Sequential Extraction of Cadmium in Edible Parts of Portunus trituberculatus and the Molecular Weight Distribution

Extraction and isolation of different cadmium complex from aquatic products are essential for the purpose of safety evaluation and analysis. In this study, the methods involving Tris-HCl, HCl and alkali protease for extraction of cadmium in edible tissues of Portunus trituberculatus were investigated and compared precisely, and coupled with ultrafiltration and dialysis, cadmium complexes with peculiar molecular weights distribution were also isolated. The results indicated that Tris-HCl soluble cadmium contributed most of the total cadmium, especially in gonad and hepatopancre with extraction efficiency of 90% or more. However, about 37% of the cadmium in muscle need alkaline protease treatment for effective dissolution due to its strong binding with tissues. Additionally, the complex with molecular weight>10 kDa accounted for the highest percentage of total cadmium in Tris-HCl extraction, which indicated the possibility of significant conjugation of cadmium with various proteins, lipids and polysaccharide, other than metallothionine-Cd. Besides, cadmium complex with molecular weight lower than 500 Da accounted for about 4%~25% of Tris-HCl extracted cadmium, maybe consisting of free Cd²⁺ and the cadmium-cysteine or-glutathione complex. These results exhibited that other mechanism, differed with metallothionine-induced pathway, for the accumulation and metabolism of cadmium in marine animals existed. Meanwhile, considering the large percentage of cadmium complex, the risk analysis based on the traditional total cadmium maybe much overestimated.