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Low CO 2 permeability of cholesterol‐containing liposomes detected by stopped‐flow fluorescence spectroscopy
Author(s) -
Tsiavaliaris Georgios,
Itel Fabian,
Hedfalk Kristina,
AlSamir Samer,
Meier Wolfgang,
Gros Gerolf,
Endeward Volker
Publication year - 2015
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fj.14-263988
Subject(s) - chemistry , vesicle , liposome , phosphatidylcholine , membrane , biophysics , pco2 , fluorescence spectroscopy , fluorescence , carbonic anhydrase , analytical chemistry (journal) , chromatography , biochemistry , phospholipid , biology , medicine , enzyme , physics , quantum mechanics
Here we ask the following: 1) what is the CO 2 permeability (P co 2 ) 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 CO 2 channel aquaporin (AQP)1 cause a significant increase in membrane P co 2 ? Presently, a drastic discrepancy exists between the answers to these two questions obtained from mass‐spectrometric 18 O‐exchange measurements (Chol reduces P co 2 100‐fold, AQP1 increases P co 2 10‐fold) vs. from stopped‐flow approaches observing CO 2 uptake (no effects of either Chol or AQP1). A novel theory of CO 2 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 CO 2 partial pressure (pCO 2 ) 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 P co 2 = 0.01cm/s, and Chol‐free vesicles exhibit ~1 cm/s, and 2) the P co 2 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 P co 2 that can be raised when functionally necessary by incorporating protein‐gas channels such as AQP1.—Tsiavaliaris, G., Itel, F., Hedfalk, K., Al‐Samir, S., Meier, W., Gros, G., Endeward, V. Low CO 2 permeability of cholesterol‐containing liposomes detected by stopped‐flow fluorescence spectroscopy. FASEB J. 29, 1780‐1793 (2015). www.fasebj.org