An isolated oral bacterial polypeptide having amidolytic activity for cleavage of denatured polypeptides and nondenatured serpin polypeptides and particularly a human &agr;1-proteinase inhibitor polypeptide is provided. The mature polypeptide of the invention has a molecular weight of about 70 kD to about 80 kD. Also provided is an isolated nucleic acid sequence encoding the oral bacterial polypeptide of the invention, methods for identifying inhibitors of the polypeptide and compositions such as immunogenic compositions and inhibitor compositions.