Mechanisms of Cu Solubilization during Deposit Feeding

Abstract

Large amounts of Cu in contaminated sediments can be dissolved by digestive fluids of deposit feeders during an in vitro digestion (Mayer et al. Environ. Sci. Technol. 1996, 30, 2641 and may subsequently become available to those organisms. The mechanisms of this digestive dissolution were investigated by comparing Cu release potentials of modified and unmodified gut fluids. Microwave treatments indicated that complexation rather than enzymatic action likely accounts for enhanced Cu solubilization by gut fluids rich in protein. Solutions of a common protein, bovine serum albumin, with similar AA concentrations mimicked digestive fluids in releasing sedimentary Cu. Chemical modification of histidine residues in gut fluids drastically reduced Cu release, suggesting their important role in Cu bioavailability. Using an ion-selective electrode, we found an abundance of gut ligand sites with conditional stability constants >10 10 , consistent with histidine complexation. Histidine concentrations among various molecular weight fractions of gut fluids can be predictive of the amount of Cu released during in vitro digestion. Complexation-enhanced metal remobilization by gut fluids thus has significant impact on metal availability during deposit feeding.

Sedimentary metal bioavailability determined by the digestive constraints of marine deposit feeders:gut retention time and dissolved amino acids

“…The importance of food ingestion and digestion in bioavailability was well illustrated with in vitro digestion experiments by incubating digestive fluids of marine deposit feeders with Cu-contaminated sediments; digestive fluid released 2 to 3 orders of magnitude more sedimentary Cu than seawater controls (Mayer et al 1996). A follow-up study showed that complexation of sedimentary Cu with amino acids (AA) in gut fluid was responsible for the marked dissolution, while the nearly neutral pH in gut fluids of these marine deposit feeders ruled out the possibility of an H+-mediated process (Chen & Mayer 1998b). Although some of the solubilized metals may not be subsequently bioaccumulated, this process does expose guts to elevated concentrations of metals and can lead to digestive enzyme toxification (Chen & Mayer 1998a).…”

“…Gut fluid from multiple individuals (>10) of the 3 species were pooled and stored in 1.5 m1 plastic vials at -80°C until experiments. Concentrations of total dissolved AA in gut fluid, an important parameter in metal remobilization (Chen & Mayer 1998b), were analyzed by hydrolyzing samples in 6 N HCl, derivatizing the AA with orthophthaldialdehyde, and assaying fluorometrically using a Hitachi FL4500 fluorometer. This method typically yields results with coefficients of variation <10%.…”

“…During the kinetic experiments, incubations were stopped periodically by centrifugation at 8000 X g for 30 min at 4°C. Metal contents in fluids were measured by GFAAS with a coefficient of variation < 5 % (Chen & Mayer 1998b). Final metal concentratlons in gut fluid after incubation were corrected by subtracting those in seawater, assuming that seawater releases only metals associated with pore water or loosely adsorbed fractions.…”

“…4 h) varied among gut fluids of different species of organisms (Mayer et al 1996, Chen & Mayer 1998b). This species-specific variation correlated with lumina1 concentrations of AA (Chen 1997, Chen & Mayer 1998b.…”

“…4 h) varied among gut fluids of different species of organisms (Mayer et al 1996, Chen & Mayer 1998b). This species-specific variation correlated with lumina1 concentrations of AA (Chen 1997, Chen & Mayer 1998b. In the present study, we focus on another factor with potential impact on the solubilization and bioavailability of sedimentary metals-the digestion time, or gut retention time (GRT) during in vivo digestion.…”