Distinguishing among HCO 3 - , CO 3 = , and H + as Substrates of Proteins That Appear To Be "Bicarbonate" Transporters.

BACKGROUND: Differentiating among HCO3 - , CO3 = , and H+ movements across membranes has long seemed impossible. We now seek to discriminate unambiguously among three alternate mechanisms: the inward flux of 2 HCO3 - (mechanism 1), the inward flux of 1 CO3 = (mechanism 2), and the CO2 /HCO3 - -stimulated outward flux of 2 H+ (mechanism 3).

METHODS: As a test case, we use electrophysiology and heterologous expression in Xenopus oocytes to examine SLC4 family members that appear to transport "bicarbonate" ("HCO3 - ").

RESULTS: First, we note that cell-surface carbonic anhydrase should catalyze the forward reaction CO2 +OH- →HCO3 - if HCO3 - is the substrate; if it is not, the reverse reaction should occur. Monitoring changes in cell-surface pH ( Δ pHS ) with or without cell-surface carbonic anhydrase, we find that the presumed Cl-"HCO3 " exchanger AE1 (SLC4A1) does indeed transport HCO3 - (mechanism 1) as long supposed, whereas the electrogenic Na/"HCO3 " cotransporter NBCe1 (SLC4A4) and the electroneutral Na+ -driven Cl-"HCO3 " exchanger NDCBE (SLC4A8) do not. Second, we use mathematical simulations to show that each of the three mechanisms generates unique quantities of H+ at the cell surface (measured as Δ pHS ) per charge transported (measured as change in membrane current, ΔI m ). Calibrating ΔpHS /Δ I m in oocytes expressing the H+ channel HV 1, we find that our NBCe1 data align closely with predictions of CO3 = transport (mechanism 2), while ruling out HCO3 - (mechanism 1) and CO2 /HCO3 - -stimulated H+ transport (mechanism 3).

CONCLUSIONS: Our surface chemistry approach makes it possible for the first time to distinguish among HCO3 - , CO3 = , and H+ fluxes, thereby providing insight into molecular actions of clinically relevant acid-base transporters and carbonic-anhydrase inhibitors.