Application of high-performance liquid chromatography-inductively coupled plasma mass spectrometry to the investigation of cadmium speciation in pig kidney following cooking and in vitro gastro-intestinal digestion

Abstract

The speciation of cadmium in retail pig kidney has been examined by size-exclusion chromatography (SEC) coupled directly to inductively coupled plasma mass spectrometry (ICP-MS). Approximately 35% of the cadmium from uncooked kidney was soluble after aqueous extraction at pH 8 and SEC-ICP-MS revealed three discrete peaks whose retention times corresponded to estimated relative molecular masses of 1.2 × 10⁶, 7.0 × 10⁴ and 6 × 10³–9 × 10³. In the cooked kidney, 35% of the Cd was soluble and was all associated with a peak of a relative molecular mass (M_r) of 6 × 10³–9 × 10³. After simulated gastric digestion of cooked pig kidney at pH 2.5, 60% of the cadmium was solubilised and associated with a species of M_r < 1 × 10³. When the digest was also subjected to simulated intestinal digestion at pH 6.8, a single peak, which corresponded to 20% of the original cadmium, was eluted. This peak co-eluted with the single peak extracted at pH 8.0 from the cooked kidney. It was also of similar estimated M_r to the single broad peak observed after simulated gastro-intestinal digestion of equine renal metallothionein (M_r= 1.1 × 10⁴). The results suggest that the majority of soluble cadmium in retail pig kidney is associated with a metallothionein-like protein that survives both cooking and simulated in vitro gastro-intestinal digestion.