RED-CELL ECHINOCYTOGENESIS IS CORRELATED TO THE RECRUITMENT OF EXTERNAL BAND-3 CONFORMATIONS

For human erythrocytes the bilayer-couple model predicts a reversible crenation for increasing ratios of external to internal membrane monol ayer area. This can be proven experimentally by changing the ratio of lipids in the two monolayers. Under physiological conditions with no a ddition of membrane-active compounds, active lipid translocation and p assive flip-flop are comparatively slow. Nevertheless, individual eryt hrocytes underwent dramatic shape changes within seconds when the susp ension medium was changed. After cell suspension in low ionic strength solutions a rapid formation of echinocytes was observed. This suspens ion effect was also found at high ionic concentrations when Cl- was re placed by SO42-, an ion transported very slowly by band 3. The band-3 anion-exchange protein occupies about 10% of the total membrane area. Band-3 conformers are asymmetrically distributed with about 90% of the transport sites facing the inside. Specific band-3 inhibitors can ind uce echinocytes. These inhibitors are known to recruit the band-3 conf ormation with he transport site exposed to the external medium, e.g. 4 ,4'-diisothio-cyanatostilbene-2,2'-disulfonic acid, t-amido-4'-isothio cyana-tostilbene-2,2'-disulfonic acid, flufenamic acid, furosemide and salicylic acid. Other inhibitors, e.g. phenylglyoxal and niflumic aci d, which do not recruit a certain conformation, are not echinocytogeni c. Physiologically, band 3 ensures the equilibration of internal and e xternal anions like Cl- and HCO3- which are distributed according to t he transmembrane potential (TMP). In the literature a correlation of t he TMP and the band-3 conformation, as well as a correlation of the TM P and the erythrocyte shape, is described. In the proposed model, conf ormational changes may significantly alter the monolayer area ratio. L ow external Cl- concentrations, negative TMPs or inhibitors may recrui t the band-3 transport site towards the outside solution. It is propos ed that this increases the bulk of the protein in the external monolay er and subsequently induces an echinocytogenic shape transformation ac cording to the bilayer couple model.