Binding of endogenous and exogenous cadmium to glutelin in rice grains as studied by HPLC/ICP-MS with use of a stable isotope.

Cadmium (Cd) in polished rice grains was extracted under various conditions and the chemical forms of the metal in soluble fractions were determined together with copper, zinc and other metals by HPLC with on-line detection by inductively coupled argon plasma (ICP)--mass spectrometry (MS). Cd (753 ng/g) in rice grains grown in Cd-contaminated rice fields was mostly bound to glutelin in soluble fractions. Binding of Cd in vitro to constituents in rice grains was examined by incubating CdCl2 enriched with 113Cd (96.3% enrichment) in a suspension of powdered rice grains. Distributions of exogenous 113Cd in the soluble fraction of Cd-contaminated rice grains was identical with that of endogenous Cd on a size-exclusion column, and the metal was shown to bind to glutelin up to 5.0 micrograms/g in the control rice grains, indicating that exogenous Cd can be bound to glutelin up to this capacity. Simultaneous speciation of exogenous 113Cd and endogenous 111Cd was demonstrated to be highly effective for comparing the binding of Cd natural and artificial origins.