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¹ College of Environmental Technology, Muroran Institute of Technology, 27-1 Mizumoto, Muroran 050-8585, Japan DOI : 10.4194/1303-2712-v14_1_26 Viewed : 4051 - Downloaded : 2796 We have previously reported the free radical scavenging activity of a protein with a molecular weight of 90 kDa (90- kDa protein) isolated from the scallop shell. In this study, we found that the 90-kDa protein also shows protease activity. The protein was most active at an alkali pH and at 60°C, and its activity was inhibited by serine protease inhibitors, phenylmethylsulfonyl fluoride and diisopropyl fluorophosphate. Its activity was maintained at approximately 90% of the initial activity, even in the presence of denaturants such as 1% sodium dodecyl sulfate (SDS) and 6 M urea. Substrate specificity analysis performed using synthetic peptides showed that the 90-kDa protein cleaves preferentially at Lys-X and Arg-X bonds. A portion of Phe-X bond was also cleaved by the 90-kDa protein. When casein was treated with the 90-kDa protein, it was digested at the Arg-X, Lys-X, and Phe-X bonds. The 90-kDa protein may be useful for proteome analysis because it retains its activity even in the presence of 1% SDS. To the best of our knowledge, this is the first report of a protease found in scallop shell. Keywords : Alkaline protease, scallop shell, shell matrix protein, thermostable