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Articles

Sulfhydryl Group Involvement in Plasmalemma Transport of HCO₃^– and OH^– in Chara corallina¹

Department of Botany, University of California, Davis, California 95616

The effect of the sulfhydryl reagents (—SH) p-chloromercuribenzene-sulfonic acid (PCMBS), N-ethylmaleimide (NEM), and inorganic mercury on H¹⁴CO₃^– assimilation in Chara corallina is reported. Commercial grade PCMBS caused severe inhibition of H¹⁴CO₃^– 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 HgCl₂. This chemical caused a dramatic inhibition of H¹⁴CO₃^– assimilation, while it had little effect on cellular ¹⁴CO₂ fixation. Reversal of the Hg²⁺ inhibition of H¹⁴CO₃^– 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 H¹⁴CO₃^– 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 HCO₃^– transport system to such an extent that the H¹⁴CO₃^– assimilation versus HCO₃ 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 HCO₃^– efflux increased significantly.

The influence of these —SH reagents on the OH^– efflux system was also investigated. The H¹⁴CO₃^– assimilation and OH^– efflux results demonstrate the involvement of —SH groups in the plasmalemma transport of both HCO₃^– 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.