5. Arsenic and Cd uptake mechanism
After Sephadex G-50 gel filtration of leaf extract from the plant treated with1000 mg L-1 As and Cd, the maximum amounts of As (Fig. 4a) and Cd (Fig. 5a) were found at 90–108 mL and 80–104 mL of eluents, respectively. Absorbances at 280 nm were also counted for each fraction because proteins give the maximum absorbance at this wavelength. According to Fig. 4a, small amounts of As were scarcely observed in the high molecula rweight fraction, whereas Cd-binding substance(s) might be those other than proteins, as they gave different peaks (Fig. 5a) in the G-50 column. Later each fraction was treated with Ellman's reagent and SH contents were measured by recording the absorbance at 480 nm (Ellman, 1959). Data represented in Fig. 4b show that the maximum amount of thiol (9 mM) was found in the fraction having the maximum amount of As. On the contrary, control leaf eluent samples have lower thiol content (2.8 mM) when treated with the 0 mg As L-1 solution (Fig. 4c). In the case of Cd, no thiol formation occurred in high Cd-containing eluents (Fig. 5b). For confirmation of whether Cd formd any substances having thiol compounds or not, we again purified the eluents containing high levels of Cd through a Sephadex G-15 gel filtration column and after performic acid treatment we measured the cysteic acid/cysteine (thiol containing peptides) and other amino acids. The data in Supplementary Ta- ble 4 indicate that only 0.95 nmol of cysteic acid which is 2.13% of the total amino acids were present in the eluents containing high levels of Cd. This means that Cd might follow different uptake mechanism than that for As.