Evaluating the biotic ligand model for toxicity and the alleviation of toxicity in terms of cell membrane surface potential

Abstract

The electrostatic nature of plant cell membrane (CM) plays significant roles in ionic interactions at the CM surface and hence in the biotic effects of metal ions. Increases in major cations (commonly Ca2+, Mg2+, H+, Na+, K+, etc.) in bulk-phase medium reduce the negativity of CM surface electrical potential (psi0), but these slightly increase the driving force of a metal ion crossing CMs (surface-to-surface transmembrane potential difference, Em,surf). Toxicologists commonly attributes the interactions between heavy metals and common cations (e.g., H+, Ca2+, and Mg2+) to competitions for binding sites at a hypothetical CM surface ligand. The psi0 effects are likely to be more important to metal toxicity and the alleviation of toxicity than site-specific competition. Models that do not consider psi0, such as the biotic ligand model (BLM) and the free ion activity model (FIAM), as usually employed are likely to lead to false conclusions about competition for binding at CM surface ligands. In the present study a model incorporating psi0 effects and site-specific competition effects was developed to evaluate metal (Cu2+, Co2+, and Ni2+) toxicities threshold (EA50, causing 50% inhibition) for higher plants. In addition, the mechanisms for the effects of common cations on toxicity of metals were also explored in terms of CM surface electrical potential.