No facilitator required for membrane transport of hydrogen sulfide

Hydrogen sulfide (H₂S) 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 H₂O, it was hypothesized that aquaporins may facilitate H₂S 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 H2S flux. To measure H₂O and H₂S fluxes, respectively, sodium ion dilution and buffer acidification by proton release (H₂S ⇆ 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 H₂S, P_M,H2S> 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,H2Sremained 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 H₂S diffusion. The fact that cholesterol and sphingomyelin reconstitution did not turn these membranes into an H₂S barrier indicates that H₂S transport through epithelial barriers, endothelial barriers, and membrane rafts also occurs by simple diffusion and does not require facilitation by membrane channels.