Hydrogen sulfide (H ₂ S) has emerged as a new and important member in the group of gaseous signaling molecules. However, the molecular transport mechanism has not yet been identified. Because of structural similarities with H ₂ O, it was hypothesized that aquaporins may facilitate H ₂ S transport across cell membranes. We tested this hypothesis by reconstituting the archeal aquaporin AfAQP from sulfide reducing bacteria Archaeoglobus fulgidus into planar membranes and by monitoring the resulting facilitation of osmotic water flow and H ₂ S flux. To measure H ₂ O and H ₂ S fluxes, respectively, sodium ion dilution and buffer acidification by proton release (H ₂ S ⇆ H ⁺ + HS ⁻ ) were recorded in the immediate membrane vicinity. Both sodium ion concentration and pH were measured by scanning ion-selective microelectrodes. A lower limit of lipid bilayer permeability to H ₂ S, P M,H ₂ S ≥ 0.5 ± 0.4 cm/s was calculated by numerically solving the complete system of differential reaction diffusion equations and fitting the theoretical pH distribution to experimental pH profiles. Even though reconstitution of AfAQP significantly increased water permeability through planar lipid bilayers, P M,H ₂ S remained unchanged. These results indicate that lipid membranes may well act as a barrier to water transport although they do not oppose a significant resistance to H ₂ S diffusion. The fact that cholesterol and sphingomyelin reconstitution did not turn these membranes into an H ₂ S barrier indicates that H ₂ S transport through epithelial barriers, endothelial barriers, and membrane rafts also occurs by simple diffusion and does not require facilitation by membrane channels.

The transport of ammonia/ammonium is fundamental to nitrogen metabolism in all forms of life. So far, no clear picture has emerged as to whether a protein channel is capable of transporting exclusively neutral NH₃ while excluding H⁺ and . Our research is the first stoichiometric study to show the selective transport of NH₃ by a membrane channel. The purified water channel protein aquaporin-8 was reconstituted into planar bilayers, and the exclusion of or H⁺ was established by ensuring a lack of current under voltage clamp conditions. The single channel water permeability coefficient of 1.2 × 10^–14 cm³/subunit/s was established by imposing an osmotic gradient across reconstituted planar bilayers, and resulting minute changes in ionic concentration close to the membrane surface were detected. It is more than 2-fold smaller than the single channel ammonia permeability (2.7 × 10^–14 cm³/subunit/s) that was derived by establishing a transmembrane ammonium concentration gradient and measuring the resulting concentration increases adjacent to the membrane. This permeability ratio suggests that electrically silent ammonia transport may be the main function of AQP8.