Membrane processes associated with the osmotic-pulse incorporation of inositol hexaphosphate

Membrane behavior as influenced by partitioning of amphiphiles during drying: a comparative study in anhydrobiotic plant systems

During cellular desiccation, reduction in volume can in principle cause amphiphilic compounds to partition from the cytoplasm into membranes, with structural perturbance as the result. Here, we studied the effect of partitioning of endogenous amphiphiles on membrane surface dynamics in desiccation-tolerant and -intolerant, higher and lower plant systems, using electron paramagnetic resonance (EPR) spin probe techniques. Labeling cells with the amphiphilic spin probe perdeuterated TEMPONE (PDT) enabled partitioning into the various phases to be followed. During drying, PDT molecules preferentially partitioned from the aqueous cytoplasm into the membrane surface and, at advanced stages of water loss, also into oil bodies. There was no specific partition behavior that could be correlated with lower/higher plants or with desiccation-tolerance. In vivo labeling with 5-doxylstearate (5-DS) enabled membrane surface fluidity to be characterized. In hydrated plants, the 5-DS spectra contained an immobile and a fluid component. The characteristics of the immobile component could not be specifically correlated with either lower or higher plants, or with desiccation tolerance. The relative contribution of the fluid component to the 5-DS spectra was higher in lower plants than in higher plants, but considerably decreased with drying in all desiccation-tolerant organisms. In contrast, the proportion of the fluid component in desiccation-sensitive wheat seedling root was higher than that in desiccation-tolerant wheat axis and considerably increased at the onset of water loss. We suggest that partitioning of amphipaths fluidize the membrane surface, but that in desiccation-tolerant systems the membranes are protected from excessive fluidization.

Effect of endotoxin on lipid order and motion in erythrocyte membranes

Electron paramagnetic resonance employing a lipid-specific spin label has been used to investigate the molecular effects of endotoxin on the physical state of bilayer lipids in rat erythrocyte membranes. When added at a concentration as low as 40 micrograms/ml to whole blood (plasma plus leukocytes present), decreased membrane lipid motion was found in subsequently washed and spin-labeled intact erythrocytes (P < 0.02). However, if endotoxin were added to washed, plasma plus leukocyte-free intact erythrocytes, no change in the motion of the spin label was found, suggesting that plasma-soluble substances and/or leukocytes are required to produce the change in the physical state of lipids. The decreased lipid motion found in these studies is discussed with reference to the known decreased deformability of endotoxin-treated red cells and to the pathogenesis of sepsis.

Electron paramagnetic resonance studies of the effects of methoxyacetic acid, a teratologic toxin, on human erythrocyte membranes

Methoxyacetic acid (MAA), a teratogenic toxin, is the major metabolite of ethylene glycol monomethyl ether (EGME, also referred to as 2-methoxyethanol, 2-ME). MAA causes a wide range of toxic effects in laboratory animals including reproductive and developmental toxicity, as well as hematotoxicity, by mechanisms that are not clear. In this study, we employed electron paramagnetic resonance (EPR) spin-labeling techniques in conjunction with spin labels specific for cytoskeletal proteins, bilayer lipids, cell-surface sialic acid, or cell-surface galactose and N-acetylgalactosamine residues of human erythrocyte membranes in order to gain insight into the mechanism of MAA toxicity. The major findings are: (1) MAA significantly increases the protein-protein interactions of skeletal proteins in a concentration-dependent manner (P < 0.001), while 2-ME has no effect (at even a 2.5-fold higher concentration). (2) Addition of MAA leads to significant increase in the rotational motion of spin-labeled terminal galactose and N-acetylgalactosamine residues (2.0 mM MAA, 38% decrease of the apparent rotational correlation time tau a, P < 0.01). (3) The rotational motion of spin-labeled sialic acid, 70% of which is on the major transmembrane sialoglycoprotein (glycophorin A or PAS 1), was not affected by MAA treatment. (4) MAA has no effect on the lipid bilayer fluidity, since no change in the motion of a lipid bilayer specific spin label (5-NS) in the erythrocyte membrane was observed. These results suggest that MAA may lead to teratologic toxicity by interacting not with lipid components but with certain, perhaps specific, protein components, i.e., transport proteins, cytoskeleton proteins or neurotransmitter receptors.

Interaction of tacrine and velnacrine with neocortical synaptosomal membranes: relevance to Alzheimer's disease

The acridine-based, potential Alzheimer's disease therapeutic agents, tacrine and velnacrine, were incubated with rat or gerbil neocortical synaptosomal membranes. Electron paramagnetic resonance employing a protein-specific spin label was used to monitor this interaction. Analogous to their effects in erythrocyte membranes [Butterfield and Rangachari (1992) Biochem. Biophys. Res. Commun. 185: 596-603], in the present studies both agents decreased segmental motion of spin labeled synaptosomal membrane proteins, consistent with increased cytoskeletal protein-protein interactions (0.001 < P < 0.005), and tacrine was more potent than velnacrine. These results are discussed with possible relevance to molecular actions of the agents and molecular alterations in Alzheimer's disease.

Acetylcarnitine increases membrane cytoskeletal protein-protein interactions

Electron paramagnetic resonance has been used to investigate the effects of interaction of acetylcarnitine with cytoskeletal proteins in human erythrocyte membranes. This compound, currently in clinical trials as a potential therapeutic agent for Alzheimer's disease, caused a highly significant increase in cytoskeletal protein-protein interactions. Carnitine, the parent compound, also increased cytoskeletal protein-protein interactions, suggesting that the acetyl group is not hydrophobic enough to direct acetylcarnitine to the bilayer phase of the membrane. Consistent with this suggestion, no change in lipid order or dynamics with acetylcarnitine was observed. These results are discussed in terms of possible implications to Alzheimer's disease treatment.