Hirustasin Structural and biochemical investigations into the leech protease inhibitor hirustasin. The hirustasin-kallikrein complex crystal structure reveals a new class of serine protease inhibitors. CTA, Novartis AG., CH-4002 Basel, Switzerland. It belongs to a class of serine protease inhibitors characterized by a well conserved pattern of cysteine residues. The conservation of the cysteine pattern and the significant sequence similarity imply that these inhibitors possess a three-dimensional structure similar to hirustasin.
Tissue kallikreins belong to the group of trypsin-like serine proteases.
They are mainly involved in kinin liberation from their natural targets, the kininogens and exert via the kinins a broad range of biological activities. Prostate specific antigen, an abundant marker for prostate cancer thought to be involved in tumor growth and metastasis, is structurally related to glandular kallikrein-1. In order to define the reactive site of the inhibitor, the interaction of hirustasin with kallikrein, chymotrypsin and trypsin was investigated by monitoring complex formation in solution as well as proteolytic cleavage of the inhibitor. During incubation with high and equimolar concentration of tissue kallikrein, hirustasin was cleaved mainly at the peptide bond between Arg30 and Ile31, the putative reactive site, to yield a modified inhibitor. In the corresponding complex with chymotrypsin, mainly uncleaved hirustasin was found and cleaved hirustasin species accumulated only slowly.