incorporation of drug and glutaraldehyde into human erythrocytes also caused conformational alterations of membrane cytoskeletal proteins and changes in the internal viscosity of the cells. Our data suggest that glutaraldehyde in idarubicin-pretreated erythrocytes may potentiate the drug toxicity leading to significant perturbations in the organization of the plasma membrane lipids and proteins.

Electron paramagnetic resonance spectroscopy for the investigation of the fluidity of human spermatozoa plasma membranes: a feasibility study

Spermatozoal membrane perturbations may play a role in abnormal sperm functions. The objective of this investigation was to study the feasibility of measuring membrane fluidity of isolated human sperm by electron paramagnetic resonance (EPR) spectroscopy and to compare the order parameter of spectra obtained from the sperm plasma membranes of living sperm of fertile men with that of infertile men. Ejaculates of infertile and fertile men were washed and the spermatozoa labelled with 5-doxylstearic acid (5-DSA) and 16-doxylstearic-acid (16-DSA) (10 nmol per 4 x 10(7) sperm). The reporter group of 5-DSA partitions into the outer, hydrophilic part of the sperm plasma membrane, whereas that of 16-DSA is distributed in the inner hydrophobic part. The following results were obtained: (i) the lowest measurable cell count was 3.6 to 7 x 10(6) sperm and the interassay variance of the order-parameter s was < 1%; (ii) swim-up experiments revealed a higher fluidity of sperm with a higher percentage of motility; (iii) sperm membranes of infertile patients exhibited a decreased fluidity of their plasma membranes in the polar interface region of 5-DSA compared with volunteer semen donors and fertile men (P=0.002). No difference of membrane fluidity was found between the different groups using 16-DSA. It is concluded that EPR spectroscopy can be used to study the fluidity of sperm plasma membranes in fertile and infertile men.

The effect of daunorubicin and glutaraldehyde treatment on the structure of erythrocyte membrane

The effect of daunorubicin (DNR) and glutaraldehyde on erythrocyte membrane structure was examined by Electron Spin Resonance spectroscopy. Human erythrocytes were incubated with daunorubicin and then with glutaraldehyde to prevent drug efflux. We have demonstrated that DNR alone caused changes in membrane fluidity mainly in the hydrophobic regions of the lipid bilayer. When DNR-preincubated erythrocytes were treated with glutaraldehyde, the alterations in fluidity were observed in the polar regions as well as in the deeper regions of the cell membrane. The incorporation of drug and glutaraldehyde into human erythrocytes also caused conformational alterations in membrane cytoskeletal proteins and changes in the internal viscosity of the cells. The results suggest that glutaraldehyde in the drug-pretreated erythrocytes may lead to significant perturbations in the organization of the plasma membrane lipids and proteins.

Surface charge, fluidity, and calcium uptake by rat intestinal brush-border vesicles

Biological membrane outer surfaces are negatively charged and interact with positively charged calcium ion during calcium uptake. Positively charged polycations such as polyarginine bind to membranes with high affinity, displacing bound calcium from the membrane. We tested the effect of polyarginine on uptake of calcium by brush-border membrane vesicles and examined the responses in terms of membrane fluidity by electron paramagnetic resonance (EPR). Polyarginine inhibited the saturable component of calcium uptake by a mechanism combining inhibition characteristics of strontium (competitive) and magnesium (non-competitive). Unlike the inhibition of non-saturable calcium uptake by strontium and magnesium, polyarginine increased kD, the rate constant for non-saturable calcium uptake, by a concentration dependent mechanism. These effects of polyarginine on calcium uptake were associated with decreased membrane fluidity at the uptake temperature. These findings are consistent with a role for surface negative charge in determining both saturable and non-saturable calcium uptake. Increased membrane fluidity is associated with decreased saturable and increased non-saturable calcium uptake. Although increased fluidity might be involved in the increased kD for non-saturable uptake, the concentration-specific stimulating effect of polyarginine suggests a gating mechanism.

Perturbation effect of eight calcium channel blockers on liposomal membranes prepared from rat brain total lipids

EPR spectroscopy of phosphatidylcholine or stearic acid labeled at the doxyl group at the 16-carbon position was used to compare the perturbation effect of eight calcium channel blockers (CB) on overall dynamics/disorder of the hydrophobic part of liposome membranes prepared from rat brain total lipids at the drug/lipid molar ratio of 1/2. Nifedipine (NIF), nimodipine, niludipine and nitrendipine had a minor effect on the dynamics/disorder of the liposome membranes, whereas the disordering effect of verapamil (VER), mepamil, gallopamil and diltiazem was more pronounced. Concentration dependence of the overall disordering effect of VER on liposomal membranes was found at the VER/lipid ratio greater than 0.02 and for the tranquilizer thioridazine greater than 0.005. VER exerted a disordering effect at the hydrophobic part of synaptosomal membranes at concentrations greater than or equal to 0.32 mmol/l, whereas NIF did not exhibit a disordering effect even at concentrations of 10-20 mmol/l.

ESR analysis with long-chain alkyl spin labels in bovine blood platelets. Relationship between the increase in membrane fluidity by alcohols and phenolic compounds and their inhibitory effects on aggregation

Four spin-labeled probes (5-doxylstearic acid (5-NS), its methyl ester (5-NMS), 16-doxylmethylstearate (16-NMS) and 4-(N,N-dimethyl-N-pentadecyl)ammonium-2,2,6,6-tetramethylpiperidine-1-ox yl (CAT-15)) were used to monitor membrane fluidity change in bovine platelets induced by three alkyl alcohols, benzyl alcohol and two phenolic compounds. The relationship between the increase in membrane fluidity induced by these compounds and their inhibitory effects on platelet aggregation was observed. Experiments with the four probes showed that n-hexyl alcohol induced decreases in the order parameter of 5-NS and apparent rotational correlation times of the other probes at the same minimal alcohol concentration. The decreases were observed in the concentration range that inhibited aggregation. n-Amyl alcohol and n-butyl alcohol decreased the values of the parameters of the above mentioned only at higher concentrations that were dependent on their hydrophobicities. Like alkyl alcohols, benzyl alcohol and phenolic compounds decreased the values of the parameters in the concentration ranges in which these compounds inhibited platelet aggregation. The concentration of these compounds causing 50% inhibition of platelet aggregation, the IC50 values, and data on 5-NS-labeled platelets indicated that they inhibited aggregation and decreased the value of the order parameter at lower concentrations relative to their Poct values in comparison to the effective concentrations of alcohols. Phenolic compounds also decreased the values of the apparent rotational correlation times of 5-NMS and 16-NMS. These results indicate that the inhibition of platelet aggregation by alcohols and phenolic compounds is due to membrane perturbation in wide range in depths within the lipid bilayer.