Differential roles of metallothionein‐like proteins in cadmium uptake and elimination by the scallop Chlamys nobilis

Abstract

Metallothionein-like proteins (MTLPs) are important inducible ligands for Cd detoxification, but whether MTLPs are involved in Cd bioaccumulation remains unclear. The present study investigated the potential interrelationships between MTLP turnover and Cd biokinetics in the scallop Chlamys nobilis with or without dissolved Cd pre-exposure. After Cd pre-exposure, increased synthesis rate and standing stock concentration of MTLPs as well as increased dissolved Cd uptake rate, were found in C. nobilis. However, the level of newly sequestrated Cd by MTLPs in the pre-exposed scallops was similar to that of the controls, indicating that MTLPs had no effect on Cd uptake from the dissolved phase. Instead, Cd uptake rate was positively related to the Cd binding rate of cellular debris and organelles. In addition, no significant difference was observed in the Cd dietary assimilation efficiency of the scallops, showing that MTLPs played a minimal role in Cd uptake from the dietary phase. Conversely, the Cd efflux rate constant decreased by 68 to 75% in the pre-exposed scallops in comparison with that in the control. Correspondingly, progressive elevations of MTLP-bound Cd and standing stock MTLP concentration were observed in the pre-exposed individuals during the Cd elimination period. In the pre-exposed scallops, the faster de novo MTLP synthesis could remobilize and recycle more Cd from other intracellular pools, effectively reducing the Cd elimination rate. The present study thus showed the differential roles of MTLPs in Cd uptake and elimination in the bivalves.

Uptake pathways and subcellular fractionation of Cd in the polychaete Nereis diversicolor

“…Based on the fractionation procedure used in the present study (Wallace and Luoma 2003), S1 fraction theoretically contains most of the metal-binding proteins, such as metallothionein (MT), which was induced for 1 week at a low Ca concentration (2.0 mM, a) and the accumulation of Cd (expressed as the actual Cd concentrations in each fraction) in the three subcellular fractions of polychaete N. diversicolor exposed to the solutions containing 10 lM CdCl 2 for 1 week at both low (2.0 mM) and high (10.0 mM) Ca concentrations (b). Fractions S1, S2 and P2 represent the cytosol, metal associated cellular debris, and metal rich granules, respectively rapidly by the organism upon short-term exposure (within 2-4 days) to metals (Liu and Wang 2011;Shi and Wang 2005;Yang and Thompson 1996). The role of MT in the sequestration and detoxification of certain metals, such as Cd, has been well documented in marine invertebrates (Pan and Wang 2008;Wallace and Luoma 2003).…”

Metallothionein-like proteins turnover, Cd and Zn biokinetics in the dietary Cd-exposed scallop Chlamys nobilis

“…A clear example comes from the nereid polychaete Perinereis aibuhitensis (Ng et al, 2008), where faster turnover but comparable MTLP concentrations have been observed in Cd-exposed animals. Our previous work has recently shown that the scallop C. nobilis with 6-24 g Cd g −1 wet weight had both higher MTLP turnover and increased MTLP concentrations following waterborne Cd exposure (Liu and Wang, 2011). Taken together, these results highlight the importance of further studies of MTLP turnover to better understand Cd detoxification as well as MTLP's utility as a biomarker of metal exposure.…”

“…A positive relationship between Cd uptake from either water or food and induced MTLP concentration has also been found in the black seabream Acanthopagrus schlegeli (Long and Wang, 2005). The importance of MTLP for Cd elimination has been demonstrated in the clam Ruditapes decusstatus (Serafim and Bebianno, 2007), the scallop Chlamys nobilis (Liu and Wang, 2011), and the freshwater bivalve Corbicula fluminea (Baudrimont et al, 2003). However, there have been inconsistent findings about the physiological roles of MTLPs in metal biokinetics Rainbow, 2005, 2010).…”

“…The Cd concentration in the scallop was expressed as g Cd g −1 wet weight. The MTLP concentrations in the whole wet tissues were determined by an 110m Ag saturation method (Scheuhammer and Cherian, 1991;Liu and Wang, 2011). A metallothionein standard (a mixture of MT-I and MT-II rabbit liver, Dalian Free Trade Zone United BoTai Bio-Tech) was used, with a recovery rate of >87% in each measurement.…”

“…Dissolved uptake, dietary assimilation and efflux of Cd and Zn, and MTLP synthesis and degradation were quantified using wellestablished radiotracer techniques (Ng et al, 2007;Liu and Wang, 2011). The influx rates of Cd and Zn were quantified in 0.22 mfiltered seawater spiked with 109 Cd (1.04 and 1.07 Ci L −1 for the 8 and 40-d Cd exposed scallops, respectively) and 65 Zn (1.05 and 1.20 Ci L −1 for the 8 and 40-d Cd exposed scallops, respectively).…”