No facilitator required for membrane transport of hydrogen sulfide

Abstract

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 H 2O, it was hypothesized that aquaporins may facilitate H 2S 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 (H 2S % 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 2S, 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, PM,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 2S diffusion. The fact that cholesterol and sphingomyelin reconstitution did not turn these membranes into an H2S barrier indicates that H 2S transport through epithelial barriers, endothelial barriers, and membrane rafts also occurs by simple diffusion and does not require facilitation by membrane channels. aquaporins ͉ gas transport ͉ membrane permeability ͉ unstirred layer ͉ signaling

Redox Biochemistry of Hydrogen Sulfide

“…The ready ionization of H 2 S at physiological pH suggests impeded permeation through the lipid bilayer when compared with other gases, viz. NO or CO. On the other hand, transport of the gas, H 2 S, across the membrane does not appear to be facilitated (2).…”

New Functional Sulfide Oxidase-Oxygen Reductase Supercomplex in the Membrane of the Hyperthermophilic Bacterium Aquifex aeolicus

“…H 2 S would in turn be used by Sqr to reduce oxygen and produce ATP. H 2 S transport across the membrane could occur by simple diffusion without facilitation by membrane channels as recently demonstrated (57). We have characterized most of the enzymes involved in the intricate sulfur metabolism in A. aeolicus, and we will now concentrate our efforts in understanding how the various energetic pathways are connected, as a way to gain insight of why A. aeolicus requires H 2 , O 2 , and a source of sulfur for growth.…”

Transport of H2S and HS− across the human red blood cell membrane: rapid H2S diffusion and AE1-mediated Cl−/HS− exchange

“…4, the sulfide-dependent k pH was ϳ6.6 min Ϫ1 (the same extracellular buffer power noted previously). To account for this k pH , P H 2 S must be Ն0.01 cm/s and could very easily be in the range estimated for lipid bilayers, i.e., Ͼ0.5 cm/s (46). With a cell water volume-to- Fig.…”

“…Two recent studies have provided strong evidence for rapid H 2 S diffusion through lipid bilayers. Mathai et al (46) measured pH changes associated with H 2 S diffusion across planar lipid bilayers and found that the permeability coefficient for H 2 S is Ն0.5 cm/s. Cuevasanta et al (7) showed that the lipid-water partition coefficient of H 2 S is ϳ2, which is consistent with a bilayer permeability coefficient of 3 cm/s.…”