Sulphydryl groups involved in Na+–Li+ exchange in human erythrocytes

Oxidative stress causes cellular injury that is thought to be due to increased cytosolic cation levels. Disturbances of a variety of mechanisms which normally maintain intracellular anion/cation homeostasis, occur during oxidative stress. Reactivity of the SH- groups essential for oubain-resistant Na+–Li+ exchange by N-ethylmaleimide (NEM) and selenite was studied in human erythrocytes. In addition, the reactivity of the substances on SH- groups and Li+ influx have been studied as a function of pH of the medium. The results show that NEM induces an irreversible inhibition of Li+ influx. It diminishes progressively with the increasing pH of the medium. Whereas we obtain increasing intracellular Li+ concentration with the rising selenite concentration in the medium. The maximum effect with this substance is reached at about pH 8.0. We can state that the -SH reagents (NEM and selenite) studied behave differently: NEM inhibits Li+ influx by modifying the essential SH-groups of the membrane proteins in such a way that the exchange is reduced, whereas it maintains the Na+ permeability almost unaltered. The slight increase in intracellular Na+ induced by selenite suggests that the oxidative changes in the intracellular sulphydryl groups may constitute an important mechanism for the regulation of the intracellular cations. Copyright © 2001 John Wiley & Sons, Ltd.