title: Transmembrane Difference in Colloid Osmotic Pressure Affects the Lipid Membrane Fluidity of Liposomes Encapsulating a Concentrated Protein Solution.
abstract: A hemoglobin vesicle (Hb-V) is an artificial oxygen carrier encapsulating a highly concentrated hemoglobin solution (40 g/dL) in a liposome. The in vivo safety and efficacy of Hb-V suspension as a transfusion alternative and structural stability during storage have been studied extensively. Because the intraliposomal Hb aqueous solution can possess colloid osmotic pressure (COP, 200-300 Torr) that is much higher than that of blood plasma (20-25 Torr), a question arises as to whether the lipid membrane senses the transmembrane difference in COP. We examined the membrane microviscosity using a fluorescence polarization technique. To avoid the interference of red Hb on the fluorescence measurement, we used human serum albumin (HSA) as a substitute for Hb. Both HSA and Hb solutions show high COP depending on the concentration. Encapsulation of HSA solution (40 g/dL) in the liposome decreased the membrane microviscosity at a lower temperature (949 ± 8 cP → 607 ± 10 cP at 25 °C). The result indicates that the transmembrane osmotic stress induced by HSA encapsulation expands the liposome maximally with increasing spherical surface area, and the membrane fluidity is increased extremely. Even for such a condition, the lowest membrane microviscosity, 377 ± 10 cP at 60 °C, is much higher than that of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine liposome (40 ± 2 cP at 60 °C). Accordingly, Hb-V as well as HSA-V maintains a spherical structure and mechanical stability under transmembrane stress caused by high COP, as described in the literature.
authors.affiliation: Department of Chemistry, Nara Medical University , 840 Shijo-cho, Kashihara, Nara 634-8521, Japan.
authors.forename: Tomoko
authors.initials: T
authors.lastname: Kure
authors.affiliation: Department of Chemistry, Nara Medical University , 840 Shijo-cho, Kashihara, Nara 634-8521, Japan.
authors.forename: Hiromi
authors.identifier: 0000-0002-0681-3032
authors.initials: H
authors.lastname: Sakai
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D002214:Capsules; D004161:Diphenylhexatriene / Q000737:chemistry; D004337:Drug Carriers / Q000737:chemistry*; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes / Q000737:chemistry; D006454:Hemoglobins / Q000737:chemistry*; D006801:Humans; D008081:Liposomes / Q000737:chemistry*; D008560:Membrane Fluidity; D009997:Osmotic Pressure; D010714:Phosphatidylethanolamines / Q000737:chemistry; D011092:Polyethylene Glycols / Q000737:chemistry; D000075462:Serum Albumin, Human / Q000737:chemistry*; D014783:Viscosity
chemical_list: D002214:Capsules; D004337:Drug Carriers; D005456:Fluorescent Dyes; D006454:Hemoglobins; D008081:Liposomes; D010714:Phosphatidylethanolamines; C107570:polyethylene glycol-distearoylphosphatidylethanolamine; D004161:Diphenylhexatriene; D015060:1,2-Dipalmitoylphosphatidylcholine; D011092:Polyethylene Glycols; D000075462:Serum Albumin, Human