title: Low CO2 permeability of cholesterol-containing liposomes detected by stopped-flow fluorescence spectroscopy.
abstract: Here we ask the following: 1) what is the CO2 permeability (Pco2) of unilamellar liposomes composed of l-α-phosphatidylcholine (PC)/l-α-phosphatidylserine (PS) = 4:1 and containing cholesterol (Chol) at levels often occurring in biologic membranes (50 mol%), and 2) does incorporation of the CO2 channel aquaporin (AQP)1 cause a significant increase in membrane Pco2? Presently, a drastic discrepancy exists between the answers to these two questions obtained from mass-spectrometric (18)O-exchange measurements (Chol reduces Pco2 100-fold, AQP1 increases Pco2 10-fold) vs. from stopped-flow approaches observing CO2 uptake (no effects of either Chol or AQP1). A novel theory of CO2 uptake by vesicles predicts that in a stopped-flow apparatus this fast process can only be resolved temporally and interpreted quantitatively, if 1) a very low CO2 partial pressure (pCO2) is used (e.g., 18 mmHg), and 2) intravesicular carbonic anhydrase (CA) activity is precisely known. With these prerequisites fulfilled, we find by stopped-flow that 1) Chol-containing vesicles possess a Pco2 = 0.01cm/s, and Chol-free vesicles exhibit ∼1 cm/s, and 2) the Pco2 of 0.01 cm/s is increased ≥ 10-fold by AQP1. Both results agree with previous mass-spectrometric results and thus resolve the apparent discrepancy between the two techniques. We confirm that biologic membranes have an intrinsically low Pco2 that can be raised when functionally necessary by incorporating protein-gas channels such as AQP1.
authors.affiliation: *Institut für Biophysikalische Chemie, Medizinische Hochschule Hannover, Hannover, Germany; Departement Chemie, Universität Basel, Basel, Switzerland; Department Chemistry & Molecular Biology, University of Gothenburg, Göteborg, Sweden; and Institut für Molekular- und Zellphysiologie, AG Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany.
authors.forename: Georgios
authors.initials: G
authors.lastname: Tsiavaliaris
authors.affiliation: *Institut für Biophysikalische Chemie, Medizinische Hochschule Hannover, Hannover, Germany; Departement Chemie, Universität Basel, Basel, Switzerland; Department Chemistry & Molecular Biology, University of Gothenburg, Göteborg, Sweden; and Institut für Molekular- und Zellphysiologie, AG Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany.
authors.forename: Fabian
authors.initials: F
authors.lastname: Itel
authors.affiliation: *Institut für Biophysikalische Chemie, Medizinische Hochschule Hannover, Hannover, Germany; Departement Chemie, Universität Basel, Basel, Switzerland; Department Chemistry & Molecular Biology, University of Gothenburg, Göteborg, Sweden; and Institut für Molekular- und Zellphysiologie, AG Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany.
authors.forename: Kristina
authors.initials: K
authors.lastname: Hedfalk
authors.affiliation: *Institut für Biophysikalische Chemie, Medizinische Hochschule Hannover, Hannover, Germany; Departement Chemie, Universität Basel, Basel, Switzerland; Department Chemistry & Molecular Biology, University of Gothenburg, Göteborg, Sweden; and Institut für Molekular- und Zellphysiologie, AG Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany.
authors.forename: Samer
authors.initials: S
authors.lastname: Al-Samir
authors.affiliation: *Institut für Biophysikalische Chemie, Medizinische Hochschule Hannover, Hannover, Germany; Departement Chemie, Universität Basel, Basel, Switzerland; Department Chemistry & Molecular Biology, University of Gothenburg, Göteborg, Sweden; and Institut für Molekular- und Zellphysiologie, AG Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Meier
authors.affiliation: *Institut für Biophysikalische Chemie, Medizinische Hochschule Hannover, Hannover, Germany; Departement Chemie, Universität Basel, Basel, Switzerland; Department Chemistry & Molecular Biology, University of Gothenburg, Göteborg, Sweden; and Institut für Molekular- und Zellphysiologie, AG Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany gros.gerolf@mh-hannover.de.
authors.forename: Gerolf
authors.initials: G
authors.lastname: Gros
authors.affiliation: *Institut für Biophysikalische Chemie, Medizinische Hochschule Hannover, Hannover, Germany; Departement Chemie, Universität Basel, Basel, Switzerland; Department Chemistry & Molecular Biology, University of Gothenburg, Göteborg, Sweden; and Institut für Molekular- und Zellphysiologie, AG Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany.
authors.forename: Volker
authors.initials: V
authors.lastname: Endeward
mesh_terms: D051398:Aquaporin 1 / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism*; D002463:Cell Membrane Permeability; D002784:Cholesterol / Q000378:metabolism*; D006801:Humans; D008081:Liposomes / Q000378:metabolism*; D010713:Phosphatidylcholines / Q000378:metabolism*; D013050:Spectrometry, Fluorescence / Q000379:methods*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: C496020:AQP1 protein, human; D008081:Liposomes; D010713:Phosphatidylcholines; D002245:Carbon Dioxide; D051398:Aquaporin 1; D002784:Cholesterol
keywords: BCECF; aquaporin 1; carbonic anhydrase; carboxyfluorescein; phospholipid vesicles