Simple Method to Reduce Interference from Excess Magnesium in Cadmium Immunoassays

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• 作者：Kazuhiro Sasaki ; Shinichi Oguma ; Thomas Glass ; Yukie Namiki ; Hideo Sugiyama ; Naoya Ohmura ; Diane A. Blake
• 刊名：Journal of Agricultural and Food Chemistry
• 出版年：2008
• 出版时间：September 10, 2008
• 年：2008
• 卷：56
• 期：17
• 页码：7613-7616
• 全文大小：126K
• 年卷期：v.56,no.17(September 10, 2008)
• ISSN：1520-5118

文摘

In order to develop a rapid inexpensive test for cadmium in rice, we identified an antibody specific for cadmium-EDTA complexes; this antibody binds to cadmium-EDTA with a K_d of ∼10⁻⁸ M. Although the antibody’s cross reactivity to magnesium was minimal (K_d ≈ 10⁻⁵ M), the high toxicity of cadmium coupled with the high natural occurrence of magnesium in rice resulted in a situation where magnesium interfered with cadmium determination and resulted in falsely elevated estimates of cadmium. Fortunately, the formation constant of EDTA for cadmium is approximately 5 × 10⁷ times higher (at pH 7) than the formation constant of EDTA for magnesium, and we were able to eliminate the magnesium interference by judicious selection of the EDTA concentration used in the assay. The resulting equilibria are complex, but we show that a relatively simple two-step model in which cadmium and magnesium compete for EDTA followed by cadmium-EDTA and magnesium-EDTA competing for antibody provided a good fit to the measured data. These analyses enabled appropriate selection of the optimum EDTA concentration for an immunoassay with improved selectivity.