Sulfhydryl Group Involvement in Plasmalemma Transport of HCO3− and OH− in Chara corallina

Abstract

The effect of the sulfhydryl reagents (-SH) p-chloromercuribenzene-sulfonic acid (PCMBS), N-ethylmaleimide (NEM), and inorganic mercury on H(14)CO(3) (-) assimilation in Chara corallina is reported. Commercial grade PCMBS caused severe inhibition of H(14)CO(3) (-) assimilation. Results obtained using purified PCMBS (stock solution passed through a chelating resin) indicated that inhibition observed using unpurified PCMBS was due predominantly to the presence of inorganic mercury (as a contaminant). The inhibitory role of inorganic mercury was verified using HgCl(2). This chemical caused a dramatic inhibition of H(14)CO(3) (-) assimilation, while it had little effect on cellular (14)CO(2) fixation. Reversal of the Hg(2+) inhibition of H(14)CO(3) (-) assimilation (in presence of 1.0 millimolar dithioerythritol) was extremely slow, requiring 2 to 3 hours for the reestablishment of control rates. This slow recovery may reflect de novo synthesis of transport proteins.Almost complete (irreversible) inhibition of H(14)CO(3) (-) assimilation was observed after cells were briefly (2 min) exposed to 0.1 to 0.3 millimolar NEM. At critical concentrations, both inorganic mercury and NEM perturbed the HCO(3) (-) transport system to such an extent that the H(14)CO(3) (-) assimilation versus HCO(3) concentration profiles changed shape. This could reflect a conversion of the transport system to a facilitated diffusion mode. Alternatively, the efficiency of the transport system may have been affected such that HCO(3) (-) efflux increased significantly.The influence of these -SH reagents on the OH(-) efflux system was also investigated. The H(14)CO(3) (-) assimilation and OH(-) efflux results demonstrate the involvement of -SH groups in the plasmalemma transport of both HCO(3) (-) and OH(-) in C. corallina. Greater transport susceptibility to inorganic mercury and NEM compared to purified PCMBS suggested that the sulfhydryl groups were not located on the peripheral surface of the plasmalemma.