Stratum corneum protein mobility as evaluated by a spin label maleimide derivative

Electron paramagnetic resonance study of lipid and protein membrane components of erythrocytes oxidized with hydrogen peroxide

Electron paramagnetic resonance (EPR) spectroscopy of spin labels was used to monitor membrane dynamic changes in erythrocytes subjected to oxidative stress with hydrogen peroxide (H(2)O(2)). The lipid spin label, 5-doxyl stearic acid, responded to dramatic reductions in membrane fluidity, which was correlated with increases in the protein content of the membrane. Membrane rigidity, associated with the binding of hemoglobin (Hb) to the erythrocyte membrane, was also indicated by a spin-labeled maleimide, 5-MSL, covalently bound to the sulfhydryl groups of membrane proteins. At 2% hematocrit, these alterations in membrane occurred at very low concentrations of H(2)O(2) (50 µM) after only 5 min of incubation at 37°C in azide phosphate buffer, pH 7.4. Lipid peroxidation, suggested by oxidative hemolysis and malondialdehyde formation, started at 300 µM H(2)O(2) (for incubation of 3 h), which is a concentration about six times higher than those detected with the probes. Ascorbic acid and α-tocopherol protected the membrane against lipoperoxidation, but did not prevent the binding of proteins to the erythrocyte membrane. Moreover, the antioxidant (+)-catechin, which also failed to prevent the cross-linking of cytoskeletal proteins with Hb, was very effective in protecting erythrocyte ghosts from lipid peroxidation induced by the Fenton reaction. This study also showed that EPR spectroscopy can be useful to assess the molecular dynamics of red blood cell membranes in both the lipid and protein domains and examine oxidation processes in a system that is so vulnerable to oxidation.

Dynamics and partitioning of spin-labeled stearates into the lipid domain of stratum corneum

The EPR spectra of the positional isomers n-doxyl stearic acids (n-DSA), with n=5, 12 and 16, and 5-doxyl methyl stearate (5-DMS) structured in the lipid domain of intact stratum corneum (SC), are characterized by the thermodynamic equilibrium of two distinct spectral components provided by two different motional states of the spin-labeled chains. A two-component model used in the EPR spectra simulations provided the relative populations of the components, allowing for the calculation of the thermodynamic profile. Based on a detailed investigation, the more motionally restricted population of spin labels (component 1) is found to arise when the spin label is hydrogen-bonded to the polar surfaces of the membranes, while the less motionally restricted population (component 2) is generated by spin labels nonhydrogen-bonded and more deeply inserted in the hydrophobic core. The 5-DSA is bound tightly to the polar surfaces (DeltaG(o)2 --> 1=-1.75 kcal/mol and DeltaH(o)2 --> 1=-13.8 kcal/mol), whereas the more lipophilic 5-DMS has a major spin population stabilized in the hydrophobic core (DeltaG(o)2 --> 10.57 kcal/mol and DeltaH(o)2 --> 1=-9.1 kcal/mol). Upon lipid-depleting SC increases the interactions of the probe with the polar surfaces, thereby decreasing its rotational diffusion. In contrast, the treatment of SC with oleic acid, a permeation enhancer, drastically increases the mobility of the spin labels, particularly that of component 1, and the thermodynamic equilibrium shifts towards the formation of component 2. A mechanism for water permeation in SC is also proposed.

Dynamics of proteins and lipids in the stratum corneum: Effects of percutaneous permeation enhancers

We have spin labeled the stratum corneum (SC) with a lysine specific reagent, succinimidyl-2,2,5,5-tetramethyl-3-pirroline-1-oxyl-carboxylate spin label (SSL), to assess the dynamics and hydration degree of SC proteins by electron paramagnetic resonance (EPR) spectroscopy taking measurements directly from the intact tissue. Treating the SC with two percutaneous penetration enhancers, 8 M urea or 20% (v/v) 1-methyl-2-pyrrolidone (1 MP), destabilizes the proteins thus promoting more mobile and solvent-exposed protein conformations. Upon SC lipid depletion the nitroxide side chain becomes more solvent exposed, suggesting that the removal of hygroscopic substances in the extraction process favors more hydrated protein conformations. On the other hand, the treatments with 8 M urea or 40% (v/v) 1 MP did not alter significantly the fluidity in the SC lipid domain as assessed by the probe 5-doxyl stearic acid; these permeation enhancers, specially 1 MP, seem to increase the probe solubility in the solvent leading to a considerable fraction of spin label to be removed from the lipid domain.

Hydration effects on the protein dynamics in stratum corneum as evaluated by EPR spectroscopy

The uppermost layer of the epidermis, the stratum corneum (SC), was spin-labeled with a sulfhydryl-specific nitroxide reagent to investigate the water content effects upon the protein dynamics directly in the intact tissue. A two-state model for the nitroxide side chain described the coexistence of two spectral components in the electron paramagnetic resonance (EPR) spectra. The so-called strongly immobilized component, S, is associated with the EPR signal of a motionally restricted nitroxide fraction having its N-O group hydrogen bonded to protein (rigid structure) while the weakly immobilized component, W, corresponds to the signal provided by the spin labels with higher mobility (approximately 10 times greater) exposed to the aqueous environment. The relative populations between these two mobility states, S and W, are in thermodynamic equilibrium. The standard Gibbs free energy, enthalpy and entropy changes for transferring the nitroxide side chain from the state contacting the solvent, W, to the one contacting protein, S, indicated that the reduction of the SC water content to below approximately h 0.69 g H(2)O/g dry SC, stabilizes the protein interacting state, S. Upon decreasing the SC hydration level below approximately h 0.69 the segmental motion of the polypeptide chains and the rotational motion of the spin-labeled side chain were also constrained. This work can also be useful to improve the spectral analysis of site-directed spin labeling, especially for a more quantitative description in terms of thermodynamic parameters.

Stratum corneum protein dynamics as evaluated by a spin-label maleimide derivative: effect of urea

The stratum corneum (SC) protein dynamics in the sulfhydryl group regions was studied by electron paramagnetic resonance (EPR) spectroscopy of a covalently attached maleimide derivative spin label. A two-state model for the nitroxide described the coexistence of two spectral components in the EPR spectra. The so-called strongly immobilized component arises from a spin-label fraction with the nitroxide moiety hydrogen-bonded to protein (rigid structure) and the weakly immobilized component is provided by the spin labels with higher mobility (approximately 10 times greater) exposed to the aqueous environment. The relative populations between these two states are in thermodynamic equilibrium. The apparent energetic gain for the nitroxide to form a hydrogen bond with the backbone rather than to be dissolved in the local environment was approximately 10 kcal/mol in the temperature range of 2-30 degrees C and approximately 6 kcal/mol in the range of 30-70 degrees C. Urea treatment caused a drastic increase in the segmental motion of the polypeptide chains that was completely reversible by its removal. Our analyses also indicated that the urea induced unfolding of the SC proteins opening the thiol group cavities. This work can also be useful to improve the spectral analysis of site-directed spin-labeling, especially for a more quantitative description of the nitroxide side chain mobility.