The interaction of bovine pancreatic trypsin inhibitor (BPTI) and bovine tryptase, which are co-localized in the same granules of bovine mast cells, has been analyzed at 30°C in 0.1 M Tris-HCl, pH 8.0. The analysis has unravelled that the functional unit of bovine tryptase is formed of (at least) four binding sites for this inhibitor. These interaction sites display a simple binding behaviour for small inhibitors (and substrates), whereas heterogeneous properties have been observed in the binding of BPTI. Furthermore, in the presence of BPTI, a positive functional interaction can be detected among the binding sites also for a small synthetic inhibitor, like benzamidine. Such features indicate the existence of a complex functional interplay among the sites of the functional unit which is transmitted through the secondary specificity sites.
