Reconstituted aquaporin 1 water channels transport CO2 across membranes

Biological membranes provide selective barriers to a number of molecules an d gases. However, the factors that affect permeability to gases remain uncl ear because of the difficulty of accurately measuring gas movements. To det ermine the roles of lipid composition and the aquaporin 1 (AQP1) water chan nel in altering CO2 flux across membranes, we developed a fluorometric assa y to measure CO2 entry into vesicles. Maximal CO2 flux was similar to 1000- fold above control values with 0.5 mg/ml carbonic anhydrase, Unilamellar ph ospholipid vesicles of varying composition gave widely varying water permea bilities but similar CO2 permeabilities at 25 degrees C. When AQP1 purified fr om human red blood cells was reconstituted into proteoliposomes, howeve r, 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 t he water pore; under certain circumstances, gas permeation through membrane s is protein-mediated.