Inactivation of proteinaceous protease inhibitors of soybeans by isolated fungi

Abstract

Proteinaceous protease inhibitors, Kunitz Soybean Trypsin Inhibitor (KSTI) and Bowman Birk Inhibitor (BBI), in legume seeds reduce the digestibility of proteins in feed of monogastric animals. Enzymatic inactivation of these inhibitors will increase the nutritional value of the feed. The aim of this study was to obtain microorganisms which produce proteases which specifically inactivate the inhibitors under conditions that occur in the stomach. Hereto, microorganisms were enriched at pH 3.0 and 37 °C on KSTI or BBI as sole nitrogen and sulphur source. Nine fungal strains were isolated from the enrichment cultures. The isolated strains were able to inactivate both inhibitors. KSTI was inactivated more effectively than BBI. KSTI inactivation was pH dependent. At pH 5.5, only 30–45% of the KSTI was inactivated in 4 days, whereas, 80–90% of the KSTI was inactivated in 2 days at pH 3.0. The isolated strains varied in capacities to inactivate KSTI. In general, strains which inactivated KSTI to a higher extent had higher biomass yields and higher general protease activities. The KSTI specificity of the most promising strain (K1) which had a relatively high KSTI inactivation capacity and a relatively low general protease activity at pH 3.0, was studied in more detail. The KSTI specificity of the produced proteases was affected by pH and by the type of substrates used [KSTI, bovine serum albumin fraction V (BSA), cysteine or without substrate]. KSTI specificity of the proteases produced at pH 5.5 was low as compared to pH 3.0. The highest KSTI specificity, 1.7 mole KSTI per mole albumin, was found when strain K1 was grown on cysteine at pH 3.0.