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D. Taglicht is the first author of a 1993 measurement of the transport stoichiometry of NhaA, an antiporter from Escherichia coli. Purified NhaA was reconstituted into sodium-loaded proteoliposomes, allowing transport to be studied in a defined membrane preparation.^[ref]

Two approaches converged on two protons exchanged for each sodium ion over the tested pH interval $7.2$–$8.1$. One compared opposing ion fluxes; the other related the membrane potential to imposed concentration gradients under coupling and equilibrium assumptions.^[ref]

The ratio explains the word electrogenic: moving two singly positive ions one way and one singly positive ion the other transfers a net unit of positive charge per completed exchange cycle. It also separates a transporter's rate from its stoichiometry: faster exchange need not mean a different number of ions per cycle.

NhaA is a sodium/proton antiporter exemplified by the Escherichia coli membrane protein. It couples the movement of sodium ions to proton movement in the opposite direction across a cell membrane, contributing to ion and pH regulation. The name of this particular protein should not be confused with every sodium/proton exchanger.

A 1993 experiment purified NhaA and placed it in sodium-loaded proteoliposomes: artificial membrane vesicles containing the transport protein. Parallel sodium-efflux and proton-influx measurements supported two protons per sodium ion over the tested pH range 7.2–8.1. A second determination used membrane electric potential difference and equilibrium gradients, under assumptions of coupled transport.^[ref]

For an illustrative cycle with two protons entering and one sodium ion leaving, the inward transfer of electric charge is $2e-e=e$, where $e$ is the elementary charge. Reversing the cycle reverses that transfer. Opposite transport directions therefore do not imply electrical neutrality, nor do they alone determine which direction is energetically favored; the electrochemical gradients matter.

A 2005 X-ray crystallography study supplied a different kind of evidence: a structure of downregulated NhaA, including a cytoplasm-facing funnel toward a putative ion-binding region. The authors proposed a mechanism in which access changes between the two sides. A static structure helps constrain such a mechanism but does not directly measure the transported ratio or film a working exchange cycle.^[ref]

A proteoliposome is a liposome containing one or more reconstituted membrane proteins in its lipid bilayer. It combines a controlled membrane boundary with a chosen molecular machine. A solution containing detergent-solubilized protein is not automatically a proteoliposome: a closed vesicle and a membrane-incorporated protein must actually be formed.

The artificial compartment is useful because material can be placed on one side of a lipid bilayer and transport measured across it. In an originating experiment on the NhaA sodium/proton antiporter, purified protein was reconstituted into sodium-loaded proteoliposomes. Parallel sodium and proton flux measurements, together with an independent membrane-potential equilibrium method, supported a stoichiometry of two protons per sodium ion over the tested pH range 7.2–8.1.^[ref]

For a bookkeeping example,100 sodium ions moved outward through a perfectly coupled $2{\mathrm{H}}^{+}/\mathrm{N}{\mathrm{a}}^{+}$ exchanger would accompany 200 protons moving inward. The net movement is 100 positive elementary charges inward, which explains the word electrogenic. These invented counts illustrate coupling, not a measured single-vesicle trace.

Interpreting a proteoliposome assay still requires knowing which way the membrane protein faces, whether the vesicles leak, and how much protein is incorporated. An otherwise matched empty liposome helps estimate membrane background. Reconstitution simplifies a biological system; it does not automatically eliminate every alternative source of the measured flux.