Accumulation and Elimination of Cadmium and Zinc in the Asian PeriwinkleLittorinabrevicula

To elucidate the differences between the detoxification mechanisms of essential metal (Zn) and non-essential metal (Cd) in Littorina brevicula that is highly resistant to a wide range of heavy metal concentrations, Asian periwinkles were exposed to Cd (400 microg/L), Zn (3000 microg/L) and a mixture of both metals. We examined metal accumulation, elimination and subcellular distribution for binding to proteins. The metal concentration in L. brevicula increased gradually with exposure time (up to 70 days), following which accumulated levels reached saturation point. The accumulated Zn content was increased in the presence of Cd, while Cd uptake was decreased when Zn was present. During the depuration period (42 days), Cd was not removed from periwinkles, while Zn was eliminated in a rate of 2.19 microg Zn g(-1) day(-1). This elimination rate was particularly high on exposure to the metal mixture (3.80 microg Zn g(-1) day(-1)). Subcellular distribution studies on Cd and Zn revealed that most Cd (80%) was bound to cytosolic ligand, while more than 75% Zn was distributed in the membrane (insoluble) fraction. An additional difference in sequestering of metal in the cytosol was noted between Cd and Zn; most Cd in the cytosol was bound to metallothionein-like cadmium binding protein, MBP-1 (9.8 kDa), while the profile for Zn distribution revealed the presence of four Zn-binding ligand peaks, specifically, HMW (60 kDa), MBP-1 (9.8 kDa), MBP-2 (5 kDa) and LMW (<1 kDa). Our data confirm that metallothionein-like cadmium binding protein, MBP-1, has the same affinity for Zn, while MBP-2 displays comparatively higher affinity for Zn than Cd.