Purification and characterization of a putative virulence factor, serine protease, from Vibrio parahaemolyticus

A protease (protease A) was successfully purified from the extracellular proteins of Vibrio parahaemolyticus no. 93, a clinical strain carrying neither tdh nor trh genes, using phenyl-Sepharose CL-4B hydrophobic interaction chromatography. The molecular mass of protease A was 43 kDa using gel filtration, which was in agreement with the results obtained from SDS-PAGE, suggesting that protease A was a monomeric protein. Additionally, the isoelectric point of this protein was 5.0. The optimum temperature and pH of protease A ranged from 40 degrees C to 50 degrees C and pH 8, respectively. Protease A activity was inhibited by serine protease inhibitors, such as phenylmethylsulfonyl fluoride and soybean trypsin inhibitor; moreover, the activity could be blocked by treatment with 20 mM of 1,10-phenanthroline, but could not be restored by adding metal ions. These results indicated that protease A is a serine protease that requires metal. The 12 N-terminal residues of protease A showed a high degree of identity (81%) to the sequence of Vibrio metschnikovii VapT serine protease. The purified protease had significant effects on the growth of Chinese hamster ovary, HeLa, Vero and Caco-2 cells and its cytotoxic activity was not blocked by gangliosides. Protease A lysed erythrocytes well but its hemolytic activity was unstable after heat treatment, indicating that protease A is able to cause hemolysis but is a heat-labile protein. The purified protease caused tissue hemorrhage and death in mice when injected both intraperitoneally and intravenously. In conclusion, this is the first report of a serine protease purified directly from the supernatant of V. parahaemolyticus and identifying it as a potential virulence factor.