Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2006
Publication Date: 12/1/2006
Citation: Kinraide, T.B. 2006. Plasma membrane surface potential (psi) as a determinant of ion bioavailability. A critical analysis of new and published toxicological studies and a simplified method for the computation of plant psi. Environmental Toxicology and Chemistry 25(12): 3188-3198.
Interpretive Summary: Toxicology is an important discipline in agricultural, environmental, and health studies. Toxicants are always present, and some toxicants are also essential nutrients for plants and animals. Examples are copper and fluoride, each of which is deadly in excessive amounts, but essential in smaller amounts. The bioavailability of toxicants is determined by the environment, and toxicologists have long sought to understand these determinants. For example, copper in 'hard' waters (high concentrations of calcium and magnesium) is much less available (and less intoxicating) than copper in 'soft' waters. Toxicologists have recently adopted the Biotic Ligand Model (BLM) to interpret and predict toxicant bioavailability. The model states that ameliorative ions (e.g., calcium) alleviate toxicity by competing with toxicants (e.g., copper) for a cell-surface binding site, but that is often not the case. The contribution of the present article is to correct this and some other deficiencies in the BLM. In particular, the BLM must be extended to take into account cell-surface electrical properties, which can have the effect of attracting or repelling toxicants, which are themselves often electrically charged.
Technical Abstract: Plasma membranes (PMs) are negatively charged, and this creates a negative PM-surface electrical potential (PSI) that is also controlled by the ionic composition of the bathing medium. PSI controls the distribution of ions between the PM surface and the medium so that negative potentials increase the surface activity of cations and decrease the surface activity of anions. All cations reduce the negativity of PSI, and these common ions are effective in the following order: Al3+ > H+ > Cu2+ > Ca2+ = Mg2+ > Na+ = K+. These ions, especially H+, Ca2+, and Mg2+, are known to reduce the uptake and biotic effectiveness of cations and to have the opposite effects upon anions. Toxicologists commonly interpret the interactions between toxic cations (commonly metals) and ameliorative cations (commonly H+, Ca2+, and Mg2+) as competitions for binding sites at a PM-surface ligand. PSI is rarely considered in this biotic ligand model (BLM), which incorporates the free ion activity model (FIAM). The thesis of the present article is that PSI effects are likely to be more important to bioavailability than site-specific competition. Furthermore, PSI effects may give the false appearance of competition even when it does not occur. The electrostatic approach can account for the bioavailability of anions whereas the BLM cannot, and it can account for interactions among cations when competition does not occur. Finally, simplified procedures are presented for the computation of PSI for plants, and the possible use of PSI in a general assessment of the bioavailability of ions is considered.