Abnormal effect of thiol groups on erythrocyte Na/Li countertransport kinetics in adult polycystic kidney disease

Background. Evidence suggests that in adult polycystic kidney disease (APCKD) there is abnormal membrane organisation that involves cytoskeletal proteins and affects ion-transport proteins. The possibility of detecting a membrane organisation defect in erythrocytes from APCKD patients was investigated. Methods. Na/Li countertransport (CT) kinetics were measured in erythrocytes from APCKD patients compared with normal controls. The modulation of Na/Li CT kinetics by key membrane thiol groups and by dissociation of spectrin by heating was studied. Results. In erythrocytes from APCKD compared to normal subjects the Km of Na/Li CT was lower (64 SEM 3 v 90 SEM 3 mmol Na/l, P<0.001) and Vmax/Km was higher (6.8 SEM 0.6 v 5.3 SEM 0.4 (x10(-3)), P<0.05). In erythrocytes from normal subjects, after N-ethylmaleimide (NEM) treatment in choline medium, Km of Na/Li CT was reduced and Vmax/Km increased (59 SEM 5, 8.2 SEM 0.4(x 10(-3)), P<0.001) but there was no effect on these kinetic parameters in erythrocytes from APCKD. The effect of heating was similar to that of NEM in choline medium. In normal controls after NEM treatment in sodium medium both Km and Vmax were reduced whereas in APCKD Vmax was reduced but Km was unchanged. Conclusions. In APCKD the effect of a key membrane thiol group on Na association with Na/Li CT was absent and the effect of spectrin dissociation was similarily abnormal. A second thiol group effect on the Vmax of Na/Li CT was normal. The results are consistent with a thiol-protein linked to the spectrin cytoskeleton that modulates Na/Li CT and is abnormal in APCKD.