Partial Volumes of Phosphatidylcholines and Vitamin E: α-Tocopherol Prefers Disordered Membranes

Abstract

Despite its discovery over 95 years ago, the biological and nutritional roles of vitamin E remain subjects of much controversy. Though it is known to possess antioxidant properties, recent assertions have implied that vitamin E may not be limited to this function in living systems. Through densitometry measurements and small-angle X-ray scattering we observe favorable interactions between α-tocopherol and unsaturated phospholipids, with more favorable interactions correlating to an increase in lipid chain unsaturation. Our data provide evidence that vitamin E may preferentially associate with oxygen sensitive lipidsan association that is considered innate for a viable membrane antioxidant.

Vitamin E Does Not Disturb Polyunsaturated Fatty Acid Lipid Domains

“…From contrast-matched small-angle neutron scattering, it is evident that high concentrations (χ = 0.1) of tocopherol decrease domain stability and promote lipid mixing, regardless of isoform. This observation is consistent with our previous work as well as with experiments by Yang et al and simulations by Muddana et al that demonstrate a linactant-like association of αToc with domain boundaries at high concentrations. ,, In each instance, the linactant effect is observed in compositions with high line tension, and it is unclear if this association is recapitulated in the DSPC/PLiPC/Chol system studied here, as tocopherols are suspected to preferentially associate with the disordered PUFA phase due to the inherent disorder of the phytyl tail. − …”

“…The presence of nanodomains suggests a significant competing interaction to those minimizing line tension, and as such, perturbations will likely be more evident in this composition. In addition, the composition used here includes an oxidation-sensitive PUFA lipid, which is proposed to interact more preferentially with the tocopherol species. − …”

“…Vitamin E is widely recognized as a potent lipid-soluble antioxidant serving to protect cell membranes from irreversible oxidative damage. ,− The presence of oxidation-sensitive polyunsaturated fatty acid (PUFA) lipids is accompanied by increased levels of vitamin E, and work has presumed an affinity of vitamin E for these disordered environments. − Neutron diffraction studies of αToc in lipid bilayers have confirmed a depth just below the membrane interface and orientation that coincides with an antioxidant function. ,− From this position, oxidized tocopherol can be recycled by more abundant aqueous antioxidants or terminated as α-tocopherylquinone (αTocQ, Figure B) . A wealth of research endorses an antioxidant role of vitamin E; yet, a lack of in vivo evidence and perplexingly low physiological abundance raises concerns about the ability of vitamin E to be effective as the membrane’s front-line defense against oxidation. ,− …”

Understanding the Retention of Vaping Additives in the Lungs: Model Lung Surfactant Membrane Perturbation by Vitamin E and Vitamin E Acetate

“…α-Tocopherol was shown to have a stabilizing effect on disordered bilayers wherein it suppressed the bilayer thermal expansivity. 16 A direct comparison of the isotherms for a given concentration of additive is provided in Figure S1 of the Supporting Information. The isotherms of monolayers containing 0, 1, and 5 mol % additive are qualitatively similar in shape.…”

“…This is in agreement with partial molar volume measurements that show a larger tocopherol partial molar volume in a DPPC fluid phase (above its T m ) than in a POPC fluid phase at the same temperature. 16 Notably, where each lipid forms a liquid-expanded phase, we have contraction of the film (all pressures for POPG, low pressures for DPPC). This contraction is significantly greater for saturated and zwitterionic DPPC lipids.…”