Computer-Simulated Evaluation of Possible Mechanisms for Quenching Heavy Metal Ion Activity in Plant Vacuoles

Abstract

Various mechanisms have been suggested for the quenching of Cd ion activity in plant vacuoles. These include solution complexation with organic acids and sulfhydryl-containing peptides and precipitation as sulfides. Because direct experimental support for these mechanisms is lacking and difficult to obtain, we have used a computer model to evaluate the quenching role of possible organic and inorganic ligands of tobacco cultured cells exposed to Cd. Results of this thermodynamic evaluation, which assumes that a chemical equilibrium state is met in the vacuole, support the conclusion that sulfhydryl-containing peptides and certain organic acids may form soluble Cd complexes. Although complexation of malate and oxalate with Cd is predicted to be less significant, citrate in the concentration range encountered in the tobacco cultured cell vacuoles has high potential for forming soluble complexes with Cd over the entire possible vacuolar pH range, especially 4.3 to 7.0, even in the presence of low levels of Cd-binding peptides. In addition, results show that inorganic chloride, sulfide (if present), and phosphate may also act to sequester Cd ion activity in the vacuole by forming soluble Cd-Cl and insoluble CdS and Cd-phosphate.Understanding the fate ofCd in plants is of interest because of concerns of Cd transfer from plants to animals and man. Recent studies suggest that heavy metals accumulated in the higher plant are mainly compartmentalized in the vacuole (5, 7, 9, 27). Various mechanisms have been proposed to account for the accumulation of these potentially toxic heavy metal ions in the plant vacuole (18,30). In general, these mechanisms include formation of soluble metal-organic acid complexes (4, 9, 14) or metal-phytate (25, 26), formation ofmetalpeptide or metal-peptide-sulfide complexes (9, 18, 27), or precipitation of metal-sulfides (2,21,25).Support for a particular mechanism of accumulation/sequestration of an ion is gained if the compartment of accumulation/sequestration is verified and if speciation of the ion in that compartment is determined. Both direct and indirect approaches (10) used to determine compartmentation ofions, including heavy metals, can only provide qualitative and quantitative estimates of vacuole contents (28). They cannot identify the species of ion complexes occurring in vacuoles. Therefore, mechanisms proposed on the basis of compartmentation analysis alone are not sufficient to argue the validity of a mechanism of accumulation/sequestration. However, it is possible with computer assistance to simulate the ion species distribution in the plant vacuole and thus to evaluate a proposed mechanism.Computer calculations have been used to model the chemistry of xylem sap of soybean and tomato (29). Here, we use data obtained previously regarding sap composition of vacuoles from Cd-treated tobacco (Nicotiana tabacum) cultured cells and the GEOCHEM-PC computer model (16,23) to predict ion species of these vacuoles in vivo. The prediction of ion speciation in vacuoles of Zn-treate...

Surface Chemical Properties of Purified Root Cell Walls from Two Tobacco Genotypes Exhibiting Different Tolerance to Manganese Toxicity

“…The Mn trapped in plant vacuoles has been speculated to be complexed with organic acids (14). This hypothesis is supported by thermodynamic predictions using computer simulation models (24,25).…”

Computer, Simulated Evaluation of Possible Mechanisms for Sequestering Metal Ion Activity in Plant Vacuoles

“…We showed earlier that Zn is localized in vacuoles of tobacco cultured cells exposed to 300 or 2000 Mm Zn (9). Evidence for vacuolar localization of the bulk of the organic acids present in plant cells was discussed in an earlier publication (25).…”

“…Even with the same 1:1 ratio, it is evident that Zn may bind citrate in several modes. Results of a companion study to this one using a similar approach predicted that Cd-citrate is the principal form of vacuolar Cd in tobacco suspension cells exposed to low or moderate levels of Cd (25). The predominant Cd-citrate complex found was 110 over the entire range of pH 4 to 7.…”

“…Under these conditions and a vacuolar pH of 5.0, Cd-citrate was found to be the predominant species of Cd present, despite an abundance of malate. We concluded that plants exposed to low levels ofCd, as is most common in agriculture, would accumulate most intracellular Cd as Cd-citrate and that this may be a common mechanism for accommodation of plants to the presence of Cd (25).…”

“…In a companion paper to this one we utilized computer modeling to predict species of Cd-ligand complexes occurring in tobacco cultured cells that had been grown in a relatively high but not growth-inhibiting level of Cd to cause Cd-peptide accumulation to a level of 10% of total Cd present (25). Under these conditions and a vacuolar pH of 5.0, Cd-citrate was found to be the predominant species of Cd present, despite an abundance of malate.…”