N-ETHYLMALEIMIDE BLOCKS THE H+ PUMP IN THE PLASMA-MEMBRANE OF CHARA-CORALLINA INTERNODAL CELLS

The sulfhydryl (SH) modifying reagent N-ethylmaleimide (NEM) was applied to the internodal cells of Chara corallina to study the role of SH residues in the activity of the plasma membrane H+ pump. NEM (1 mu M) caused a marked depolarizing shift of the resting potential by 64+/-10 mV (n=7) together with depression of the conductance peak at around -200 mV, indicating a marked depression of the H+ pump activity. This effect of NEM was partly reversible, the membrane repolarized and the conductance peak was restored after extracellular washing. The H+ pump inhibitor, dicyclohexylcarbodiimide (DCCD), caused no additive membrane depolarization and/or depression of the H+ pump conductance, in the presence of NEM. This suggests that NEM blocks the H+ pump and that SH residues play a pivotal role in maintaining the H+ pump activity in Chara corallina.