Pepsin-digestibility of contaminated estuarine sediments

A standard method for the in vitro digestion of animal protein feeds (2% pepsin in 0.075 N HCl) has been applied to contaminated sediments in order to evaluate a 'bioavailable' or 'gut-soluble' fraction of carbon, nitrogen and mineral and trace metals. For most sediment samples, considerably more nitrogen was digested than carbon because of enzymatic digestion of proteinaceous material, and the sequence of metal 'gut-solubility' was: Cu, Zn>Mn>Fe much greater than Al. The principal mechanism of metal release appears to be hydrochloric acid digestion of inorganic hydrogenous host phases (e.g. amorphous Fe and Mn oxides), although release of Cu via surface complexation with pepsin molecules may also be significant, and the amount of metal digested enzymatically is restricted to a small and unquantifiable fraction associated with proteinaceous material. Dilute HCl alone does not, however, afford a suitable surrogate for assessing a gut-soluble fraction of metal because enzymatic and acid digestions exhibit synergistic effects, including possible re-adsorption of pepsin-metal complexes under acidic conditions, and exposure and acid attack of otherwise inaccessible hydrogenous material following enzymatic digestion of organic matter. (C) 2001 Academic Press.