Involved in the melanization cascade in response to lipopolysaccharide (LPS). In vitro, reversibly and competitively inhibits trypsin (Ki=8.6 nM) but not serine proteases chymotrypsin, elastase, subtilisin, thrombin and plasmin, cysteine peptidase papain or metallopeptidase carboxypeptidase A. Monomer. Expressed in hemocytes (at protein level). Contains 3 disulfide bonds.