Slow-binding inhibitor of serine proteases. The inhibitor rapidly binds to the protease forming a weak enzyme-inhibitor complex, and this is followed by a slow isomerization forming a tight-binding enzyme-inhibitor complex. Active against subtilisin A with a dissociation constant of 0.18 nM. Active against perkinsin. Not active against thermolysin, papain or pepsin. Detected in hemolymph (at protein level). Within the digestive gland expression is limited to the basophil cells of the digestive diverticula. Contains 6 disulfide bonds.