Effects of tributyltin and other metals on the phenoloxidase activating system of the tunicate, Styela plicata

Toxic metals, such as tributyltin (TBT), contribute substantially to anthro pogenic pollution in many estuarine environments. Animals that live in thos e environments, particularly invertebrate filter feeders like tunicates, ar e likely to be exposed to substantial metal contamination. This study inves tigates the effects of TBT and other metals on the phenoloxidase activity o f the estuarine tunicate, Styela plicata, in an effort to identify a bioche mical marker of metal pollution. Hemocytes harvested from S. plicata that w ere exposed to tributyltin or copper in aquaria had significantly enhanced phenoloxidase activities relative to non-exposed controls. This enhanced ph enoloxidase activity could be explained by an increased frequency of morula cells, which contain high levels of phenoloxidase's proenzyme, prophenolox idase. Unlike those from tunicates exposed to metals in aquaria.. the pheno loxidase activities of hemocytes incubated with tributyltin in vitro were s ignificantly reduced when compared with hemocytes cultured without tributyl tin. The ability of tributyltin to decrease phenoloxidase activity in tissu e culture may reflect its known inhibitory effects on calcium-dependent sig naling systems such as those involved in the exocytosis of prophenoloxidase from morula cells. (C) 2001 Elsevier Science B.V. All rights reserved.