Hypophosphite transport in human erythrocytes studied by overdetermined one-dimensional NMR exchange analysis

The membrane transport kinetics of the disubstituted phosphorus oxyacid, hypophosphite, were studied in human red cells under equilibrium exchange conditions. Hypophosphite is an analogue of both the bicarbonate and phosphate ions and is known to be rapidly transported across the human red cell membrane via the anion transport protein, Band 3. The hypophosphite ion is a particularly useful probe of Band 3-mediated membrane transport as the intracellular and extracellular species occur as distinct resonances in the 31P NMR spectrum; as a result the membrane potential may also be readily inferred. We applied 'overdetermined' one-dimensional exchange analysis to estimate the rates of exchange for influx and efflux. The apparent equilibrium exchange (ee) values Kmee for the hypophosphite efflux and influx were different and while the efflux parameters were not able to be unambiguously defined, the measured apparent influx kinetic transport parameters were Vmaxee = 1600 +/- 190 amol cell-1 s-1, and Kmee = 75 +/- 16 mM.