A Structural and Functional Analogue of a Bowman Birk-type Protease Inhibitor from Odorrana Schmackeri

Frog skin secretions contain complex peptidomes and peptidic protease inhibitors are one of the biologically- and structurally-described groups of components. In this study, by use of molecular “shotgun” cloning and LC MS/MS fractionation sequencing, a novel Bowman Birk-type heptadecapeptide (AALKGCWTKSIPPKPCF-amide), named OSTI, with a canonical Cys6-Cys16 disulfide bridge, was isolated and identified in Piebald Odorous Frog (Odorrana schmackeri) skin secretion. A synthetic replicate of OSTI exhibited trypsin inhibitory activity with a Ki value of 0.3 ± 0.04 nM and also a tryptase inhibitory effect with a Ki of 2.5 ± 0.6 μM. This is the first time that this property has been reported for a peptide originating from amphibian sources. In addition, substituting lysine (K) with phenylalanine (F) at the presumed P1 position, completely abrogated the trypsin and tryptase inhibition, but produced a strong chymotrypsin inhibition with a Ki of 1.0 ± 0.1 μM. Thus, the specificity of this peptidic protease inhibitor could be optimized through modifying the amino acid residue at the presumed P1 position and this novel native OSTI, along with its analogue, [Phe9]-OSTI, have expanded the potential drug discovery and development pipeline directed towards alleviation of
serine protease-mediated pathologies.