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

The sulfhydryl (SH) modifying reagent N-ethylmaleimide (NEM) was appli ed 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 e ffect of NEM was partly reversible, the membrane repolarized and the c onductance peak was restored after extracellular washing. The H+ pump inhibitor, dicyclohexylcarbodiimide (DCCD), caused no additive membran e 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 C hara corallina.