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Plant peptidase inhibitors play essential roles in the defense systems of plants. A trypsin inhibitor (PHTI) with a molecular mass of 20.5 kDa was isolated from the fresh roots of the medicinal herb, Pseudostellaria heterophylla . The purification process involved ammonium sulfate precipitation, gel filtration chromatography on Sephadex G50, and ion-exchange chromatography on DEAE 650M. The PHTI contained 3.7% α-helix, 42.1% β-sheets, 21.2% β-turns, and 33% disordered structures, which showed similarity with several Kunitz-type trypsin inhibitors. Inhibition kinetic studies indicated that PHTI was a competitive inhibitor, with a K i value of 3.01 × 10 -9 M, indicating a high affinity to trypsin. The PHTI exhibited considerable stability over a broad range of pH (2⁻10) and temperatures (20⁻70 °C); however, metal ions, including Fe 3+ , Ba 2+ , Mn 2+ , and Al 3+ , could inactivate PHTI to different degrees. Results of fluorescence spectroscopy and circular dichroism showed that Fe 3+ could bind to TI with an association constant of 2.75 × 10⁵ M -1 to form a 1:1 complex, inducing conformation changes and inactivation of PHTI. In addition, PHTI could inhibit the growth of the phytopathogens, Colletotrichum gloeosporioides and Fusarium oxysporum , through disruption of the cell membrane integrity. The present study extended research on Pseudostellaria heterophylla proteins and makes PHTI an exploitable candidate as an antifungal protein for further investigation.

Physico-Chemical and Antifungal Properties of a Trypsin Inhibitor from the Roots of Pseudostellaria heterophylla