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Despite the ecologic and economic significance of the softshell clam ( Mya arenaria ), little is known about the humoral factors involved in its host defense mechanisms. Protease inhibitors, a group of proteins believed to play a role in host defense mechanisms against infections and proliferative diseases, have recently been identified in bivalve molluscs. In the present study we provide evidence for the presence of protease inhibitors in softshell clam plasma. Levels of protease inhibitory activities against the enzymes tested varied greatly, e.g. 1 μg of plasma protein inhibited 35.3±9.69 ng pepsin (aspartic protease), 4.9±1.45 ng papain (cysteine protease) and 3.1±0.88 ng trypsin (serine protease). On the contrary, the level of anti-metalloprotease (thermolysin) activities was much lower. The sensitivity to methylamine and the ability to protect trypsin from active site trypsin inhibitors provided evidence for the presence of an α 2 -macroglobulin-like molecule in softshell clam plasma. In the Chesapeake Bay widespread epizootics of disseminated sarcoma have been described in M. arenaria populations. The impact of this lethal proliferative disorder on clam defense responses has received little attention. In this study the effects of sarcoma progression on plasma protease inhibitory activities were, therefore, assessed. Clams with early stages of sarcoma showed a non-significant decrease in protease inhibitor levels. Clams with advanced stages of sarcoma showed a significant decrease in the ability to inhibit trypsin and papain, while the protease inhibitory activity levels against aspartic and metalloprotease were completely exhausted. |
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