Transmembrane Transport of Bicarbonate Unravelled**

Abstract

Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of particular interest, as defects in transmembrane transport of bicarbonate are associated with various diseases. However, no convenient method exists to directly observe bicarbonate transport and study the mechanisms involved. Here, an assay is presented that allows the kinetics of bicarbonate transport into liposomes to be monitored directly and with great sensitivity. The assay utilises an encapsulated europium(III) complex, which exhibits a large increase in emission intensity upon binding bicarbonate. Mechanisms involving CO 2 diffusion and the dissipation of a pH gradient are shown to be able to lead to an increase in bicarbonate concentration within liposomes, without transport of the anion occurring at all. By distinguishing these alternative mechanisms from actual bicarbonate transport, this assay will inform the future development of bicarbonate transporters.

Transmembrane transport of bicarbonate by anion receptors

“…Last year, in collaboration with S. Butler, we reported the EuL1 assay, that uses a luminescent europium-based complex as HCO3 − probe for the direct monitoring of bicarbonate transport using fluorescence spectroscopy. 43 In this assay, the probe EuL1 is encapsulated inside LUVs which contain NaX (X = Cl or NO3) both inside and outside the vesicles. The transporter is added to the membranes by either preincorporation or postinsertion and the transport is triggered by an extravesicular pulse of NaHCO3 (Figure 3f).…”

“…The earliest examples of (thio)urea-based bicarbonate transporters after the cholapods contained a tris(2aminoethyl)amine core, that yielded tripodal anion receptors 4a-l, 44,46 and a trans-decalin core, which is present in compound 8a-c and represents a structural simplification of the steroid scaffold in the cholapods. 39,43,48,52 Other structural cores used to obtain tripodal transporters were the cyclohexanebased scaffold in compound 10 and the tris(aminomethyl)benzene and cyclopeptidic scaffolds present in the capsular compound 11. 53,54 Moreover, various bis-amine scaffolds were used to obtain (thio)ureas 12-15.…”

“…Decalin bis-urea 8a showed good Cl − /HCO3 − antiport activity in the lucigenin assay at 0.004 mol%, and the [I] determined for this compound was 360 s −1 . 43,48 The analogous bis-thiourea 8b, one of the most efficient chloride transporters described to date, 39 showed to be more active for both Cl − /NO3 − and Cl − /HCO3 − antiport, with a [I] of 970 s −1 for the later process. 43 Prodigiosin 2 also was studied with the lucigenin assay, giving a [I] of 110 s −1 , which revealed that decalins 8a-b are among the few synthetic transporters with higher efficiency than prodigiosin 2.…”

“…43,48 The analogous bis-thiourea 8b, one of the most efficient chloride transporters described to date, 39 showed to be more active for both Cl − /NO3 − and Cl − /HCO3 − antiport, with a [I] of 970 s −1 for the later process. 43 Prodigiosin 2 also was studied with the lucigenin assay, giving a [I] of 110 s −1 , which revealed that decalins 8a-b are among the few synthetic transporters with higher efficiency than prodigiosin 2. 84 Moreover, bambusuril 35d afforded a specific rate [I] of 1600 s −1 .…”

A Simulation Model for the Non-Electrogenic Uniport Carrier-Assisted Transport of Ions across Lipid Membranes

“…Impressive work has been completed, covering different aspects such as anion recognition, sensing, extraction, crystal engineering, self-assembling, catalysis, and transmembrane anion transport. The latter has lately attracted considerable attention, as transmembrane anion transport is crucial to several biological processes [ 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. Failure in maintaining chloride homeostasis in cells is related to various diseases [ 16 ] such as cystic fibrosis, in which a malfunctioning of the cystic fibrosis transmembrane conductance regulator (CFTR) gene causes defective chloride transport [ 17 ].…”