Saturday, 15 July 2006

Cell-Surface Electrical Potential: A Demonstration of its Importance for Ion Bioavailability and a Simplified Method for its Computation.

Thomas B. Kinraide, Agricultural Research Service, USDA, 1224 Airport Rd., Beaver, WV 25813-9423

Cell surfaces of virtually all taxa are negatively charged. This applies to both the Plasma Membranes (PMs) and the Cell Walls (CWs) or to other PM covering materials. These negative charges create negative surface potentials at the PMs (ψPM) and the CWs (ψCW), but it is the ψPM, rather than the ψCW, that plays the principal role in ionic interactions and biotic effects. These surface potentials are also controlled by the ionic composition of the bathing medium. ψPM 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 ψPM because of ionic screening and binding. The common ions Al3+, H+, Ca2+, and Mg2+ are especially effective. These ions, especially the latter three, 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. ψPM is rarely considered. This biotic ligand model (BLM) is sometimes considered to be an extension of the free ion activity model (FIAM), and for either model the precise chemical composition of the bathing media must be measured or calculated (i.e., speciation must be performed). The thesis of this presentation is that ψPM effects are likely to be more important to bioavailability than site-specific competition. Models that do not consider ψPM, such as BLM and FIAM, as usually employed, are likely to lead to false conclusions about competition for binding at PM-surface ligands. The electrostatic approach can account for the bioavailability of anions whereas the BLM cannot, and it can account for interactions among cations when site-specific competition does not occur. ψPM is measured, approximately, as a ζ potential from electrophoretic mobility or is computed with a Gouy-Chapman-Stern model for which parameter values must be obtained. Although parameter values are available for plants, computation of ψPM requires dedicated computer programs. The first equation below presents a simpler method of computation of ψPM for plants. One may replace the activity of Na+ (aNa+) with activities for K+, NH4+, and most other monovalent cations, other than H+; the activity of Ca2+ (aCa2+) may be replaced with activities for Mg2+, Sr2+, and some other divalent cations. Anions, and cations at very low activities, may be ignored except for H+ and the trivalent cations. Ion activities at the PM surface may be computed with the second equation below. In these equations, ai refers to the activity (in μM) of ion i in the bulk-phase medium, ai,PM refers to the activity (in μM) of ion i at the PM surface, Zi is the charge on ion i, and ψPM is in units mV. Ion uptake, intoxication, and the alleviation of intoxication are much better correlated with ai,PM than with ai, thus the ratio ai,PM/ai is an index of the bioavailability of an ion.

ψPM = 55.5 266/(3.08aH+1/2 + 0.100aNa+1/2 + 0.636aZn2+1/2 + 1.00aCa2+1/2 + 3.37aCu2+1/2 + 14.0aLa3+1/2 + 36.3aAl3+1/2 + 0.00323aH+2)0.306

ai,PM = ai exp(ZiFψPM/(RT)) = ai exp(ZiψPM/25.7)

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