PLASMA MEMBRANE SURFACE POTENTIAL (ψ_(PM)) AS A DETERMINANT OF ION BIOAVAILABILITY: A CRITICAL ANALYSIS OF NEW AND PUBLISHED TOXICOLOGICAL STUDIES AND A SIMPLIFIED METHOD FOR THE COMPUTATION OF PLANT ψ_(PM)

摘要

Plasma membranes (PMs) are negatively charged, and this creates a negative PM surface electrical potential (ψ_(PM)) that is also controlled by the ionic composition of the bathing medium. The ψ_(PM) 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 ψ_(PM), and these common ions are effective in the following order: Al~(3+) > H~+ > Cu~(2+) > Ca~(2+) ≈ Mg~(2+) > Na~+ ≈ K~+. These ions, especially H~+, Ca~(2+), and Mg~(2+), are known to reduce the uptake and biotic effectiveness of cations and to have the opposite effects on anions. Toxicologists commonly interpret the interactions between toxic cations (commonly metals) and ameliorative cations (commonly H~+, Ca~(2+), and Mg~(2+)) as competitions for binding sites at a PM surface ligand. The ψ_(PM) is rarely considered in this biotic ligand model, which incorporates the free ion activity model. The thesis of this article is that ψ_(PM) effects are likely to be more important to bioavailability than site-specific competition. Furthermore, ψ_(PM) effects could give the false appearance of competition even when it does not occur. The electrostatic approach can account for the bioavailability of anions, whereas the biotic ligand model cannot, and it can account for interactions among cations when competition does not occur. Finally, a simplified procedure is presented for the computation of ψ_(PM) for plants, and the possible use of ψ_(PM) in a general assessment of the bioavailability of ions is considered.