Interpretation of the electron spin resonance spectra of nitroxide-maleimide-labelled proteins and the use of this technique in the study of albumin and biomembranes

Spin-Label Electron Paramagnetic Resonance Spectroscopy Reveals Effects of Wastewater Filter Membrane Coated with Titanium Dioxide Nanoparticles on Bovine Serum Albumin

The accumulation of proteins in filter membranes limits the efficiency of filtering technologies for cleaning wastewater. Efforts are ongoing to coat commercial filters with different materials (such as titanium dioxide, TiO2) to reduce the fouling of the membrane. Beyond monitoring the desired effect of the retention of biomolecules, it is necessary to understand what the biophysical changes are in water-soluble proteins caused by their interaction with the new coated filter membranes, an aspect that has received little attention so far. Using spin-label electron paramagnetic resonance (EPR), aided with native fluorescence spectroscopy and dynamic light scattering (DLS), here, we report the changes in the structure and dynamics of bovine serum albumin (BSA) exposed to TiO2 (P25) nanoparticles or passing through commercial polyvinylidene fluoride (PVDF) membranes coated with the same nanoparticles. We have found that the filtering process and prolonged exposure to TiO2 nanoparticles had significant effects on different regions of BSA, and denaturation of the protein was not observed, neither with the TiO2 nanoparticles nor when passing through the TiO2-coated filter membranes.

Comparative studies of water permeability of red blood cells from humans and over 30 animal species: an overview of 20 years of collaboration with Philip Kuchel

NMR measurements of the diffusional permeability of the human adult red blood cell (RBC) membrane to water (P(d)) and of the activation energy (E(a,d)) of the process furnished values of P(d) ~ 4 × 10(-3) cm/s at 25 °C and ~6.1 × 10(-3) cm/s at 37 °C, and E(a,d) ~ 26 kJ/mol. Comparative NMR measurements for other species showed: (1) monotremes (echidna and platypus), chicken, little penguin, and saltwater crocodile have the lowest P(d) values; (2) sheep, cow, and elephant have P(d) values lower than human P(d) values; (3) cat, horse, alpaca, and camel have P(d) values close to those of humans; (4) guinea pig, dog, dingo, agile wallaby, red-necked wallaby, Eastern grey kangaroo, and red kangaroo have P(d) values higher than those of humans; (5) mouse, rat, rabbit, and "small and medium size" marsupials have the highest values of P(d) (>8.0 × 10(-3) cm/s at 25 °C and >10.0 × 10(-3) cm/s at 37 °C). There are peculiarities of E(a,d) values for the RBCs from different species. The maximum inhibition of diffusional permeability of RBCs induced by incubation with p-chloromercuribenzene sulfonate varied between 0% (for the chicken and little penguin) to ~50% (for human, mouse, cat, sheep, horse, camel, and Indian elephant), and ~60-75% (for rat, guinea pig, rabbit, dog, alpaca, and all marsupials). These results indicate that no water channel proteins (WCPs) or aquaporins are present in the membrane of RBCs from monotremes (echidna, platypus), chicken, little penguin and saltwater crocodile whereas WCPs from the membranes of RBCs from marsupials have peculiarities.

The first discovered water channel protein, later called aquaporin 1: molecular characteristics, functions and medical implications

After a decade of work on the water permeability of red blood cells (RBC) Benga group in Cluj-Napoca, Romania, discovered in 1985 the first water channel protein in the RBC membrane. The discovery was reported in publications in 1986 and reviewed in subsequent years. The same protein was purified by chance by Agre group in Baltimore, USA, in 1988, who called in 1991 the protein CHIP28 (CHannel forming Integral membrane Protein of 28 kDa), suggesting that it may play a role in linkage of the membrane skeleton to the lipid bilayer. In 1992 the Agre group identified CHIP28's water transport property. One year later CHIP28 was named aquaporin 1, abbreviated as AQP1. In this review the molecular structure-function relationships of AQP1 are presented. In the natural or model membranes AQP1 is in the form of a homotetramer, however, each monomer has an independent water channel (pore). The three-dimensional structure of AQP1 is described, with a detailed description of the channel (pore), the molecular mechanisms of permeation through the channel of water molecules and exclusion of protons. The permeability of the pore to gases (CO(2), NH(3), NO, O(2)) and ions is also mentioned. I have also reviewed the functional roles and medical implications of AQP1 expressed in various organs and cells (microvascular endothelial cells, kidney, central nervous system, eye, lacrimal and salivary glands, respiratory apparatus, gastrointestinal tract, hepatobiliary compartments, female and male reproductive system, inner ear, skin). The role of AQP1 in cell migration and angiogenesis in relation with cancer, the genetics of AQP1 and mutations in human subjects are also mentioned. The role of AQP1 in red blood cells is discussed based on our comparative studies of water permeability in over 30 species.

Pharmacokinetic properties of nitroxide-labeled albumin in mice

We have conjugated bovine serum albumin (BSA) with a pyrrolidinyl nitroxide and report on the in vivo pharmacokinetic properties of this conjugate in mice. In vivo EPR measurements of nitroxide were obtained after intravenous injection of 30 mg of labeled BSA by analysis of the nitroxide signal from the tails of mice. Following in vivo nitroxide measurements, the animals were sacrificed by exsanguination and organs were removed for determination of nitroxide levels. The level of nitroxide as determined by in vivo measurements declined exponentially with time and had a half-life (t1/2) of 7 hours. Blood nitroxide levels also declined exponentially with time with an initial t1/2 of 70 minutes and a terminal t1/2 of 10 hours. Nitroxide concentration varied among different organs; no nitroxide was detected within brain whereas lung had high concentrations of nitroxide. Liver and kidney both had relatively low levels of oxidized nitroxide, though total nitroxide (reduced plus oxidized) accumulated in the kidneys with time. Nitroxide-labeled BSA was well tolerated by the mice, is relatively stable, and is mainly confined to the intravascular space. Nitroxide-labeled albumin may be useful as a contrast agent for MRI or EPR imaging.

An ESR study of pathologic red blood cell membranes (RBCM)

A comparative study of red blood cell membranes from normal subjects and beta-thalassemia and sickle cell anemia patients was performed by spin labeling at the lipidic and protein phase. The results show that the quantity of bound spin label is the same for sickle, thalassemic, and normal membranes. The data from 5-doxyl stearic acid suggest an increase in fluidity for the thalassemic membrane.

Critical temperatures for the interaction of free fatty acids with the erythrocyte membrane

Non-esterified long-chain fatty acids reduce the extent of hypotonic hemolysis at a certain low concentration range but cause hemolysis at higher concentrations. This biphasic behavior was investigated at different temperatures (0-37 degrees C) for lauric (12:0), myristic (14:0), palmitoleic (16:1), oleic (cis-18:1) and elaidic (trans-18:1) acids. The results are summarized as follows: (A) the fatty acids examined exhibit a high degree of specificity in their thermotropic behavior; (B) oleic acid protects against hypotonic hemolysis even at the highest concentrations, up to 15 degrees C, when it becomes hemolytic, but only in a limited concentration range; (C) elaidic acid does not affect the osmotic stability of erythrocytes up to 20 degrees C, when it starts protecting: above 30 degrees C, it becomes hemolytic at the highest concentrations; (D) palmitoleic acid is an excellent protecting agent at all temperatures in a certain concentration range, becoming hemolytic at higher concentrations; (E) lauric acid protects up to 30 degrees C and becomes hemolytic only above this temperature; (F) myristic acid exhibits an extremely unusual behavior at 30 and 37 degrees C by having alternating concentration ranges of protecting and hemolytic effects; (G) there is a common critical temperature for hemolysis at 30 degrees C for saturated and trans-unsaturated fatty acids; (H) the initial slope of Arrhenius plots of percent hemolysis at the concentration of maximum protection is negative for cis-unsaturated fatty acids and positive for saturated and trans-unsaturated fatty acids.

Characterization of protein spin labeling by maleimide: evidence for nitroxide reduction

A quantitative determination of maleimide spin label (MAL) binding in oxi and met hemoglobin (Hb) and bovine serum albumin are investigated using double integration to the ESR signal. This determination permitted the observation that a considerable fraction of MAL is reduced, losing its paramagnetism. Experiments using the same spin label with myoglobin and Hb with blocked-SH groups, where reduction was not observed, indicate the involvement of SH groups in the process. The 4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxyl spin label (which is not able to bind in the SH group) is reduced too, but the dependence on the molar ratio is different in comparison with the MAL case. In both cases the reduction percentage depends on the molar ratio spin label to protein and to the protein concentration. In order to obtain the total SH groups labeled (two in the Hb case) it is necessary to use an excessive amount of label (around 18:1) in the 0.5 mM Hb concentration.

Spin label studies on synaptosomal membranes of rat brain cortex during aging

Membrane lipids of brain synaptosomes of 2, 12 and 24 months old rats were labeled by stearic acid spin probes with a doxyl group in the C-5 and C-16 position, respectively. We demonstrated that the hydrophobic region of synaptosomal membranes became more rigid during the life span studied, and the region located nearer the surface of membranes displays a significantly decreased fluidity only in the oldest group. Membrane proteins labeled with a nitroxide derivative of maleimide suggest that conformational changes take place during aging.

Electron spin resonance studies of the effects of lipids on the environment of proteins in mitochondrial membranes

The physical state of mitochondrial membranes has been investigated by means of stearic acid spin labels and of a maleimide spin label covalently bound to protein sulfhydryl groups. Stearic acid spin labels 5-NS and 16-NS show that n-butanol enhances the lipid fluidity of mitochondrial membranes in the whole temperature range between 4 and 37 degrees C; the effects in the hydrophobic membrane core, probed by 16-NS, are already apparent at 10 mM butanol. In liposomes formed of mitochondrial phospholipids, a fluidizing effect appears only at much higher concentration. Such results are compatible with the idea that butanol destabilizes lipid-protein interactions. On the other hand, the ratio between weakly and strongly immobilized SH groups probed by maleimide spin label is only slightly affected in the temperature range of 4-37 degrees C by addition of high concentrations of n-butanol, indicating that the environments probed are stable to agents inducing fluidity changes in the lipids. There are, however, indications that the environment probed by maleimide is affected by lipids, since the spin label, when bound to lipid-depleted mitochondria, becomes more immobilized, reconstitution of such lipid-depleted membranes with phospholipids restores the original spectra.

Effects of temperature and pH on the water exchange through erythrocyte membranes: nuclear magnetic resonance studies

The temperature and pH dependence of water exchange has been studied on isolated erythrocytes suspended in isotonic buffered solutions. At pH 7.4 a break in the Arrhenius plot of water exchange time at around 26 degrees C was found. The mean value of the apparent activation energy of the water exchange time at temperatures higher than that of the discontinuity was 5.7 kcal/mole (+/- 0.4); at lower temperatures the values of the apparent activation energy were below 1.4 kcal/mole. The pH dependence of water exchange time of isolated erythrocytes revealed a marked increase of the water exchange time values in the acid range of pH; a much smaller variation of the same parameter occurs between pH 7.0 and 8.0. These finding could be correlated with other processes involving erythrocyte membranes that showed similar pH and temperature dependence and were considered to indicate state transitions in the membranes. It is suggested that the temperature and pH effects on water diffusion indicate that conformational changes and cooperative effects are implicated in the mechanism of this transport process.