Pre-steady state transport by erythrocyte band 3 protein: uphill countertransport induced by the impermeant inhibitor H2DIDS

Pre-steady state CT efflux experiments have been performed to test directly the idea that the transport inhibitor H2DIDS (4,4'-diisothiocyanatodihydro stilbene-2,2'-disulfonate) binds preferentially to the outward-facing state of the transporter. Cells were equilibrated with a medium consisting of 15 0 mM sodium phosphate, pH 6.2, N-2 atmosphere, and 80-250 mu M Cl-36(-). Ad dition of H2DIDS (10-fold molar excess compared with band 3) induces a tran sient efflux of Cl- as expected if H2DIDS binds more tightly to outward-fac ing than to inward-facing states. The size of the H2DIDS-induced efflux dep ends on the Cl- concentration and is about 700 000 ions per cell at the hig hest concentrations tested. The size of the transient efflux is larger than would be expected if the catalytic cycle for anion exchange involved one p air of exchanging anions per band 3 dimer. These results are completely con sistent with a ping-pong mechanism of anion exchange in which the catalytic cycle consists of one pair of exchanging anions per subunit of the band 3 dimer.