Reconstituted aquaporin 1 water channels transport CO2 across membranes

Biological membranes provide selective barriers to a number of molecules and gases. However, the factors that affect permeability to gases remain unclear because of the difficulty of accurately measuring gas movements. To determine the roles of lipid composition and the aquaporin 1 (AQP1) water channel in altering CO2 flux across membranes, we developed a fluorometric assay to measure CO2 entry into vesicles. Maximal CO2 flux was approximately 1000-fold above control values with 0.5 mg/ml carbonic anhydrase. Unilamellar phospholipid vesicles of varying composition gave widely varying water permeabilities but similar CO2 permeabilities at 25 degreesC. When AQP1 purified from human red blood cells was reconstituted into proteoliposomes, however, it increased water and CO2 permeabilities markedly. Both increases were abolished with HgCl2, and the mercurial inhibition was reversible with beta-mercaptoethanol. We conclude that unlike water and small nonelectrolytes, CO2 permeation is not significantly altered by lipid bilayer composition or fluidity. AQP1 clearly serves to increase CO2 permeation, likely through the water pore; under certain circumstances, gas permeation through membranes is protein-mediated.