Transmembrane Transport of Bicarbonate Unravelled*.

Luis Martínez‐Crespo,Anthony P Davis,Hennie Valkenier,Nicola Alessandro De Simone,Sarah H Hewitt,Vladimír Šindelář,Stephen Butler

doi:10.1002/chem.202100491

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 CO2 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.

Highlights

The transport of bicarbonate is crucial to many biological processes, such as the regulation of pH[1,2] and the removal of metabolic waste.[ 3 ]. The development of synthetic HCO3− transporters could contribute to the study and understanding of various diseases linked to mutations in HCO3− transporting proteins, such as haemolytic anaemia, renal diseases, congenital chloride diarrhoea, and glaucoma.[3]
The probe responds to physiologically relevant concentrations of HCO3− and exhibits high selectivity over poorly coordinating anions that are commonly used in anion transport assays, including Cl− and NO3−.[27]
In order to use [Eu.L1]+ to monitor the transport of HCO3−, we encapsulated this probe into large unilamellar vesicles (LUVs) consisting of the lipids POPC and cholesterol in a 7:3 ratio and extruded these liposomes through a membrane with 200 nm pores, to obtain LUVs with an average hydrodynamic diameter of 183 nm (Figure S2)

Summary

Introduction

The transport of bicarbonate is crucial to many biological processes, such as the regulation of pH[1,2] and the removal of metabolic waste.[ 3 ]. Since hardly any change in the emission intensity was observed in the absence of anionophore 1 (Figure 3d, black curve), we can conclude that bambusuril 1 transports HCO3− into the liposomes and that the new EuL1 assay allows this process to be monitored.

Results

Conclusion