Chemical form of cadmium (and other heavy metals) in rice and wheat plants

Abstract

Chemical forms of heavy metals such as Cd, Cu, Ni, and Pb in rice and wheat plants grown in nutrient solution containing a heavy metal were investigated. Fractionation of an extract of Cd-treated rice plants on Sephadex G-75 showed cadmium to be associated with organic compounds of high (fraction A), intermediary (fraction B), and low molecular weight (fraction C). Material A, whose molecular weight was greater than 440,000, is probably nonspecific binding of Cd to normal cell components. Materials B and C can be classified as types of metallothionein. The molecular weight of B was 33,100. This material contains 12 mg Cd/g protein. The UV-absorption spectrum of B showed absorptions at 280 and 250 nm. Material B was not eluted even at a very high ionic strength from the DEAE-cellulose column, but it was eluted at a very low ionic strength from a CM-cellulose column, indicating a highly anionic molecule which differs from metallothionein in animals. Fraction C contains two materials: one a Cd-containing material whose molecular weight was estimated to be approximately 7000 and the other an inorganic Cd salt. In addition to cadmium, copper, lead, and nickel in rice and wheat have been studied. As a result, heavy metal-containing materials whose molecular weights were estimated to be approximately 16,000 and 8900 (Ni-treated rice plants), 7000 (Pb-treated rice plants), 5000 (Cd-treated wheat plants), and 21,000 (Cu-treated wheat plants) were isolated.